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1* SRFI-130: Cursor-based string library
2** Abstract
3[[][R5RS]] Scheme has an impoverished set of string-processing utilities, which is a problem for authors of portable code. Although [[][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.
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 [[][R7RS]].
7For more information see: [[][SRFI-130: Cursor-based string library]]
8** Procedure Index
9Here 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
52This 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.
54When SRFI 13 was defined in 1999, it was intended to provide efficient string operations on both whole strings and substrings. At that time, it was normal for strings to be sequences of 8-bit characters, or in a few cases, characters of other fixed lengths. Consequently, many of the SRFI 13 procedures accept optional exact integer arguments for each of the string arguments, indexing the beginning and the end of the substring(s) to be operated on.
56Unfortunately 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.
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 [[][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.
60Many SRFI 13 procedures accept either a predicate, a single character, or a [[][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 [[][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.
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.
64In 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
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.)
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!.
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
73All 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
75Some 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:
79string-take string-take-right
80string-drop string-drop-right
81string-pad string-pad-right
82string-trim string-trim-right string-trim-both
83string-split string-filter string-remove
86In 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 [[][R7RS]] procedures, are not permitted to return shared results. If a shared value is returned, it may be mutable or immutable.
87*** Naming conventions
88The 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.
90Procedures 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.
96When 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.
98It 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.
104Unless 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
107halts? f [x init-store] → [boolean integer]
110would 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
114sum-squares x ...  → number
116takes zero or more arguments (x ...),
117***** while the procedure with signature
119spell-check doc dict[1] dict[2] ... → string-list
122takes 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.
124Such a procedure is not even required to be consistent from call to call.
125*** Procedures
126**** Cursor operations
127These procedures are mostly taken from Chibi Scheme.
128***** string-cursor? obj → boolean
129Returns #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
132Returns the start/post-end cursor of s respectively.
133***** string-cursor-next s cursor → cursor
134***** string-cursor-prev s cursor → cursor
135Returns 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
138Returns 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
144Compares two cursors or two indexes pointing into the same string.
145***** string-cursor-diff s start end → nchars
146Returns 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
149Converts 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
152Is s the empty string?
153***** string-every pred s [start end] → value
154***** string-any pred s [start end] → value
155Checks 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].
159If 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.
167When 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
179(port->string p) = (string-unfold eof-object? values
180                                  (lambda (x) (read-char p))
181                                  (read-char p))
183(list->string lis) = (string-unfold null? car cdr lis)
185(string-tabulate f size) = (string-unfold (lambda (i) (= i size)) f add1 0)
187****** To map f over a list lis, producing a string:
188#+BEGIN_SRC scheme
189(string-unfold null? (compose f car) cdr lis)
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
196******* then
197#+BEGIN_SRC scheme
198(string-fold-right kons knil (string-unfold knull? kar kdr x)) = x
200******* and
201#+BEGIN_SRC scheme
202(string-unfold knull? kar kdr (string-fold-right kons knil s)) = s.
205The final string constructed does not share storage with either base or the value produced by make-final.
207This 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
210This is a fundamental constructor for strings.
212It 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
216string->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
218An efficient implementation of (compose list->string reverse):
220#+BEGIN_SRC scheme
221(reverse-list->string '(#\a #\B #\c)) → "cBa"
224This 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
226This procedure is a simple unparser --- it pastes strings together using the delimiter string.
228The 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.
230An empty list will produce an empty string -- note, however, that parsing an empty string with an infix or separator grammar is ambiguous.
231Is 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
238(string-join '("foo" "bar" "baz") ":")         => "foo:bar:baz"
239(string-join '("foo" "bar" "baz") ":" 'suffix) => "foo:bar:baz:"
241;; Infix grammar is ambiguous wrt empty list vs. empty string,
242(string-join '()   ":") => ""
243(string-join '("") ":") => ""
245;; but suffix & prefix grammars are not.
246(string-join '()   ":" 'suffix) => ""
247(string-join '("") ":" 'suffix) => ":"
249**** Selection
250***** string-ref/cursor s cursor → char
251Returns 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
254These 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
259string-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.
261If 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
264(string-take "Pete Szilagyi" 6) => "Pete S"
265(string-drop "Pete Szilagyi" 6) => "zilagyi"
267(string-take-right "Beta rules" 5) => "rules"
268(string-drop-right "Beta rules" 5) => "Beta "
270****** It is an error to take or drop more characters than are in the string:
271#+BEGIN_SRC scheme
272(string-take "foo" 37) => error
274***** string-pad s len [char start end] → string
275***** string-pad-right s len [char start end] → string
276Build 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.
278If len <= end-start, the returned value is allowed to share storage with s, or be exactly s (if len = end-start).
280#+BEGIN_SRC scheme
281(string-pad     "325" 5) => "  325"
282(string-pad   "71325" 5) => "71325"
283(string-pad "8871325" 5) => "71325"
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
288Trim 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?.
290If no trimming occurs, these functions may return either s or a copy of s; in some implementations, proper substrings may share memory with s.
292#+BEGIN_SRC scheme
293(string-trim-both "  The outlook wasn't brilliant,  \n\r")
294    => "The outlook wasn't brilliant,"
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
299Return 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).
301The optional start/end cursors or indexes restrict the comparison to the indicated substrings of s1 and s2.
302****** Examples
303#+BEGIN_SRC scheme
304string-prefix?    s1 s2 [start1 end1 start2 end2] → boolean
305string-suffix?    s1 s2 [start1 end1 start2 end2] → boolean
308Is 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
315string-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.
317The 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.
319The 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
321#+BEGIN_SRC scheme
322(substring/cursors s (string-skip s char-whitespace?))
325Note 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.
327These 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
330Does string s1 contain string s2?
332Returns 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.
334The returned cursor is in the range [start1,end1). A successful match must lie entirely in the [start1,end1) range of s1.
336Note 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.
338#+BEGIN_SRC scheme
339(string-contains "eek -- what a geek." "ee"
340                 12 18) ; Searches "a geek"
341    => {Cursor 15}
344The 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
347Reverse the string.
349string-reverse returns the result string and does not alter its s parameter.
351#+BEGIN_SRC scheme
352(string-reverse "Able was I ere I saw elba.")
353    => ".able was I ere I saw elbA"
354(string-reverse "Who stole the spoons?" 14 20)
355    => "snoops"
358Unicode 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
360Append the elements of string-list together into a single string. Guaranteed to return a freshly allocated string.
362Note 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
364With no optional arguments, this function is equivalent to
366#+BEGIN_SRC scheme
367(string-concatenate (reverse string-list))
370If 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.
372If 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
374#+BEGIN_SRC scheme
376  (reverse (cons (substring final-string
377                            (string-cursor-start final-string)
378                            end)
379                 string-list)))
384#+BEGIN_SRC scheme
385(string-concatenate-reverse '(" must be" "Hello, I") " going.XXXX" 7)
386  => "Hello, I must be going."
389This 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
392These are the fundamental iterators for strings.
394The left-fold operator maps the kons procedure across the string from left to right
396#+BEGIN_SRC scheme
397(... (kons s[2] (kons s[1] (kons s[0] knil))))
400In other words, string-fold obeys the (tail) recursion
402#+BEGIN_SRC scheme
403(string-fold kons knil s start end) =
404    (string-fold kons (kons s[start] knil) start+1 end)
407The right-fold operator maps the kons procedure across the string from right to left
409#+BEGIN_SRC scheme
410(kons s[0] (... (kons s[end-3] (kons s[end-2] (kons s[end-1] knil)))))
413obeying the (tail) recursion
415#+BEGIN_SRC scheme
416(string-fold-right kons knil s start end) =
417    (string-fold-right kons (kons s[end-1] knil) start end-1)
419****** Examples:
420#+BEGIN_SRC scheme
421;;; Convert a string to a list of chars.
422(string-fold-right cons '() s)
424;;; Count the number of lower-case characters in a string.
425(string-fold (lambda (c count)
426               (if (char-lower-case? c)
427                   (+ count 1)
428                   count))
429             0
430             s)
432;;; Double every backslash character in S.
433(let* ((ans-len (string-fold (lambda (c sum)
434                               (+ sum (if (char=? c #\\) 2 1)))
435                             0 s))
436       (ans (make-string ans-len)))
437  (string-fold (lambda (c i)
438                 (let ((i (if (char=? c #\\)
439                              (begin (string-set! ans i #\\) (+ i 1))
440                              i)))
441                   (string-set! ans i c)
442                   (+ i 1)))
443               0 s)
444  ans)
446****** The right-fold combinator is sometimes called a "catamorphism."
447***** string-for-each-cursor proc s [start end] → unspecified
448Apply 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:
450#+BEGIN_SRC scheme
451(let ((s "abcde") (v '()))
452  (string-for-each-cursor
453    (lambda (cur) (set! v (cons (char->integer (string-ref/cursor s cur)) v)))
454    s)
455  v) => (101 100 99 98 97)
457***** string-replicate s from to [start end] → string
458This is an "extended substring" procedure that implements replicated copying of a substring of some string.
460S 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
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 ...
467string-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:
477string-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
479Return a count of the number of characters in s that satisfy the pred argument.
480***** string-replace s1 s2 start1 end1 [start2 end2] → string
483#+BEGIN_SRC scheme
484(string-append (substring/cursors s1 (string-cursor-start s1) start1)
485               (substring/cursors s2 start2 end2)
486               (substring/cursors s1 end1 (string-cursor-end s1)))
489That 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
492(string-replace "The TCL programmer endured daily ridicule."
493                "another miserable perl drone" 4 7 8 22 ) =>
494    "The miserable perl programmer endured daily ridicule."
496(string-replace "It's easy to code it up in Scheme." "lots of fun" 5 9) =>
497    "It's lots of fun to code it up in Scheme."
499(define (string-insert s i t) (string-replace s t i i))
501(string-insert "It's easy to code it up in Scheme." 5 "really ") =>
502    "It's really easy to code it up in Scheme."
504***** string-split s delimiter [grammar limit start end] → list
505Returns 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.
507Grammar 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.
509If 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.
511Use 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
514Filter the string s, retaining only those characters that satisfy / do not satisfy pred.
516If the string is unaltered by the filtering operation, these functions may return either s or a copy of s.
517****** Compatibility note:
518string-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
520This 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:
522I 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.
524The 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.
526The 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.
528The 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.
530The 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.
532In 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.
534Another 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
536Thanks 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:
538The design of this library benefited greatly from the feedback provided during the SRFI discussion phase. Among those contributing thoughtful commentary and suggestions, both on the mailing list and by private discussion, were Paolo Amoroso, Lars Arvestad, Alan Bawden, Jim Bender, Dan Bornstein, Per Bothner, Will Clinger, Brian Denheyer, Mikael Djurfeldt, Kent Dybvig, Sergei Egorov, Marc Feeley, Matthias Felleisen, Will Fitzgerald, Matthew Flatt, Arthur A. Gleckler, Ben Goetter, Sven Hartrumpf, Erik Hilsdale, Richard Kelsey, Oleg Kiselyov, Bengt Kleberg, Donovan Kolbly, Bruce Korb, Shriram Krishnamurthi, Bruce Lewis, Tom Lord, Brad Lucier, Dave Mason, David Rush, Klaus Schilling, Jonathan Sobel, Mike Sperber, Mikael Staldal, Vladimir Tsyshevsky, Donald Welsh, and Mike Wilson. I am grateful to them for their assistance.
540I am also grateful to the authors, implementors and documentors of all the systems mentioned in the introduction. Aubrey Jaffer and Kent Pitman should be noted for their work in producing Web-accessible versions of the R5RS and Common Lisp spec, which was a tremendous aid.
542This is not to imply that these individuals necessarily endorse the final results, of course.
544During 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]
547Common Lisp: the Language.
548Guy L. Steele Jr. (editor).
549Digital Press, Maynard, Mass., second edition 1990.
550Available at
552The Common Lisp "HyperSpec," produced by Kent Pitman, is essentially the ANSI spec for Common Lisp:
554***** [MIT-Scheme]
556***** [R5RS]
557Revised^5 report on the algorithmic language Scheme.
558R. Kelsey, W. Clinger, J. Rees (editors).
559Higher-Order and Symbolic Computation, Vol. 11, No. 1, September, 1998.
560and ACM SIGPLAN Notices, Vol. 33, No. 9, October, 1998.
561Available at
562***** [R7RS]
563Revised^7 report on the algorithmic language Scheme.
564A. Shinn, J. Cowan, A. Gleckler (editors).
565Available at
566***** [SRFI]
567The SRFI web site.
569***** [SRFI-13]
570SRFI-13: String libraries.
572***** [SRFI-14]
573SRFI-14: Character-set library.
575The SRFI 14 char-set library defines a character-set data type, which is used by some procedures in this library.
576** Author
577John Cowan
578Ported and packaged to Chicken 5 by Sergey Goldgaber
579** Copyright
580Copyright (C) Olin Shivers (1998, 1999, 2000) and John Cowan (2016).
582Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
584The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
587** Version history
588*** 0.1 - SRFI-130 ported to Chicken 5.2.0
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