Changeset 39136 in project


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Timestamp:
11/07/20 02:42:04 (3 weeks ago)
Author:
gnosis
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Fixed formatting snafu

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  • wiki/eggref/5/srfi-130

    r39135 r39136  
    1 * SRFI-130: Cursor-based string library
    2 ** Abstract
    3 [[https://srfi.schemers.org/srfi-130/srfi-130.html#R5RS][R5RS]] Scheme has an impoverished set of string-processing utilities, which is a problem for authors of portable code. Although [[https://srfi.schemers.org/srfi-130/srfi-130.html#R7RS][R7RS]] provides some extensions and improvements, it is still very incomplete. This SRFI proposes a coherent and comprehensive set of string-processing procedures; it is accompanied by a portable sample implementation of the spec.
    4 
    5 This SRFI is derived from SRFI 13. The biggest difference is that it allows subsequences of strings to be specified by cursors as well as the traditional string indexes. In addition, it omits the comparison, case-mapping, and mutation operations of SRFI 13, as well as all procedures already present in [[https://srfi.schemers.org/srfi-130/srfi-130.html#R7RS][R7RS]].
    6 
    7 For more information see: [[https://srfi.schemers.org/srfi-130/][SRFI-130: Cursor-based string library]]
    8 ** Procedure Index
     1== SRFI-130: Cursor-based string library
     2=== Abstract
     3[[https://srfi.schemers.org/srfi-130/srfi-130.html#R5RS|R5RS]] Scheme has an impoverished set of string-processing utilities, which is a problem for authors of portable code. Although [[https://srfi.schemers.org/srfi-130/srfi-130.html#R7RS|R7RS]] provides some extensions and improvements, it is still very incomplete. This SRFI proposes a coherent and comprehensive set of string-processing procedures; it is accompanied by a portable sample implementation of the spec.
     4
     5This SRFI is derived from SRFI 13. The biggest difference is that it allows subsequences of strings to be specified by cursors as well as the traditional string indexes. In addition, it omits the comparison, case-mapping, and mutation operations of SRFI 13, as well as all procedures already present in [[https://srfi.schemers.org/srfi-130/srfi-130.html#R7RS|R7RS]].
     6
     7For more information see: [[https://srfi.schemers.org/srfi-130/|SRFI-130: Cursor-based string library]]
     8=== Procedure Index
    99Here is a list of the procedures provided by this SRFI.
    10 *** Cursor operations
    11 **** string-cursor?
    12 **** string-cursor-start    string-cursor-end
    13 **** string-cursor-next     string-cursor-prev
    14 **** string-cursor-forward  string-cursor-back
    15 **** string-cursor=?
    16 **** string-cursor<?        string-cursor>?
    17 **** string-cursor<=?       string-cursor>=?
    18 **** string-cursor-diff
    19 **** string-cursor->index   string-index->cursor
    20 *** Predicates
    21 **** string-null?
    22 **** string-every string-any
    23 *** Constructors
    24 **** string-tabulate
    25 **** string-unfold   string-unfold-right
    26 *** Conversion
    27 **** string->list/cursors string->vector/cursors
    28 **** reverse-list->string string-join
    29 *** Selection
    30 **** string-ref/cursor
    31 **** substring/cursors  string-copy/cursors
    32 **** string-take        string-take-right
    33 **** string-drop        string-drop-right
    34 **** string-pad         string-pad-right
    35 **** string-trim        string-trim-right string-trim-both
    36 *** Prefixes & suffixes
    37 **** string-prefix-length    string-suffix-length
    38 **** string-prefix?          string-suffix?
    39 *** Searching
    40 **** string-index     string-index-right
    41 **** string-skip      string-skip-right
    42 **** string-contains  string-contains-right
    43 *** The whole string
    44 **** string-reverse
    45 **** string-concatenate  string-concatenate-reverse
    46 **** string-fold         string-fold-right
    47 **** string-for-each-cursor
    48 **** string-replicate    string-count
    49 **** string-replace      string-split
    50 **** string-filter       string-remove
    51 ** Rationale
     10==== Cursor operations
     11* string-cursor?
     12* string-cursor-start    string-cursor-end
     13* string-cursor-next     string-cursor-prev
     14* string-cursor-forward  string-cursor-back
     15* string-cursor=?
     16* string-cursor<?        string-cursor>?
     17* string-cursor<=?       string-cursor>=?
     18* string-cursor-diff
     19* string-cursor->index   string-index->cursor
     20==== Predicates
     21* string-null?
     22* string-every string-any
     23==== Constructors
     24* string-tabulate
     25* string-unfold   string-unfold-right
     26==== Conversion
     27* string->list/cursors string->vector/cursors
     28* reverse-list->string string-join
     29==== Selection
     30* string-ref/cursor
     31* substring/cursors  string-copy/cursors
     32* string-take        string-take-right
     33* string-drop        string-drop-right
     34* string-pad         string-pad-right
     35* string-trim        string-trim-right string-trim-both
     36==== Prefixes & suffixes
     37* string-prefix-length    string-suffix-length
     38* string-prefix?          string-suffix?
     39==== Searching
     40* string-index     string-index-right
     41* string-skip      string-skip-right
     42* string-contains  string-contains-right
     43==== The whole string
     44* string-reverse
     45* string-concatenate  string-concatenate-reverse
     46* string-fold         string-fold-right
     47* string-for-each-cursor
     48* string-replicate    string-count
     49* string-replace      string-split
     50* string-filter       string-remove
     51=== Rationale
    5252This SRFI defines a rich set of operations for manipulating strings. These are frequently useful for scripting and other text-manipulation applications. The library's design was influenced by the string libraries found in MIT Scheme, Gambit, RScheme, MzScheme, SLIB, Common Lisp, Bigloo, Guile, Chez, APL, Java, and the SML standard basis. All functionality is available in substring and full-string forms.
    5353
     
    5656Unfortunately for this design, Unicode has become much more widely used, and it is now fairly common for implementations to store strings internally as UTF-8 or UTF-16 code unit sequences, which means that indexing operations are potentially O(n) rather than O(1). Using opaque cursors makes it possible to iterate much more efficiently through such strings compared to incrementing or decrementing indexes; however, for backward compatibility, the procedures defined in this SRFI accept either cursors or indexes. The results returned are always cursors: the use of indexes is preserved mainly for the sake of existing code and for implementer convenience.
    5757
    58 The operations provided here are entirely independent of the character repertoire supported by the implementation. In particular, this means that the comparison and case conversion procedures of SRFI 13 are excluded. There is also no provision for [[http://www.r6rs.org/final/html/r6rs-lib/r6rs-lib-Z-H-2.html#node_idx_54][R6RS normalization procedures]] or for a string->integer procedure that was proposed for SRFI 13 but not included. These may appear in future SRFIs. Furthermore, string mutation can be extremely expensive if the storage used for the string needs to be expanded, particularly if the implementation does not use an indirect pointer to it (as in Chicken), so this SRFI does not provide for it. The low-level procedures of SRFI 13 are specific to the sample implementation, and have been removed to make other implementations simpler and easier.
    59 
    60 Many SRFI 13 procedures accept either a predicate, a single character, or a [[https://srfi.schemers.org/srfi-14/srfi-14.html][SRFI 14]] character set. In this SRFI, only support for predicates is required, though implementations may also support the other two alternatives. In that case, a single character is interpreted as a predicate which returns true if its argument is the same (in the sense of eqv?) to that character; a character set is interpreted as a predicate which returns true if its argument belongs to that character set. In SRFI 13, character sets are inherently more efficient than predicates [[https://srfi.schemers.org/srfi-13/mail-archive/msg00052.html][because testing them is fast and free of side effects]], though how fast character sets actually are if they support full Unicode is very implementation-dependent. The only procedure that absolutely requires character set support, string-tokenize, has been replaced here by the more usual string-split procedure provided by Perl, Python, Java, JavaScript, and other languages.
    61 
    62 The search procedures in SRFI 13 return either an index or #f if the search fails. Their counterparts in this SRFI return cursors. Left-to-right searches return a cursor representing the leftmost matching character, or the post-end cursor if there is no match; right-to-left searches return a cursor representing the successor of the rightmost matching character, or the start cursor if there is no match. This convention was devised by Alan Watson and implemented in Chibi Scheme.
     58The operations provided here are entirely independent of the character repertoire supported by the implementation. In particular, this means that the comparison and case conversion procedures of SRFI 13 are excluded. There is also no provision for [[http://www.r6rs.org/final/html/r6rs-lib/r6rs-lib-Z-H-2.html#node_idx_54|R6RS normalization procedures]] or for a string->integer procedure that was proposed for SRFI 13 but not included. These may appear in future SRFIs. Furthermore, string mutation can be extremely expensive if the storage used for the string needs to be expanded, particularly if the implementation does not use an indirect pointer to it (as in Chicken), so this SRFI does not provide for it. The low-level procedures of SRFI 13 are specific to the sample implementation, and have been removed to make other implementations simpler and easier.
     59
     60Many SRFI 13 procedures accept either a predicate, a single character, or a [[https://srfi.schemers.org/srfi-14/srfi-14.html|SRFI 14]] character set. In this SRFI, only support for predicates is required, though implementations may also support the other two alternatives. In that case, a single character is interpreted as a predicate which returns true if its argument is the same (in the sense of eqv?) to that character; a character set is interpreted as a predicate which returns true if its argument belongs to that character set. In SRFI 13, character sets are inherently more efficient than predicates [[https://srfi.schemers.org/srfi-13/mail-archive/msg00052.html|because testing them is fast and free of side effects]], though how fast character sets actually are if they support full Unicode is very implementation-dependent. The only procedure that absolutely requires character set support, string-tokenize, has been replaced here by the more usual string-split procedure provided by Perl, Python, Java, JavaScript, and other languages.
     61
     62The search procedures in SRFI 13 return either an index or {{#f}} if the search fails. Their counterparts in this SRFI return cursors. Left-to-right searches return a cursor representing the leftmost matching character, or the post-end cursor if there is no match; right-to-left searches return a cursor representing the successor of the rightmost matching character, or the start cursor if there is no match. This convention was devised by Alan Watson and implemented in Chibi Scheme.
    6363
    6464In short, this SRFI is intended to help move the practice of Scheme programming away from mutable strings, string indexes, and SRFI 13, while largely maintaining backward compatibility. It does not require any particular run-time efficiencies from its procedures.
    65 ** Specification
    66 *** String cursors
    67 While indexes are exact integers ranging from 0 to the length of the string they refer to, cursors are opaque objects that point into strings. However, they are not required to belong to a disjoint type, as long as they are either disjoint from indexes or identical to indexes. For example, they may be negative exact integers representing indexes into a byte array underlying the string. It is also possible to implement cursors as a record type or an implementation-specific primitive type. Additionally, in implementations where no provision has been made for cursors, or there is no benefit in implementing them separately because strings are in fact arrays of fixed-length characters, it is useful to allow indexes and cursors to be the same thing. (Cursors must also be disjoint from #f.)
    68 
    69 It is an error to make any use of a cursor referring to a string after the string, or any string that shares storage with it, has been mutated by a procedure like string-set!, string-copy!, or string-fill!.
    70 
    71 Given a string of length n, there are n + 1 valid cursors that refer to it: one for each character in the string, and one for the position just after the last character, known as the "post-end cursor". The cursor for the first (or zeroth) position in the string is known as the "start cursor". The post-end cursor is provided because when creating a string from cursors the second cursor argument is exclusive. It is an error if a cursor argument is not one of the valid cursors for the string argument. The index analogue of the post-end cursor is n.
    72 *** Calling predicates
     65=== Specification
     66==== String cursors
     67While indexes are exact integers ranging from 0 to the length of the string they refer to, cursors are opaque objects that point into strings. However, they are not required to belong to a disjoint type, as long as they are either disjoint from indexes or identical to indexes. For example, they may be negative exact integers representing indexes into a byte array underlying the string. It is also possible to implement cursors as a record type or an implementation-specific primitive type. Additionally, in implementations where no provision has been made for cursors, or there is no benefit in implementing them separately because strings are in fact arrays of fixed-length characters, it is useful to allow indexes and cursors to be the same thing. (Cursors must also be disjoint from {{#f}}.)
     68
     69It is an error to make any use of a cursor referring to a string after the string, or any string that shares storage with it, has been mutated by a procedure like {{string-set!}}, {{string-copy!}}, or {{string-fill!}}.
     70
     71Given a string of length {{n}}, there are {{n + 1}} valid cursors that refer to it: one for each character in the string, and one for the position just after the last character, known as the "post-end cursor". The cursor for the first (or zeroth) position in the string is known as the "start cursor". The post-end cursor is provided because when creating a string from cursors the second cursor argument is exclusive. It is an error if a cursor argument is not one of the valid cursors for the string argument. The index analogue of the post-end cursor is n.
     72==== Calling predicates
    7373All predicates passed to procedures defined in this SRFI may be called in any order and any number of times, except as otherwise noted. This is not the case in SRFI 13.
    74 *** Shared storage
     74==== Shared storage
    7575Some Scheme implementations, e.g. Guile, provide ways to construct substrings that share storage with other strings. SRFI 130 provides only minimal support for such shared substrings. The following SRFI 130 procedures are allowed to return a result which shares storage with one or more of their string arguments:
    7676
    77 #+BEGIN_EXAMPLE
    78 substring/cursors
    79 string-take string-take-right
    80 string-drop string-drop-right
    81 string-pad string-pad-right
    82 string-trim string-trim-right string-trim-both
    83 string-split string-filter string-remove
    84 #+END_EXAMPLE
    85 
    86 In particular, if the result is the same (in the sense of string=?) as any of the arguments, any implementation of the above procedures may return the string argument without copying it. Other procedures such as string-copy/cursors, as well as all the [[https://srfi.schemers.org/srfi-130/srfi-130.html#R7RS][R7RS]] procedures, are not permitted to return shared results. If a shared value is returned, it may be mutable or immutable.
    87 *** Naming conventions
     77 substring/cursors
     78 string-take string-take-right
     79 string-drop string-drop-right
     80 string-pad string-pad-right
     81 string-trim string-trim-right string-trim-both
     82 string-split string-filter string-remove
     83
     84In particular, if the result is the same (in the sense of {{string=?}}) as any of the arguments, any implementation of the above procedures may return the string argument without copying it. Other procedures such as {{string-copy/cursors}}, as well as all the [[https://srfi.schemers.org/srfi-130/srfi-130.html#R7RS|R7RS]] procedures, are not permitted to return shared results. If a shared value is returned, it may be mutable or immutable.
     85==== Naming conventions
    8886The procedures of this SRFI follow a consistent naming scheme, and are consistent with the conventions developed in SRFI 1. The names are composed of smaller lexemes in a regular way that exposes the structure and relationships between the procedures. This should help the programmer to recall or reconstitute the name of the desired procedure. In particular, the order of common parameters is consistent across the different procedures.
    8987
    9088Procedures that have left/right directional variants use no suffix to specify left-to-right operation, -right to specify right-to-left operation, and -both to specify both. This is a general convention that has been established in other SRFIs; the value of a convention is proportional to the extent of its use.
    91 *** Notation
    92 **** In the following procedure specifications:
    93 ***** An s parameter is a string.
    94 ***** A char parameter is a character.
    95 ***** Start and end parameters are half-open string cursors or indexes specifying a substring within a string parameter, and typically restrict a procedure's action to the indicated substring.
    96 When omitted, they default to 0 and the length of the string, respectively; or from another point of view, they default to the start cursor and the post-end cursor, respectively. For indexes, it must be the case that 0 <= start <= end <= (string-length s), for the corresponding parameter s when start and end are indexes, and the corresponding relationship must hold when they are cursors. It is an error unless start and end are both cursors or both indexes.
    97 ***** A pred parameter is a unary character predicate procedure, returning a true/false value when applied to a character.
    98 It is an error if a pred is not pure and functional.
    99 ***** A cursor parameter is either a cursor or an exact non-negative integer specifying an index into a string.
    100 ***** Len and nchars parameters are exact non-negative integers specifying a length of a string or some number of characters.
    101 ***** An obj parameter may be any value at all.
    102 **** Passing values to procedures with these parameters that do not satisfy these types is an error.
    103 **** Parameters given in square brackets are optional.
    104 Unless otherwise noted in the text describing the procedure, any prefix of these optional parameters may be supplied, from zero arguments to the full list. When a procedure returns multiple values, this is shown by listing the return values in square brackets, as well.
    105 ***** So, for example, the procedure with signature
    106 #+BEGIN_EXAMPLE
    107 halts? f [x init-store] → [boolean integer]
    108 #+END_EXAMPLE
    109 
    110 would take one (f), two (f, x) or three (f, x, init-store) input parameters, and return two values, a boolean and an integer.
    111 **** A parameter followed by "..." means zero or more elements.
    112 ***** So the procedure with the signature
    113 #+BEGIN_EXAMPLE
    114 sum-squares x ...  → number
    115 #+END_EXAMPLE
    116 takes zero or more arguments (x ...),
    117 ***** while the procedure with signature
    118 #+BEGIN_EXAMPLE
    119 spell-check doc dict[1] dict[2] ... → string-list
    120 #+END_EXAMPLE
    121 
    122 takes two required parameters (doc and dict[1]) and zero or more optional parameters (dict[2] ...).
    123 **** If a procedure's return value is said to be "unspecified," this means that the procedure returns a single arbitrary value.
     89==== Notation
     90===== In the following procedure specifications:
     91* An {{s}} parameter is a string.
     92* A {{char}} parameter is a character.
     93* {{Start}} and {{end}} parameters are half-open string cursors or indexes specifying a substring within a string parameter, and typically restrict a procedure's action to the indicated substring.
     94
     95When omitted, they default to 0 and the length of the string, respectively; or from another point of view, they default to the start cursor and the post-end cursor, respectively. For indexes, it must be the case that 0 <= {{start}} <= {{end}} <= {{(string-length s)}}, for the corresponding parameter s when {{start}} and {{end}} are indexes, and the corresponding relationship must hold when they are cursors. It is an error unless {{start}} and {{end}} are both cursors or both indexes.
     96* A {{pred}} parameter is a unary character predicate procedure, returning a true/false value when applied to a character.
     97* It is an error if a {{pred}} is not pure and functional.
     98* A cursor parameter is either a cursor or an exact non-negative integer specifying an index into a string.
     99* {{Len}} and {{nchars}} parameters are exact non-negative integers specifying a length of a string or some number of characters.
     100* An {{obj}} parameter may be any value at all.
     101* Passing values to procedures with these parameters that do not satisfy these types is an error.
     102* Parameters given in square brackets are optional.
     103* Unless otherwise noted in the text describing the procedure, any prefix of these optional parameters may be supplied, from zero arguments to the full list.
     104* When a procedure returns multiple values, this is shown by listing the return values in square brackets, as well.
     105====== So, for example, the procedure with signature
     106{{halts? f [x init-store] → [boolean integer]}}
     107
     108would take one {{(f)}}, two {{(f, x)}} or three {{(f, x, init-store)}} input parameters, and return two values, a boolean and an integer.
     109===== A parameter followed by "..." means zero or more elements.
     110====== So the procedure with the signature
     111{{sum-squares x ...  → number}}
     112
     113takes zero or more arguments {{(x ...)}},
     114====== while the procedure with signature
     115{{spell-check doc dict[1] dict[2] ... → string-list}}
     116
     117takes two required parameters ({{doc}} and {{dict[1]}}) and zero or more optional parameters ({{dict[2]}} ...).
     118===== If a procedure's return value is said to be "unspecified," this means that the procedure returns a single arbitrary value.
    124119Such a procedure is not even required to be consistent from call to call.
    125 *** Procedures
    126 **** Cursor operations
     120==== Procedures
     121===== Cursor operations
    127122These procedures are mostly taken from Chibi Scheme.
    128 ***** string-cursor? obj → boolean
    129 Returns #t if obj can be a string cursor, and #f otherwise. In implementations where cursors and indexes are the same thing, #t is returned on any cursor or index; where they are disjoint, #t is returned on cursors, #f on indexes. If obj is neither a cursor nor an index, string-cursor? will always return #f.
    130 ***** string-cursor-start s → cursor
    131 ***** string-cursor-end s → cursor
     123
     124<procedure>string-cursor? obj → boolean</procedure>
     125
     126Returns {{#t}} if {{obj}} can be a string cursor, and {{#f}} otherwise. In implementations where cursors and indexes are the same thing, {{#t}} is returned on any cursor or index; where they are disjoint, {{#t}} is returned on cursors, {{#f}} on indexes. If obj is neither a cursor nor an index, string-cursor? will always return {{#f}}.
     127
     128<procedure>string-cursor-start s → cursor</procedure>
     129
     130
     131<procedure>string-cursor-end s → cursor</procedure>
     132
    132133Returns the start/post-end cursor of s respectively.
    133 ***** string-cursor-next s cursor → cursor
    134 ***** string-cursor-prev s cursor → cursor
     134
     135<procedure>string-cursor-next s cursor → cursor</procedure>
     136
     137
     138<procedure>string-cursor-prev s cursor → cursor</procedure>
     139
    135140Returns the cursor into s following/preceding cursor. If cursor is an index, returns one more/less than cursor. It is an error if cursor is the post-end/start cursor of s.
    136 ***** string-cursor-forward s cursor nchars → cursor
    137 ***** string-cursor-back s cursor nchars → cursor
    138 Returns the cursor into s which follows/precedes cursor by nchars characters. If cursor is an index, returns nchars more/less than cursor. It is an error if the result would be an invalid cursor or index.
    139 ***** string-cursor=? cursor[1] cursor[2] → boolean
    140 ***** string-cursor<? cursor[1] cursor[2] → boolean
    141 ***** string-cursor>? cursor[1] cursor[2] → boolean
    142 ***** string-cursor<=? cursor[1] cursor[2] → boolean
    143 ***** string-cursor>=? cursor[1] cursor[2] → boolean
     141
     142<procedure>string-cursor-forward s cursor nchars → cursor</procedure>
     143
     144
     145<procedure>string-cursor-back s cursor nchars → cursor</procedure>
     146
     147Returns the cursor into s which follows/precedes cursor by {{nchars}} characters. If cursor is an index, returns {{nchars}} more/less than cursor. It is an error if the result would be an invalid cursor or index.
     148
     149<procedure>string-cursor=? cursor[1] cursor[2] → boolean</procedure>
     150
     151
     152<procedure>string-cursor<? cursor[1] cursor[2] → boolean</procedure>
     153
     154
     155<procedure>string-cursor>? cursor[1] cursor[2] → boolean</procedure>
     156
     157
     158<procedure>string-cursor<=? cursor[1] cursor[2] → boolean</procedure>
     159
     160
     161<procedure>string-cursor>=? cursor[1] cursor[2] → boolean</procedure>
     162
    144163Compares two cursors or two indexes pointing into the same string.
    145 ***** string-cursor-diff s start end → nchars
    146 Returns the number of characters between start and end in string s. Note that the result is always non-negative if start and end are a valid start-end pair.
    147 ***** string-cursor->index s cursor → index
    148 ***** string-index->cursor s index → cursor
     164
     165<procedure>string-cursor-diff s start end → nchars</procedure>
     166
     167Returns the number of characters between {{start}} and {{end}} in string s. Note that the result is always non-negative if {{start}} and {{end}} are a valid {{start-end}} pair.
     168
     169<procedure>string-cursor->index s cursor → index</procedure>
     170
     171
     172<procedure>string-index->cursor s index → cursor</procedure>
     173
    149174Converts a cursor/index into s into the corresponding index/cursor. If the argument is already an index/cursor, it is returned unchanged.
    150 **** Predicates
    151 ***** string-null? s → boolean
     175===== Predicates
     176
     177<procedure>string-null? s → boolean</procedure>
     178
    152179Is s the empty string?
    153 ***** string-every pred s [start end] → value
    154 ***** string-any pred s [start end] → value
    155 Checks to see if every/any character in s satisfies pred proceeding from left (index start) to right (index end).
    156 ****** The predicate is "witness-generating":
    157 ******* If string-any returns true, the returned true value is the one produced by the application of the predicate.
    158 ******* If string-every returns true, the returned true value is the one produced by the final application of the predicate to s[end-1].
    159 If string-every is applied to an empty sequence of characters, it simply returns #t.
    160 ****** The names of these procedures do not end with a question mark -- this is to indicate that they do not return a simple boolean (#t or #f), but a general value.
    161 **** Constructors
    162 ***** string-tabulate proc len → string
    163 ****** Proc is an integer → char procedure.
    164 ****** Construct a string of size len by applying proc to each value from 0 (inclusive) to len (exclusive) to produce the corresponding string element.
    165 ****** The order in which proc is applied to the indexes is not specified.
    166 ****** Note that the order of arguments is not the same as SRFI 1's list-tabulate, but is the same as tabulation functions in other SRFIs.
     180
     181<procedure>string-every pred s [start end] → value</procedure>
     182
     183
     184<procedure>string-any pred s [start end] → value</procedure>
     185
     186Checks to see if every/any character in s satisfies {{pred}} proceeding from left (index {{start}}) to right (index {{end}}).
     187======= The predicate is "witness-generating":
     188* If {{string-any}} returns true, the returned true value is the one produced by the application of the predicate.
     189* If {{string-every}} returns true, the returned true value is the one produced by the final application of the predicate to {{s[end-1]}}.
     190If {{string-every}} is applied to an empty sequence of characters, it simply returns {{#t}}.
     191======= The names of these procedures do not end with a question mark -- this is to indicate that they do not return a simple boolean ({{#t}} or {{#f}}), but a general value.
     192===== Constructors
     193
     194<procedure>string-tabulate proc len → string</procedure>
     195
     196* {{Proc}} is an integer → char procedure.
     197* Construct a string of size {{len}} by applying {{proc}} to each value from {{0}} (inclusive) to {{len}} (exclusive) to produce the corresponding string element.
     198* The order in which {{proc}} is applied to the indexes is not specified.
     199* Note that the order of arguments is not the same as SRFI 1's list-tabulate, but is the same as tabulation functions in other SRFIs.
    167200When this discrepancy was discovered in SRFI 13, it was too late to change SRFI 1.
    168 ***** string-unfold stop? mapper successor seed [base make-final] → string
    169 ****** This is a fundamental constructor for strings.
    170 ****** Successor is used to generate a series of "seed" values from the initial seed:
    171 ******* seed, (successor seed), (successor^2 seed), (successor^3 seed), ...
    172 ****** Stop? tells us when to stop -- when it returns true when applied to one of these seed values.
    173 ****** Mapper maps each seed value to the corresponding character in the result string. These chars are assembled into the string in a left-to-right order.
    174 ****** Base is the optional initial/leftmost portion of the constructed string; it defaults to the empty string "".
    175 ****** Make-final is applied to the terminal seed value (on which stop? returns true) to produce the final/rightmost portion of the constructed string. It defaults to (lambda (x) "").
    176 ****** string-unfold is a fairly powerful string constructor -- you can use it to convert a list to a string, read a port into a string, reverse a string, copy a string, and so forth.
    177 ***** Examples:
    178 #+BEGIN_SRC scheme
     201
     202<procedure>string-unfold stop? mapper successor seed [base make-final] → string</procedure>
     203
     204* This is a fundamental constructor for strings.
     205
     206<parameter>Successor</parameter>
     207is used to generate a series of "seed" values from the initial seed:
     208
     209 seed, (successor seed), (successor^2 seed), (successor^3 seed), ...
     210
     211<parameter>Stop?</parameter>
     212tells us when to stop -- when it returns true when applied to one of these seed values.
     213
     214<parameter>Mapper</parameter>
     215maps each seed value to the corresponding character in the result string. These chars are assembled into the string in a left-to-right order.
     216
     217<parameter>Base</parameter>
     218is the optional initial/leftmost portion of the constructed string; it defaults to the empty string "".
     219
     220<parameter>Make-final</parameter>
     221is applied to the terminal seed value (on which stop? returns true) to produce the final/rightmost portion of the constructed string. It defaults to {{(lambda (x) "")}}.
     222
     223* {{string-unfold}} is a fairly powerful string constructor -- you can use it to convert a list to a string, read a port into a string, reverse a string, copy a string, and so forth.
     224====== Examples:
     225<enscript highlight="scheme">
    179226(port->string p) = (string-unfold eof-object? values
    180227                                  (lambda (x) (read-char p))
     
    184231
    185232(string-tabulate f size) = (string-unfold (lambda (i) (= i size)) f add1 0)
    186 #+END_SRC
    187 ****** To map f over a list lis, producing a string:
    188 #+BEGIN_SRC scheme
     233</enscript>
     234======= To map f over a list lis, producing a string:
     235<enscript highlight="scheme">
    189236(string-unfold null? (compose f car) cdr lis)
    190 #+END_SRC
    191 ****** Interested functional programmers may enjoy noting that string-fold-right and string-unfold are in some sense inverses.
    192 ******* That is, given operations knull?, kar, kdr, kons, and knil satisfying
    193 #+BEGIN_SRC scheme
    194 (kons (kar x) (kdr x)) = x  and (knull? knil) = #t
    195 #+END_SRC
    196 ******* then
    197 #+BEGIN_SRC scheme
     237</enscript>
     238======= Interested functional programmers may enjoy noting that string-fold-right and string-unfold are in some sense inverses.
     239======== That is, given operations knull?, kar, kdr, kons, and knil satisfying
     240<enscript highlight="scheme">
     241(kons (kar x) (kdr x)) = x  and (knull? knil) = {{#t}}
     242</enscript>
     243======== then
     244<enscript highlight="scheme">
    198245(string-fold-right kons knil (string-unfold knull? kar kdr x)) = x
    199 #+END_SRC
    200 ******* and
    201 #+BEGIN_SRC scheme
     246</enscript>
     247======== and
     248<enscript highlight="scheme">
    202249(string-unfold knull? kar kdr (string-fold-right kons knil s)) = s.
    203 #+END_SRC
     250</enscript>
    204251
    205252The final string constructed does not share storage with either base or the value produced by make-final.
    206253
    207254This combinator sometimes is called an "anamorphism."
    208 ****** Note: implementations should take care that runtime stack limits do not cause overflow when constructing large (e.g., megabyte) strings with string-unfold.
    209 ***** string-unfold-right stop? mapper successor seed [base make-final] → string
     255======= Note: implementations should take care that runtime stack limits do not cause overflow when constructing large (e.g., megabyte) strings with string-unfold.
     256
     257<procedure>string-unfold-right stop? mapper successor seed [base make-final] → string</procedure>
     258
    210259This is a fundamental constructor for strings.
    211260
    212261It is equivalent to string-unfold, except that the results of mapper are assembled into the string in a right-to-left order, base is the optional rightmost portion of the constructed string, and make-final produces the leftmost portion of the constructed string.
    213 **** Conversion
    214 ***** string->list/cursors s [start end] → char-list
    215 ***** string->vector/cursors s [start end] → char-vector
    216 string->list/cursors and string->vector/cursors return a newly allocated list or vector of the characters that make up the given string. They differ from the R7RS procedures string->list and string->vector by accepting either cursors or indexes.
    217 ***** reverse-list->string char-list → string
    218 An efficient implementation of (compose list->string reverse):
    219 
    220 #+BEGIN_SRC scheme
     262===== Conversion
     263
     264<procedure>string->list/cursors s [start end] → char-list</procedure>
     265
     266
     267<procedure>string->vector/cursors s [start end] → char-vector</procedure>
     268
     269{{string->list/cursors}} and {{string->vector/cursors}} return a newly allocated list or vector of the characters that make up the given string. They differ from the R7RS procedures {{string->list}} and {{string->vector}} by accepting either cursors or indexes.
     270====== reverse-list->string char-list → string
     271An efficient implementation of (compose {{list->string}} reverse):
     272
     273<enscript highlight="scheme">
    221274(reverse-list->string '(#\a #\B #\c)) → "cBa"
    222 #+END_SRC
    223 
    224 This is a common idiom in the epilog of string-processing loops that accumulate an answer in a reverse-order list. (See also string-concatenate-reverse for the "chunked" variant.)
    225 ***** string-join string-list [delimiter grammar] → string
     275</enscript>
     276
     277This is a common idiom in the epilog of string-processing loops that accumulate an answer in a reverse-order list. (See also {{string-concatenate-reverse}} for the "chunked" variant.)
     278
     279<procedure>string-join string-list [delimiter grammar] → string</procedure>
     280
    226281This procedure is a simple unparser --- it pastes strings together using the delimiter string.
    227282
    228 The grammar argument is a symbol that determines how the delimiter is used, and defaults to 'infix.
    229 ****** 'infix means an infix or separator grammar: insert the delimiter between list elements.
     283The {{grammar}} argument is a symbol that determines how the delimiter is used, and defaults to {{'infix}}.
     284* {{'infix}} means an infix or separator {{grammar:}} insert the delimiter between list elements.
    230285An empty list will produce an empty string -- note, however, that parsing an empty string with an infix or separator grammar is ambiguous.
    231286Is it an empty list, or a list of one element, the empty string?
    232 ****** 'strict-infix means the same as 'infix, but will signal an error if given an empty list.
    233 ****** 'suffix means a suffix or terminator grammar: insert the delimiter after every list element. This grammar has no ambiguities.
    234 ****** 'prefix means a prefix grammar: insert the delimiter before every list element. This grammar has no ambiguities.
    235 ****** The delimiter is the string used to delimit elements; it defaults to a single space " ".
    236 ****** Examples
    237 #+BEGIN_SRC scheme
     287* {{'strict-infix}} means the same as 'infix, but will signal an error if given an empty list.
     288* {{'suffix}} means a suffix or terminator {{grammar:}} insert the delimiter after every list element. This grammar has no ambiguities.
     289* {{'prefix}} means a prefix {{grammar:}} insert the delimiter before every list element. This grammar has no ambiguities.
     290* The delimiter is the string used to delimit elements; it defaults to a single space " ".
     291* Examples
     292<enscript highlight="scheme">
    238293(string-join '("foo" "bar" "baz") ":")         => "foo:bar:baz"
    239294(string-join '("foo" "bar" "baz") ":" 'suffix) => "foo:bar:baz:"
     
    246301(string-join '()   ":" 'suffix) => ""
    247302(string-join '("") ":" 'suffix) => ":"
    248 #+END_SRC
    249 **** Selection
    250 ***** string-ref/cursor s cursor → char
    251 Returns character s[i] using a valid cursor or index of s. It differs from the R7RS procedure string-ref by accepting either a cursor or an index.
    252 ***** substring/cursors s start end → string
    253 ***** string-copy/cursors s [start end] → string
    254 These procedures return a string whose contents are the characters of s beginning with index start (inclusive) and ending with index end (exclusive). If substring/ cursors produces the entire string, it may return either s or a copy of s; in some implementations, proper substrings may share memory with s. However, string-copy /cursors always returns a newly allocated string. They differ from the R7RS procedures substring and string-copy by accepting either cursors or indexes.
    255 ***** string-take s nchars → string
    256 ***** string-drop s nchars → string
    257 ***** string-take-right s nchars → string
    258 ***** string-drop-right s nchars → string
    259 string-take returns the first nchars of s; string-drop returns all but the first nchars of s. string-take-right returns the last nchars of s; string-drop-right returns all but the last nchars of s.
    260 
    261 If these procedures produce the entire string, they may return either s or a copy of s; in some implementations, proper substrings may share memory with s.
    262 ****** Examples
    263 #+BEGIN_SRC scheme
     303</enscript>
     304===== Selection
     305====== string-ref/cursor s cursor → char
     306Returns character {{s[i]}} using a valid cursor or index of {{s}}. It differs from the R7RS procedure {{string-ref}} by accepting either a cursor or an index.
     307====== substring/cursors s start end → string
     308====== string-copy/cursors s [start end] → string
     309These procedures return a string whose contents are the characters of s beginning with index {{start}} (inclusive) and ending with index {{end}} (exclusive). If {{substring/cursors}} produces the entire string, it may return either {{s}} or a copy of {{s}}; in some implementations, proper substrings may share memory with {{s}}. However, string-copy /cursors always returns a newly allocated string. They differ from the R7RS procedures substring and string-copy by accepting either cursors or indexes.
     310====== string-take s nchars → string
     311====== string-drop s nchars → string
     312====== string-take-right s nchars → string
     313====== string-drop-right s nchars → string
     314string-take returns the first {{nchars}} of {{s}}; string-drop returns all but the first {{nchars}} of {{s}}. string-take-right returns the last {{nchars}} of s; string-drop-right returns all but the last {{nchars}} of {{s}}.
     315
     316If these procedures produce the entire string, they may return either {{s}} or a copy of {{s}}; in some implementations, proper substrings may share memory with {{s}}.
     317======= Examples
     318<enscript highlight="scheme">
    264319(string-take "Pete Szilagyi" 6) => "Pete S"
    265320(string-drop "Pete Szilagyi" 6) => "zilagyi"
     
    267322(string-take-right "Beta rules" 5) => "rules"
    268323(string-drop-right "Beta rules" 5) => "Beta "
    269 #+END_SRC
    270 ****** It is an error to take or drop more characters than are in the string:
    271 #+BEGIN_SRC scheme
     324</enscript>
     325======= It is an error to take or drop more characters than are in the string:
     326<enscript highlight="scheme">
    272327(string-take "foo" 37) => error
    273 #+END_SRC
    274 ***** string-pad s len [char start end] → string
    275 ***** string-pad-right s len [char start end] → string
    276 Build a string of length len comprised of s padded on the left (right) by as many occurrences of the character char as needed. If s has more than len chars, it is truncated on the left (right) to length len. Char defaults to #\space.
    277 
    278 If len <= end-start, the returned value is allowed to share storage with s, or be exactly s (if len = end-start).
    279 
    280 #+BEGIN_SRC scheme
     328</enscript>
     329
     330<procedure>string-pad s len [char start end] → string</procedure>
     331
     332
     333<procedure>string-pad-right s len [char start end] → string</procedure>
     334
     335Build a string of length {{len}} comprised of s padded on the left (right) by as many occurrences of the character char as needed. If {{s}} has more than {{len}} chars, it is truncated on the left (right) to length {{len}}. Char defaults to {{#\space}}.
     336
     337If {{len <= end-start}}, the returned value is allowed to share storage with {{s}}, or be exactly {{s}} (if {{len = end-start}}).
     338
     339<enscript highlight="scheme">
    281340(string-pad     "325" 5) => "  325"
    282341(string-pad   "71325" 5) => "71325"
    283342(string-pad "8871325" 5) => "71325"
    284 #+END_SRC
    285 ***** string-trim       s [pred start end] → string
    286 ***** string-trim-right s [pred start end] → string
    287 ***** string-trim-both  s [pred start end] → string
    288 Trim s by skipping over all characters on the left / on the right / on both sides that satisfy the second parameter pred: pred defaults to char-whitespace?.
    289 
    290 If no trimming occurs, these functions may return either s or a copy of s; in some implementations, proper substrings may share memory with s.
    291 
    292 #+BEGIN_SRC scheme
     343</enscript>
     344
     345<procedure>string-trim       s [pred start end] → string</procedure>
     346
     347
     348<procedure>string-trim-right s [pred start end] → string</procedure>
     349
     350
     351<procedure>string-trim-both  s [pred start end] → string</procedure>
     352
     353Trim {{s}} by skipping over all characters on the left / on the right / on both sides that satisfy the second parameter {{pred}}: {{pred}} defaults to {{char-whitespace?}}.
     354
     355If no trimming occurs, these functions may return either {{s}} or a copy of {{s}}; in some implementations, proper substrings may share memory with {{s}}.
     356
     357<enscript highlight="scheme">
    293358(string-trim-both "  The outlook wasn't brilliant,  \n\r")
    294359    => "The outlook wasn't brilliant,"
    295 #+END_SRC
    296 **** Prefixes & suffixes
    297 ***** string-prefix-length    s1 s2 [start1 end1 start2 end2] → integer
    298 ***** string-suffix-length    s1 s2 [start1 end1 start2 end2] → integer
    299 Return the length of the longest common prefix/suffix of the two strings. For prefixes, this is equivalent to the "mismatch index" for the strings (modulo the start cursors).
    300 
    301 The optional start/end cursors or indexes restrict the comparison to the indicated substrings of s1 and s2.
    302 ****** Examples
    303 #+BEGIN_SRC scheme
    304 string-prefix?    s1 s2 [start1 end1 start2 end2] → boolean
    305 string-suffix?    s1 s2 [start1 end1 start2 end2] → boolean
    306 #+END_SRC
    307 
    308 Is s1 a prefix/suffix of s2?
    309 ****** The optional start/end cursors or indexes restrict the comparison to the indicated substrings of s1 and s2.
    310 **** Searching
    311 ***** string-index s pred [start end] → cursor
    312 ***** string-index-right s pred [start end] → cursor
    313 ***** string-skip s pred [start end] → cursor
    314 ***** string-skip-right s pred [start end] → cursor
    315 string-index searches through s from the left, returning the cursor of the first occurrence of a character which satisfies the predicate pred. If no match is found, it returns end. string-index-right searches through s from the right, returning the cursor of the successor of the first occurrence of a character which satisfies the predicate pred. If no match is found, it returns start.
    316 
    317 The start and end parameters specify the beginning and end cursors or indexes of the search; the search includes the start, but not the end. Be careful of "fencepost" considerations: when searching right-to-left, the first position considered is (string-cursor-prev end), whereas when searching left-to-right, the first index considered is start. That is, the start/end indexes describe the same half-open interval [start,end) in these procedures that they do in all the other SRFI 130 procedures.
    318 
    319 The skip functions are similar, but use the complement of the criteria: they search for the first char that doesn't satisfy pred. E.g., to skip over initial whitespace, say
    320 
    321 #+BEGIN_SRC scheme
     360</enscript>
     361===== Prefixes & suffixes
     362
     363<procedure>string-prefix-length    s1 s2 [start1 end1 start2 end2] → integer</procedure>
     364
     365
     366<procedure>string-suffix-length    s1 s2 [start1 end1 start2 end2] → integer</procedure>
     367
     368Return the length of the longest common prefix/suffix of the two strings. For prefixes, this is equivalent to the "mismatch index" for the strings (modulo the {{start}} cursors).
     369
     370The optional {{start/end}} cursors or indexes restrict the comparison to the indicated substrings of {{s1}} and {{s2}}.
     371
     372
     373<procedure>string-prefix?    s1 s2 [start1 end1 start2 end2] → boolean</procedure>
     374
     375
     376<procedure>string-suffix?    s1 s2 [start1 end1 start2 end2] → boolean</procedure>
     377
     378Is {{s1}} a prefix/suffix of {{s2}}?
     379* The optional {{start/end}} cursors or indexes restrict the comparison to the indicated substrings of {{s1}} and {{s2}}.
     380===== Searching
     381
     382<procedure>string-index s pred [start end] → cursor</procedure>
     383
     384
     385<procedure>string-index-right s pred [start end] → cursor</procedure>
     386
     387
     388<procedure>string-skip s pred [start end] → cursor</procedure>
     389
     390
     391<procedure>string-skip-right s pred [start end] → cursor</procedure>
     392
     393{{string-index}} searches through {{s}} from the left, returning the cursor of the first occurrence of a character which satisfies the predicate {{pred}}. If no match is found, it returns {{end}}. string-index-right searches through {{s}} from the right, returning the cursor of the successor of the first occurrence of a character which satisfies the predicate {{pred}}. If no match is found, it returns start.
     394
     395The {{start}} and {{end}} parameters specify the beginning and end cursors or indexes of the search; the search includes the {{start}}, but not the {{end}}. Be careful of "fencepost" considerations: when searching right-to-left, the first position considered is {{(string-cursor-prev end)}}, whereas when searching left-to-right, the first index considered is start. That is, the {{start/end}} indexes describe the same half-open interval {{[start,end)}} in these procedures that they do in all the other SRFI 130 procedures.
     396
     397The skip functions are similar, but use the complement of the criteria: they search for the first char that doesn't satisfy {{pred}}. E.g., to skip over initial whitespace, say
     398
     399<enscript highlight="scheme">
    322400(substring/cursors s (string-skip s char-whitespace?))
    323 #+END_SRC
    324 
    325 Note that the result is always a cursor, even when start and end are indexes. Use string-cursor->index to convert the result to an index. Therefore, these four functions are not entirely compatible with their SRFI 13 counterparts, which return #f on failure.
     401</enscript>
     402
     403Note that the result is always a cursor, even when {{start}} and {{end}} are indexes. Use {{string-cursor->index}} to convert the result to an index. Therefore, these four functions are not entirely compatible with their SRFI 13 counterparts, which return {{#f}} on failure.
    326404
    327405These functions can be trivially composed with string-take and string-drop to produce take-while, drop-while, span, and break procedures without loss of efficiency.
    328 ***** string-contains s1 s2 [start1 end1 start2 end2] → cursor
    329 ***** string-contains-right s1 s2 [start1 end1 start2 end2] → cursor
    330 Does string s1 contain string s2?
    331 
    332 Returns the cursor in s1 referring to the first character of the first/last instance of s2 as a substring, or #f if there is no match. The optional start/end indexes restrict the operation to the indicated substrings.
    333 
    334 The returned cursor is in the range [start1,end1). A successful match must lie entirely in the [start1,end1) range of s1.
    335 
    336 Note that the result is always a cursor, even when start1 and end1 are indexes. Use string-cursor->index to convert a cursor result to an index.
    337 
    338 #+BEGIN_SRC scheme
     406
     407<procedure>string-contains s1 s2 [start1 end1 start2 end2] → cursor</procedure>
     408
     409
     410<procedure>string-contains-right s1 s2 [start1 end1 start2 end2] → cursor</procedure>
     411
     412Does string {{s1}} contain string {{s2}}?
     413
     414Returns the cursor in {{s1}} referring to the first character of the first/last instance of {{s2}} as a substring, or {{#f}} if there is no match. The optional {{start/end}} indexes restrict the operation to the indicated substrings.
     415
     416The returned cursor is in the range {{[start1,end1)}}. A successful match must lie entirely in the {{[start1,end1)}} range of {{s1}}.
     417
     418Note that the result is always a cursor, even when {{start1}} and {{end1}} are indexes. Use {{string-cursor->index}} to convert a cursor result to an index.
     419
     420<enscript highlight="scheme">
    339421(string-contains "eek -- what a geek." "ee"
    340422                 12 18) ; Searches "a geek"
    341423    => {Cursor 15}
    342 #+END_SRC
    343 
    344 The name of this procedure does not end with a question mark -- this is to indicate that it does not return a simple boolean (#t or #f). Rather, it returns either false (#f) or a cursor.
    345 **** The whole string
    346 ***** string-reverse  s [start end] -> string
     424</enscript>
     425
     426The name of this procedure does not end with a question mark -- this is to indicate that it does not return a simple boolean ({{#t}} or {{#f}}). Rather, it returns either false ({{#f}}) or a cursor.
     427===== The whole string
     428
     429<procedure>string-reverse  s [start end] -> string</procedure>
     430
    347431Reverse the string.
    348432
    349 string-reverse returns the result string and does not alter its s parameter.
    350 
    351 #+BEGIN_SRC scheme
     433{{string-reverse}} returns the result string and does not alter its {{s}} parameter.
     434
     435<enscript highlight="scheme">
    352436(string-reverse "Able was I ere I saw elba.")
    353437    => ".able was I ere I saw elbA"
    354438(string-reverse "Who stole the spoons?" 14 20)
    355439    => "snoops"
    356 #+END_SRC
    357 
    358 Unicode note: Reversing a string simply reverses the sequence of code-points it contains. So a combining diacritic a coming after a base character b in string s would come out before b in the reversed result.
    359 ***** string-concatenate string-list → string
     440</enscript>
     441
     442Unicode note: Reversing a string simply reverses the sequence of code-points it contains. So a combining diacritic a coming after a base character {{b}} in string {{s}} would come out before {{b}} in the reversed result.
     443
     444<procedure>string-concatenate string-list → string</procedure>
     445
    360446Append the elements of string-list together into a single string. Guaranteed to return a freshly allocated string.
    361447
    362448Note that the (apply string-append string-list) idiom is not robust for long lists of strings, as some Scheme implementations limit the number of arguments that may be passed to an n-ary procedure.
    363 ***** string-concatenate-reverse string-list [final-string end] → string
     449
     450<procedure>string-concatenate-reverse string-list [final-string end] → string</procedure>
     451
    364452With no optional arguments, this function is equivalent to
    365453
    366 #+BEGIN_SRC scheme
     454<enscript highlight="scheme">
    367455(string-concatenate (reverse string-list))
    368 #+END_SRC
    369 
    370 If the optional argument final-string is specified, it is consed onto the beginning of string-list before performing the list-reverse and string-concatenate operations.
    371 
    372 If the optional argument end is given, only the characters up to but not including end in final-string are added to the result, thus producing
    373 
    374 #+BEGIN_SRC scheme
     456</enscript>
     457
     458If the optional argument final-string is specified, it is consed onto the beginning of {{string-list}} before performing the {{list-reverse}} and {{string-concatenate}} operations.
     459
     460If the optional argument {{end}} is given, only the characters up to but not including {{end}} in final-string are added to the result, thus producing
     461
     462<enscript highlight="scheme">
    375463(string-concatenate
    376464  (reverse (cons (substring final-string
     
    378466                            end)
    379467                 string-list)))
    380 #+END_SRC
     468</enscript>
    381469
    382470E.g.
    383471
    384 #+BEGIN_SRC scheme
     472<enscript highlight="scheme">
    385473(string-concatenate-reverse '(" must be" "Hello, I") " going.XXXX" 7)
    386474  => "Hello, I must be going."
    387 #+END_SRC
     475</enscript>
    388476
    389477This procedure is useful in the construction of procedures that accumulate character data into lists of string buffers, and wish to convert the accumulated data into a single string when done.
    390 ***** string-fold kons knil s [start end] → value
    391 ***** string-fold-right kons knil s [start end] → value
     478
     479<procedure>string-fold kons knil s [start end] → value</procedure>
     480
     481
     482<procedure>string-fold-right kons knil s [start end] → value</procedure>
     483
    392484These are the fundamental iterators for strings.
    393485
    394 The left-fold operator maps the kons procedure across the string from left to right
    395 
    396 #+BEGIN_SRC scheme
     486The left-fold operator maps the {{kons}} procedure across the string from left to right
     487
     488<enscript highlight="scheme">
    397489(... (kons s[2] (kons s[1] (kons s[0] knil))))
    398 #+END_SRC
    399 
    400 In other words, string-fold obeys the (tail) recursion
    401 
    402 #+BEGIN_SRC scheme
     490</enscript>
     491
     492In other words, {{string-fold}} obeys the (tail) recursion
     493
     494<enscript highlight="scheme">
    403495(string-fold kons knil s start end) =
    404496    (string-fold kons (kons s[start] knil) start+1 end)
    405 #+END_SRC
    406 
    407 The right-fold operator maps the kons procedure across the string from right to left
    408 
    409 #+BEGIN_SRC scheme
     497</enscript>
     498
     499The {{right-fold}} operator maps the {{kons}} procedure across the string from right to left
     500
     501<enscript highlight="scheme">
    410502(kons s[0] (... (kons s[end-3] (kons s[end-2] (kons s[end-1] knil)))))
    411 #+END_SRC
     503</enscript>
    412504
    413505obeying the (tail) recursion
    414506
    415 #+BEGIN_SRC scheme
     507<enscript highlight="scheme">
    416508(string-fold-right kons knil s start end) =
    417509    (string-fold-right kons (kons s[end-1] knil) start end-1)
    418 #+END_SRC
    419 ****** Examples:
    420 #+BEGIN_SRC scheme
     510</enscript>
     511======= Examples:
     512<enscript highlight="scheme">
    421513;;; Convert a string to a list of chars.
    422514(string-fold-right cons '() s)
     
    443535               0 s)
    444536  ans)
    445 #+END_SRC
    446 ****** The right-fold combinator is sometimes called a "catamorphism."
    447 ***** string-for-each-cursor proc s [start end] → unspecified
    448 Apply proc to each cursor of s, in order, excluding the post-end cursor. The optional start/end pairs restrict the endpoints of the loop. This is simply a method of looping over a string that is guaranteed to be safe and correct. Example:
    449 
    450 #+BEGIN_SRC scheme
     537</enscript>
     538======= The right-fold combinator is sometimes called a "catamorphism."
     539
     540<procedure>string-for-each-cursor proc s [start end] → unspecified</procedure>
     541
     542Apply {{proc}} to each cursor of s, in order, excluding the post-end cursor. The optional {{start/end}} pairs restrict the endpoints of the loop. This is simply a method of looping over a string that is guaranteed to be safe and correct.
     543
     544====== Example:
     545<enscript highlight="scheme">
    451546(let ((s "abcde") (v '()))
    452547  (string-for-each-cursor
     
    454549    s)
    455550  v) => (101 100 99 98 97)
    456 #+END_SRC
    457 ***** string-replicate s from to [start end] → string
     551</enscript>
     552
     553<procedure>string-replicate s from to [start end] → string</procedure>
     554
    458555This is an "extended substring" procedure that implements replicated copying of a substring of some string.
    459556
    460 S is a string; start and end are optional arguments that demarcate a substring of s, defaulting to 0 and the length of s (i.e., the whole string). Replicate this substring up and down index space, in both the positive and negative directions. For example, if s = "abcdefg", start=3, and end=6, then we have the conceptual bidirectionally-infinite string
    461 
    462 #+BEGIN_EXAMPLE
    463 ...  d  e  f  d  e  f  d  e  f d  e  f  d  e  f  d  e  f  d ...
    464 ... -9 -8 -7 -6 -5 -4 -3 -2 -1 0 +1 +2 +3 +4 +5 +6 +7 +8 +9 ...
    465 #+END_EXAMPLE
    466 
    467 string-replicate returns the substring of this string beginning at index from, and ending at to. Note that these arguments cannot be cursors. It is an error if from is greater than to.
    468 ****** You can use string-replicate to perform a variety of tasks:
    469 ******* To rotate a string left: (string-replicate "abcdef" 2 8) => "cdefab"
    470 ******* To rotate a string right: (string-replicate "abcdef" -2 4) => "efabcd"
    471 ******* To replicate a string: (string-replicate "abc" 0 7) => "abcabca"
    472 ****** Note that
    473 ******* The from/to indexes give a half-open range -- the characters from index from up to, but not including, index to.
    474 ******* The from/to indexes are not in terms of the index space for string s. They are in terms of the replicated index space of the substring defined by s, start, and end.
    475 ****** It is an error if start=end -- although this is allowed by special dispensation when from=to.
    476 ****** Compatibility note:
    477 string-replicate is identical to the xsubstring procedure of SRFI 13, except that the to argument is required.
    478 ***** string-count s pred [start end] → integer
    479 Return a count of the number of characters in s that satisfy the pred argument.
    480 ***** string-replace s1 s2 start1 end1 [start2 end2] → string
     557{{S}} is a string; {{start}} and {{end}} are optional arguments that demarcate a substring of {{s}}, defaulting to {{0}} and the length of {{s}} (i.e., the whole string). Replicate this substring up and down index space, in both the positive and negative directions. For example, if {{s = "abcdefg"}}, {{start=3}}, and {{end=6}}, then we have the conceptual bidirectionally-infinite string
     558
     559 ...  d  e  f  d  e  f  d  e  f d  e  f  d  e  f  d  e  f  d ...
     560 ... -9 -8 -7 -6 -5 -4 -3 -2 -1 0 +1 +2 +3 +4 +5 +6 +7 +8 +9 ...
     561
     562string-replicate returns the substring of this string beginning at index {{from}}, and ending at {{to}}. Note that these arguments cannot be cursors. It is an error if {{from}} is greater than {{to}}.
     563======= You can use string-replicate to perform a variety of tasks:
     564* To rotate a string left:
     565<enscript highlight="scheme">
     566(string-replicate "abcdef" 2 8) => "cdefab
     567</enscript>"
     568* To rotate a string right:
     569<enscript highlight="scheme">
     570(string-replicate "abcdef" -2 4) => "efabcd
     571</enscript>"
     572* To replicate a string:
     573<enscript highlight="scheme">
     574(string-replicate "abc" 0 7) => "abcabca
     575</enscript>"
     576======= Note that
     577* The from/to indexes give a half-open range -- the characters from index from up to, but not including, index to.
     578* The from/to indexes are not in terms of the index space for string {{s}}. They are in terms of the replicated index space of the substring defined by {{s}}, {{start}}, and {{end}}.
     579* It is an error if {{start=end}} -- although this is allowed by special dispensation when from=to.
     580======= Compatibility note:
     581{{string-replicate}} is identical to the {{xsubstring}} procedure of SRFI 13, except that the to argument is required.
     582
     583<procedure>string-count s pred [start end] → integer</procedure>
     584
     585Return a count of the number of characters in {{s}} that satisfy the {{pred}} argument.
     586
     587<procedure>string-replace s1 s2 start1 end1 [start2 end2] → string</procedure>
     588
    481589Returns
    482590
    483 #+BEGIN_SRC scheme
     591<enscript highlight="scheme">
    484592(string-append (substring/cursors s1 (string-cursor-start s1) start1)
    485593               (substring/cursors s2 start2 end2)
    486594               (substring/cursors s1 end1 (string-cursor-end s1)))
    487 #+END_SRC
    488 
    489 That is, the segment of characters in s1 from start1 to end1 is replaced by the segment of characters in s2 from start2 to end2. If start1=end1, this simply splices the s2 characters into s1 at the specified index.
    490 ****** Examples:
    491 #+BEGIN_SRC scheme
     595</enscript>
     596
     597That is, the segment of characters in {{s1}} from {{start1}} to {{end1}} is replaced by the segment of characters in {{s2}} from {{start2}} to end2. If {{start1=end1}}, this simply splices the {{s2}} characters into {{s1}} at the specified index.
     598======= Examples:
     599<enscript highlight="scheme">
    492600(string-replace "The TCL programmer endured daily ridicule."
    493601                "another miserable perl drone" 4 7 8 22 ) =>
     
    501609(string-insert "It's easy to code it up in Scheme." 5 "really ") =>
    502610    "It's really easy to code it up in Scheme."
    503 #+END_SRC
    504 ***** string-split s delimiter [grammar limit start end] → list
    505 Returns a list of the words contained in the substring of string from start (inclusive) to end (exclusive). Delimiter specifies a string that is to be used as the word separator. This will often be a single character, but multiple characters are allowed for cases like splitting on "\r\n". The returned list will then have one more item than the number of non-overlapping occurrences of the delimiter in the string. If delimiter is an empty string, then the returned list contains a list of strings, each of which contains a single character.
    506 
    507 Grammar is a symbol with the same meaning as in the string-join procedure. If it is infix, which is the default, processing is done as described above, except that an empty s produces the empty list; if it is strict-infix, an empty s signals an error. The values prefix and suffix cause a leading/trailing empty string in the result to be suppressed.
    508 
    509 If limit is a non-negative exact integer, at most that many splits occur, and the remainder of string is returned as the final element of the list (thus, the result will have at most limit+1 elements). If limit is not specified or is #f, then as many splits as possible are made. It is an error if limit is any other value.
     611</enscript>
     612
     613<procedure>string-split s delimiter [grammar limit start end] → list</procedure>
     614
     615Returns a list of the words contained in the substring of string from {{start}} (inclusive) to {{end}} (exclusive). Delimiter specifies a string that is to be used as the word separator. This will often be a single character, but multiple characters are allowed for cases like splitting on "\r\n". The returned list will then have one more item than the number of non-overlapping occurrences of the delimiter in the string. If delimiter is an empty string, then the returned list contains a list of strings, each of which contains a single character.
     616
     617Grammar is a symbol with the same meaning as in the {{string-join}} procedure. If it is infix, which is the default, processing is done as described above, except that an empty {{s}} produces the empty list; if it is strict-infix, an empty {{s}} signals an error. The values {{prefix}} and {{suffix}} cause a leading/trailing empty string in the result to be suppressed.
     618
     619If {{limit}} is a non-negative exact integer, at most that many splits occur, and the remainder of string is returned as the final element of the list (thus, the result will have at most {{limit+1}} elements). If {{limit}} is not specified or is {{#f}}, then as many splits as possible are made. It is an error if {{limit}} is any other value.
    510620
    511621Use SRFI 115's regexp-split to split on a regular expression rather than a simple string.
    512 ***** string-filter pred s [start end] → string
    513 ***** string-remove pred s [start end] → string
    514 Filter the string s, retaining only those characters that satisfy / do not satisfy pred.
    515 
    516 If the string is unaltered by the filtering operation, these functions may return either s or a copy of s.
    517 ****** Compatibility note:
    518 string-remove is identical to the string-delete procedure of SRFI 13, but the name string-delete is inconsistent with the conventions of SRFI 1 and other SRFIs.
    519 **** Sample implementation
    520 This SRFI comes with a sample implementation, which can be found in the repository of this SRFI. It is a cut-down version of the sample implementation of SRFI 13, with the addition of the cursor operations procedures, the */cursors procedures, string-contains-right, and string-split. Here are Olin's original implementation notes:
    521 
    522 I have placed this source on the Net with an unencumbered, "open" copyright. The prefix/suffix and comparison routines in this code had (extremely distant) origins in MIT Scheme's string lib, and were substantially reworked by myself. Being derived from that code, they are covered by the MIT Scheme copyright, which is a generic BSD-style open-source copyright. See the source file for details.
    523 
    524 The KMP string-search code was influenced by implementations written by Stephen Bevan, Brian Denheyer and Will Fitzgerald. However, this version was written from scratch by myself.
    525 
    526 The remainder of the code was written by myself for scsh or for this SRFI; I have placed this code under the scsh copyright, which is also a generic BSD-style open-source copyright.
    527 
    528 The code is written for portability and should be straightforward to port to any Scheme. The source comments contains detailed notes describing the non-R5RS dependencies.
    529 
    530 The library is written for clarity and well-commented. Fast paths are provided for common cases. This is not to say that the implementation can't be tuned up for a specific Scheme implementation. There are notes in the comments addressing ways implementors can tune the reference implementation for performance.
    531 
    532 In short, I've written the reference implementation to make it as painless as possible for an implementor -- or a regular programmer -- to adopt this library and get good results with it.
    533 
    534 Another implementation, derived from Chibi Scheme's SRFI 130, is present in the foof subdirectory. This implementation is smaller but may be slower. It can be more easily adapted to Schemes that differentiate between indexes and cursors.
    535 **** Acknowledgements
     622
     623<procedure>string-filter pred s [start end] → string</procedure>
     624
     625
     626<procedure>string-remove pred s [start end] → string</procedure>
     627
     628Filter the string {{s}}, retaining only those characters that satisfy / do not satisfy {{pred}}.
     629
     630If the string is unaltered by the filtering operation, these functions may return either {{s}} or a copy of {{s}}.
     631======= Compatibility note:
     632{{string-remove}} is identical to the {{string-delete}} procedure of SRFI 13, but the name {{string-delete}} is inconsistent with the conventions of SRFI 1 and other SRFIs.
     633===== Sample implementation
     634This SRFI comes with a sample implementation, which can be found in the repository of this SRFI. It is a cut-down version of the sample implementation of SRFI 13, with the addition of the cursor operations procedures, the {{*/cursors}} procedures, {{string-contains-right}}, and {{string-split}}. Here are Olin's original implementation notes:
     635
     636''I have placed this source on the Net with an unencumbered, "open" copyright. The prefix/suffix and comparison routines in this code had (extremely distant) origins in MIT Scheme's string lib, and were substantially reworked by myself. Being derived from that code, they are covered by the MIT Scheme copyright, which is a generic BSD-style open-source copyright. See the source file for details.''
     637
     638''The KMP string-search code was influenced by implementations written by Stephen Bevan, Brian Denheyer and Will Fitzgerald. However, this version was written from scratch by myself.''
     639
     640''The remainder of the code was written by myself for scsh or for this SRFI; I have placed this code under the scsh copyright, which is also a generic BSD-style open-source copyright.''
     641
     642''The code is written for portability and should be straightforward to port to any Scheme. The source comments contains detailed notes describing the non-R5RS dependencies.''
     643
     644''The library is written for clarity and well-commented. Fast paths are provided for common cases. This is not to say that the implementation can't be tuned up for a specific Scheme implementation. There are notes in the comments addressing ways implementors can tune the reference implementation for performance.''
     645
     646''In short, I've written the reference implementation to make it as painless as possible for an implementor -- or a regular programmer -- to adopt this library and get good results with it.''
     647
     648''Another implementation, derived from Chibi Scheme's SRFI 130, is present in the foof subdirectory. This implementation is smaller but may be slower. It can be more easily adapted to Schemes that differentiate between indexes and cursors.''
     649===== Acknowledgements
    536650Thanks to the members of the SRFI 130 mailing list who made this SRFI what it now is, including Per Bothner, Arthur Gleckler, Shiro Kawai, Jim Rees, and especially Alex Shinn, whose idea it was to make cursors and indexes disjoint, and who provided the foof implementation. The following acknowledgements by Olin Shivers are taken from SRFI 13:
    537651
     
    543657
    544658During this document's long development period, great patience was exhibited by Mike Sperber, who is the editor for the SRFI, and by Hillary Sullivan, who is not.
    545 **** References & links
    546 ***** [CommonLisp]
     659===== References & links
     660====== [CommonLisp]
    547661Common Lisp: the Language.
    548662Guy L. Steele Jr. (editor).
     
    552666The Common Lisp "HyperSpec," produced by Kent Pitman, is essentially the ANSI spec for Common Lisp:
    553667http://www.lispworks.com/documentation/HyperSpec/Front/index.htm.
    554 ***** [MIT-Scheme]
     668====== [MIT-Scheme]
    555669http://www.swiss.ai.mit.edu/projects/scheme/
    556 ***** [R5RS]
     670====== [R5RS]
    557671Revised^5 report on the algorithmic language Scheme.
    558672R. Kelsey, W. Clinger, J. Rees (editors).
     
    560674and ACM SIGPLAN Notices, Vol. 33, No. 9, October, 1998.
    561675Available at http://www.schemers.org/Documents/Standards/.
    562 ***** [R7RS]
     676====== [R7RS]
    563677Revised^7 report on the algorithmic language Scheme.
    564678A. Shinn, J. Cowan, A. Gleckler (editors).
    565679Available at http://r7rs.org.
    566 ***** [SRFI]
     680====== [SRFI]
    567681The SRFI web site.
    568682http://srfi.schemers.org/
    569 ***** [SRFI-13]
     683====== [SRFI-13]
    570684SRFI-13: String libraries.
    571685http://srfi.schemers.org/srfi-13/
    572 ***** [SRFI-14]
     686====== [SRFI-14]
    573687SRFI-14: Character-set library.
    574688http://srfi.schemers.org/srfi-14/
    575689The SRFI 14 char-set library defines a character-set data type, which is used by some procedures in this library.
    576 ** Author
     690=== Author
    577691John Cowan
    578692Ported and packaged to Chicken 5 by Sergey Goldgaber
    579 ** Copyright
     693=== Copyright
    580694Copyright (C) Olin Shivers (1998, 1999, 2000) and John Cowan (2016).
    581695
     
    585699
    586700THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
    587 ** Version history
    588 *** 0.1 - SRFI-130 ported to Chicken 5.2.0
     701=== Version history
     702==== 0.1 - SRFI-130 ported to Chicken 5.2.0
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