source: project/wiki/chicken-for-ruby-programmers @ 31126

Last change on this file since 31126 was 31126, checked in by Mario Domenech Goulart, 7 years ago

Properly capitalize CHICKEN on the wiki directory (only first level).

I used the following shell script to change things:

while IFS= read -d $'\0' -r file ; do

sed -i 's/Chicken/CHICKEN/g' "$file"

done < <(find wiki -maxdepth 1 -type f -print0 )

Some files have been manually reverted after that, since some
substitutions don't apply:

  • friedly-chicken (repl banner)
  • survey2011 (Chicken in URI paths)
  • chickenista-guide (Chickenista)

I hope the link canonicalization thing will be on my side.

File size: 30.5 KB
1== CHICKEN for Ruby programmers
5If you don't know much about CHICKEN yet, please take a moment to read
6the introductory part of [[/manual|The User's Manual]].  You're back?  Good!
8=== Paradigm independence
10The most important design feature of Ruby is that it is purely
11object-oriented; everything is an object.  Scheme is ''not'' an
12object-oriented language.  In fact, it does not commit to any
13particular programming paradigm -- it offers ''complete and total
14freedom to the programmer''.  If you decide (a part of) a program is
15best implemented in an object-oriented fashion, you can choose to use
16one of the many object systems.  Have a quick glance at the
17[[|OOP category]] of
18the egg index to get an impression of the
19diversity of styles of object oriented programming you can use with
20CHICKEN.  By the way, the list on that page shows all the available
21''eggs'' for CHICKEN.  We'll explain all about these [[#Eggs|later]].
23Besides object-oriented programming, you can also program in a
24procedural fashion (like you would in Pascal, for example) or in a
25functional style (a bit like Haskell or ML) and you can even
26experiment with message-passing like in Erlang, logic programming
27like you would with Prolog, or stack languages like Forth and Factor.
29=== Origins
31Ruby's origins are firmly rooted in Lisp.  It takes many things and
32ideas from Lisp (symbols, lambdas, eval, metaprogramming, DSLs etc).
33What it doesn't take from Lisp it takes from Smalltalk, which was
34itself inspired by Lisp's clean syntax.  All this means that once
35you're over the initial hump of grokking the syntax, you'll find
36yourself in pretty familiar territory.
38Originally Ruby began as an implementation-defined language.  That is,
39there was only one implementation (Matz's) and whatever that implementation
40did '''was''' the language spec.  Nowadays, Ruby
41has [[|rubyspec]], an attempt to
42standardize the Ruby language.
44Likewise, Scheme is a specification-defined language.  There
45is one official language specification which says what Scheme is and
46how it works.  CHICKEN is simply an implementation of that
47specification.  There is one thing that is important to know right
48now: The Scheme specification is ''extremely'' minimal in design.  It
49tries to define as few language constructs as possible, but these few
50should be so powerful that you will not need any more to make
51programs.  This results in an extremely small spec and a very elegant
52and clean language with very few rules to remember, which makes it
53easy to learn.
55However, in the real world you will need much more than just
56programming constructs, you need things to interact with the operating
57system, networking libraries, etc.  That's where the difference
58between Scheme and other languages comes in: Every implementation of
59Scheme defines the things it thinks are necessary to build real
60programs with.  Unfortunately, this means most Scheme programs are
61inherently not portable from one implementation to another, but it also means that Scheme implementations
62are free to experiment how they want and explore new language
63territory.  This gives each Scheme implementation its uniqueness.
65Fortunately, most Scheme implementations including CHICKEN are fairly portable to modern hardware architectures and operating systems.  You may or may not be able to port a CHICKEN program to another Scheme, but you can port it from Windows to Mac OS X or Linux, or from the 32-bit Intel 386 to the 64-bit SPARC architecture, because CHICKEN runs in all those environments.  Usually "porting" just means "running it on that system", because just like Ruby CHICKEN has procedures that are platform-independent, with multiple implementations where the platform itself differs.
67CHICKEN's power is in how it extends the Scheme standard.  It has a
68very comfortable interface to C that does not require you to touch a
69single line of C code in order to create bindings to existing C
70libraries, but it also gives you the freedom to embed CHICKEN in C
71code or C in CHICKEN code as you want.  It offers a TCP/IP networking
72layer, it has great POSIX interoperability so you can interact with
73the OS. And most importantly: It can compile Scheme code to very
74efficient C code which can itself be compiled to machine code, giving
75you the best of both worlds: a dynamic language which allows you to
76program in the flexible way you are used to with Ruby, but this can be
77compiled for maximum efficiency.
79== Syntax
81=== The basics
83The one thing that is most strikingly different between Ruby and
84Scheme is of course the syntax.  Ruby has an extremely baroque syntax
85that allows you many freedoms in how you would like to write down
86things. Scheme, on the other hand, has only one way in which to write
87a given expression.  Let's start by looking at an example.  First we
88start by firing up an {{irb}} session and typing a little program:
90  irb(main):001:0> # My first program
91  irb(main):002:0* [1, 2, 3, 4].map{|x| x + 1}
92  => [2, 3, 4, 5]
93  irb(main):003:0>
95Now, we fire up a {{csi}} (chicken scheme interpreter) session:
97  #;1> ; My first program
98  (map add1 (list 1 2 3 4))
99  (2 3 4 5)
100  #;2>
102In Scheme, lists are delimited with parentheses with its elements
103separated by spaces. As we can see, everything in Scheme is a list,
104even the expressions that you use to tell it what to do!  An
105expression in Scheme is always a list with the operator on its first
106position and the operands following it.  Procedures that accept no
107arguments are simply a list with only the procedure in it, for example
108{{(newline)}}, which simply displays a newline character.
110This simple rule also means that ''every parenthesis has a meaning''.
111You can't add more parentheses or leave off parentheses like you can
112in most Ruby expressions.  Adding extra parentheses simply applies
113the resulting expression as if it were a procedure:
115  #;2> ((newline))
117  Error: call of non-procedure: #<unspecified>
119          Call history:
121          <syntax>                ((newline))
122          <syntax>                (newline)
123          <eval>          ((newline))
124          <eval>          (newline)       <--
126If {{(newline)}} returned a procedure, it would be called.  But as it
127happens, {{newline}} simply returns an unspecified value which is not
128a procedure and thus can't be applied.  We can also use the result of
129calling a procedure in the call to another procedure:
131  #;3> (add1 2)
132  3
133  #;4> (- 10 (add1 2) 1)
134  6
136We see that arithmetic is a little different from Ruby, as a result of
137the simple rules Scheme has for its syntax.  This may be a little
138awkward, especially with complex calculations:
140  #;5> (* (+ 1 (- 6 2) 2) (- 10 5))
141  35
143In Ruby (and other languages with algebraic syntax) this would have
146  irb(main):002:0> (1 + (6 - 2) + 2) * (10 - 5)
147  => 35
148  irb(main):003:0> # Alternatively:
149  irb(main):004:0* (1 + 6 - 2 + 2) * (10 - 5)
150  irb(main):005:0> # or even:
151  irb(main):006:0* (((1) + ((((6 - 2)))) + 2) * (((10) - ((5)))))
152  => 35
154Both types of syntax have their advantages and disadvantages: The
155Ruby-like syntax is more natural, but you have to think about operator
156precedence rules.  CHICKEN does not need operator precedence rules
157because the precedence can be determined from the way it's nested, but
158it's less natural for most people (though you get used to it very
161Actually, right now you know almost all there is to know
162about Scheme's syntax!  CHICKEN has a couple of extensions to the
163basic Scheme syntax, but we won't go into detail here.  Later you'll
164see a couple of handy shortcuts, but this is basically it.
166=== Variables
168Variables are names for things.  CHICKEN has vary lax rules for
169naming variables.  Actually, ''any string'' is a valid identifier
170as long as you quote it correctly.
174  #;1> x = 10
175  => 10
176  #;2> x
177  => 10
178  #;3> x-y-z = 10
179  NameError: undefined local variable or method `x' for main:Object
180        from (irb):2
184  #;1> (define x 10)
185  #;2> x
186  10
187  #;3> (define x-y-z 10)
188  #;4> x-y-z
189  10
190  #;5> (define %x% 1)
191  #;6> %x%
192  1
193  #;7> (define |a b c| 5)
194  #;8> |a b c|
195  5
197As you can see, because of Scheme's operator rules, symbols that would
198normally be separate tokens designating operators have no special
199meaning so we can use it in the middle of a name.  The convention in
200Scheme is to use the minus sign as word separator (in Ruby, you would
201use an underscore for separating words).  The final example shows how
202any string is allowed as a variable name: if the string contains
203syntax that would mean something else to Scheme you can enclose the
204variable name in pipe symbols.  The pipe symbol itself can be escaped
205with a backslash, if you need it to be part of a variable.
207To assign to a pre-existing variable we can also use {{set!}}:
209  #;1> (define x 10)
210  #;2> x
211  10
212  #;3> (set! x 20)
213  #;4> x
214  20
216Top-level variables can also be overwritten by simply redefining them,
217but in some cases you need {{set!}}.  However, set! is a typical
218artifact of an imperative programming style and in clean code you
219want to avoid using it.
221Scheme also allows the ''binding'' of local variables. Bound variables
222behave like the defined variables above, however, they are only valid
223within a local scope. The top-level variables we've seen are the equivalent
224of a global in Ruby.
226The most common binding constructs are {{let}} and {{let*}}.
227{{let}} allows for any number of bindings, none of which are related.
229<enscript highlight=scheme>
230(let ((a 5)
231      (b 10))
232  (+ a b))
235{{let*}} is like let, except that the bindings are evaluated in order,
236so subsequent bindings can reference earlier bindings.
238<enscript highlight=scheme>
239(let* ((a 5)
240       (b (* 2 a)))
241  (+ a b))
244There are other binding forms, such as {{letrec}} and the so-called
245''named let''. More information about these forms can be found in
246the [[|Scheme specification]].
248=== Procedures
250Of course using simple expressions like this is not enough.  You'll
251need procedures too.  In Ruby, named procedures are actually methods
252on objects, but we can forget about that little detail for now:
256<enscript highlight=ruby>
257def pythagoras(a, b)
258  Math.sqrt(a**2 + b**2)
264<enscript highlight=scheme>
265(define pythagoras
266  (lambda (a b)
267    (sqrt (+ (* a a) (* b b)))))
270Now that's interesting!  Procedures are just regular variables in
271Scheme (a bit like functions in Javascript).  We assign a lambda to it.
272We can do that in Ruby too, but it's not pretty:
275  some_class.send(:define_method, :pythagoras) {|a, b| Math.sqrt(a**2 + b**2) }
277Just like in Ruby the {{def foo}} is shorter than the above, in Scheme
278we have a convenient shorthand for defining procedures too:
280<enscript highlight=scheme>
281  (define (pythagoras a b)
282    (sqrt (+ (* a a) (* b b))))
284==== Recursion and tail-call optimization
286In Scheme, recursion is a very important tool.  In fact, it is so
287important that the Scheme standard ''demands'' tail call optimization
288(TCO), which basically means that you can have infinite recursion as
289long as the recursive procedure does not need to do anything after it
290returns.  That sounds a bit strange, so here's an example:
294  irb(main):010:0> def add_up_to(num)
295  irb(main):011:1>   if
296  irb(main):012:2>     0
297  irb(main):013:2>   else
298  irb(main):014:2*     add_up_to(num - 1) + num
299  irb(main):015:2>   end
300  irb(main):016:1> end
301  => nil
302  irb(main):017:0> add_up_to(4)
303  => 10
304  irb(main):018:0> add_up_to(9999)
305  SystemStackError: stack level too deep
309  #;2> (define (add-up-to x)
310         (if (zero? x)
311             0
312             (+ (add-up-to (sub1 x)) x)))
313  #;3> (add-up-to 4)
314  10
315  #;4> (add-up-to 9999)
316  49995000
318Note the {{+}} in front of the {{(add-up-to (sub1 ...))}}. That is a cue that this is not tail-recursive code: each level of recursion must eventually come back 'up a level' in order to complete the addition, and so the program must keep a live reference to every level of recursion until the final result is computed.
320In most other Schemes, however, this will break just like in Ruby,
321because when the {{(+ (add-up-to (sub1 x)) x)}} expression is
322evaluated, the recursive call to {{add-up-to}} creates a new stack
323frame so that when it returns the x can be added to the result.
325[This code will 'break' in CHICKEN too, but only for much
326larger numbers. Although CHICKEN doesn't have an arbitrary stack
327depth, if you try (add-up-to) on a large enough number, you'll use up
328all your system memory before getting an answer. Read on for a better
329way to write it.]
331All Schemes know that when there is nothing that needs to be done
332after a procedure returns, there is no point in returning to the
333procedure that called it at all: instead it can just return directly
334to the procedure that called the current procedure. So the call can
335be optimized to become a ''goto'', replacing the current stack frame.
337Here is a tail-recursive version. Ruby still can't handle it:
339  irb(main):027:0> def add_up_to(x)
340  irb(main):028:1>   def inner(x, y)
341  irb(main):029:2>     if
342  irb(main):030:3>       y
343  irb(main):031:3>     else
344  irb(main):032:3*       inner(x-1, x+y)
345  irb(main):033:3>     end
346  irb(main):034:2>   end
347  irb(main):035:1>   inner(x, 0)
348  irb(main):036:1> end
349  => nil
350  irb(main):037:0> add_up_to(9999)
351  SystemStackError: stack level too deep
353But Scheme can (this works in all Schemes):
355  #;2> (define (add-up-to x)
356         (define (inner x y)
357           (if (zero? x)
358               y
359               (inner (sub1 x) (+ y x))))
360         (inner x 0))
361  #;3> (add-up-to 4)
362  10
363  #;4> (add-up-to 9999)
364  49995000
366Note that the recursive call to {{inner}} isn't nested inside another function call, such as the {{(+ (add-up-to ...))}} in the first version. This is the hallmark of a tail-recursive program. (The astute reader might note that it actually *is* nested inside an {{(if ...)}} procedure, but conditional forms like {{if}} are handled intelligently in tail-recursion. The {{if}} statement itself is not nested inside a procedure call, so all is well.)
368As you'll notice, this version is a lot faster in CHICKEN too because
369it does not have to travel back through all those empty "stack
370frames".  In the first example, CHICKEN's memory usage increases upon
371every recursion: for large numbers, it will break because it can't
372allocate any more. But in the second example, memory usage will stay
373constant and simply loop forever.
375=== Blocks
377Ruby programmers will be familiar with ''blocks.'' Classic example in
378Ruby is the {{map}} method used to iterate over a collection, executing
379a ''block'' of code for each item in the collection.
382  >> [1, 2, 3, 4, 5].map { |x| x * x }
383  => [1, 4, 9, 16, 25]
385Scheme also contains blocks, though we call them anonymous procedures
386usually. Procedures are created using the {{(lambda args body...)}} body
387form. This syntax is a little more verbose than Ruby's, but the trade off
388is that more than one procedure can be passed as an argument, whereas Ruby
389generally only allows one.
392  #;1> (map (lambda (x) (* x x)) '(1 2 3 4 5))
393  (1 4 9 16 25)
395A more complicated example involves opening and closing files. Say we
396wanted to create a utility like {{wc -l}} that counts the number of
397lines in a file. In Ruby, it might look something like:
399<enscript highlight=ruby>
400count = 0"myfile", 'r') { |fh|
402    count += 1 while fh.gets
404puts count
407Similarly, Scheme uses anonymous procedures to create the same behavior:
409<enscript highlight=scheme>
410(with-input-from-file "filename"
411  (lambda () (port-fold (lambda (line lines-so-far) (+ 1 lines-so-far)) 0 read-line)))
414This Scheme code also showcases some typical functional style, using
415a ''fold'' operation instead of incrementing the value of a variable.
418== Data types
420Now we have a basic grasp of Scheme's syntax, we can have a look at the
421different data types CHICKEN has to offer.  We will do this from a Ruby
424=== Arrays
426In Ruby we use arrays for storing lists of things.  The obvious Scheme
427equivalent type is the list, you'd think.  This is sort of true:
431<enscript highlight=ruby>
432x = [1, 2, 3, 4]{|y| y + 10 }
434x.each{|y| puts y }
439<enscript highlight=scheme>
440(define x '(1 2 3))
441(map (lambda (x) (+ x 10)) x)
442(for-each print x)
445Note that Scheme does not have the block scoping bug.  Another thing
446that we should note is the first line.  We create a list by
447''quoting'' it.  This allows us to enter the list in such a way that
448CHICKEN knows the list is just that; a list, and not a procedure
449application of the procedure called {{1}} on the arguments {{2}} and
450{{3}}.  The apostrophe takes care of that.
452However, we must always remember that the Scheme list is more like a
453linked list.  This means that it is very flexible in how we can add
454things to it and alter it, but it also means that traversing it takes
455more time as more items are added to it.  Accessing an element is an
456O(n) operation, where n is the position of the element.
458If we want O(1) operations on our lists, we can use a ''vector'':
460  #;1> (define x (vector 1 2 3 4))
461  #;2> (vector-ref x 2)
462  3
463  #;3> (define y (list 1 2 3 4))
464  #;4> (list-ref y 2)
465  3
467Adding new elements to a vector requires resizing or even copying the
468vector, just like it would in Ruby.  So whenever you're contemplating
469using a list type, think about the properties you want the list type
470to have.  This may sound odd, but in fact this gives you much more
471flexibility than Ruby, where you have the choice of using an Array,
472or... using an Array.  However, as Knuth famously said: "Premature
473optimization is the root of all evil", and you should probably take
474the list solution until it's proven that you ''need'' vectors.  Also,
475because Lisp was built on lists, it is very good at manipulating them,
476so they're most likely the most convenient datatype.
478CHICKEN also offers you several other types of array-like types, each
479with their own unique time and space properties.  Which you'll use
480depends on the task at hand and the situations your system will be
481used under.
483==== List procedures
485Lists are, as mentioned before, linked lists.  This means they always
486consist of two parts: a head and a tail.  We've seen the {{list}}
487procedure which creates lists, but this works on lower primitives:
489  #;1> (list 1)
490  (1)
491  #;2> (cons 1 '())
492  (1)
494The {{()}} is the empty list.  It is itself a list, but it is also a
495single symbol.  It serves as the ''end of list marker''.  That's why
496the list construction procedure, {{cons}}, can create longer lists too:
498  #;1> (list 1 2 3 4)
499  (1 2 3 4)
500  #;2> (cons 1 (cons 2 (cons 3 (cons 4 '()))))
501  (1 2 3 4)
503To take the head/tail of these lists we have two procedures:
505  #;1> (car '(1 2 3 4))
506  1
507  #;2> (cdr '(1 2 3 4))
508  (2 3 4)
509  #;3> (cdr (cdr '(1 2 3 4)))
510  (3 4)
511  #;4> (car (cdr (cdr '(1 2 3 4))))
512  3
513  #;5> (caddr '(1 2 3 4)) ; combination of car cdr cdr
514  3
515  #;6> (car (car '(1 2 3 4)))
516  Error: (car) bad argument type: 1
517  #;7> (cdr (cdr '(1)))
518  Error: (cdr) bad argument type: ()
520Actually, {{cons}} just sticks two things together, so we could also
521stick together two numbers:
523  #;1> (cons 1 2)
524  (1 . 2)
525  #;2> (car (cons 1 2))
526  1
527  #;3> (cdr (cons 1 2))
528  2
530Two things stuck together are called a ''pair''.  By sticking together
531more things without an end of list marker, we can create an ''improper
534  #;1> (cons 1 (cons 2 (cons 3 4)))
535  (1 2 3 . 4)
537You should not use lists like these unless you know what you're doing,
538because ''all'' list library procedures expect ''proper lists'': lists
539with end markers.  CHICKEN supports the full
540[[|SRFI-1]] out of the box.
541Have a look at that document and compare it to the Ruby standard
542Enumerator and Array methods.  Most of the procedures in srfi-1 will
543look ''very'' familiar to you.  Here are some examples:
545  #;1> (use srfi-1)  ;; Not needed in Ruby
546  ; loading library srfi-1 ...
547  #;2> ;; [1, 2, 3] + [4, 5, 6] / [1, 2, 3].concat([4, 5, 6])
548  (append '(1 2 3) '(4 5 6))
549  (1 2 3 4 5 6)
550  #;3> (map add1 '(1 2 3 4)) ;; [1, 2, 3, 4].map{|x| x + 1}
551  (2 3 4 5)
552  #;4> ;; No equivalent because map works on one object:
553  (map + '(1 2 3 4) '(5 6 7 8))
554  (6 8 10 12)
555  #;5> ;; [1, 2, 3, 4].each{|x| puts x}
556  (for-each (lambda (x) (printf "~A\n" x)) '(1 2 3 4))
557  1
558  2
559  3
560  4
561  #;6> ;; [1, 2, 3, 4, 5, 6].select{|x| x.even? }
562  (filter even? '(1 2 3 4 5 6))
563  (2 4 6)
564  #;7> ;; [1, 2, 3, 4].inject(""){|str, x| str + x.to_s}
565  (fold (lambda (x str) (conc str x)) "" '(1 2 3 4))
566  "1234"
568=== Symbols
570Luckily, you are a Ruby programmer, so we will not have to go through
571the whole "explaining what symbols exactly are" again :)
572Actually, Ruby borrowed symbols from Lisp.
576<enscript highlight=ruby>
584<enscript highlight=scheme>
586(string->symbol "foo")
587(symbol->string 'foo)
590As we can see, a symbol is only a quoted variable name!  This is the
591origin of symbols and also the reason you can {{send}} symbols
592representing method names to objects in Ruby.  Symbols have all the
593same semantics as Ruby's symbols: they can be compared in constant
594time and they take up very little memory space.
596=== Strings
598Strings are simple.  Just like in Ruby, we have strings enclosed by
599double quotes: {{"foo"}} works the same in Ruby as it does in CHICKEN.
600CHICKEN's double quoted strings work more like Ruby's single-quoted
601strings, though.  There is no string interpolation and other things;
602a string is just a string.
606<enscript highlight=ruby>
607x = 10
608y = "x contains #{x}"
609z = "x contains " + x.to_s
614<enscript highlight=scheme>
615  (define x 10)
616  (define y (sprintf "x contains ~A" x))
617  (define z (conc "x contains " x))
618  ; Conc automatically converts its arguments to strings. We also could do:
619  (define z (string-append "x contains " (->string x)))
622Note that {{->string}} is simply the name of a procedure, including
623the arrow.
625It may be important to know that Scheme also has a ''character'' data
626type, unlike Ruby:
630  irb(main):001:0> "foo"[0]
631  => 102
633  #;1> (string-ref "foo" 0)
634  #\f
635  #;2> (char->integer #\f)
636  102
638You will probably not need this data type for your first few Scheme
639programs so we won't go into it deeper here.
641==== String procedures
643CHICKEN comes shipped with
644[[|SRFI-13]], which is a
645library of string procedures which is intended to be a lot like
646SRFI-1, which we already looked at [[#List procedures|a few sections
649  #;1> (use srfi-13) ;; Not needed in Ruby
650  ; loading library srfi-13 ...
651  #;2> ;; "abc" + "def"
652  (string-append "abc" "def")
653  "abcdef"
654  #;3> ;; "abcdef"[-3..-1]
655  (string-take-right "abcdef" 2)
656  "ef"
657  #;4> ;; "abcdef".rjust(10)
658  (string-pad "abcdef" 10)
659  "    abcdef"
660  #;5> ;; ["this", "is", "very", "cool"].join(" ")
661  (string-join '("this" "is" "very" "cool"))
662  "this is very cool"
663  #;6> ;; "this is very cool".split(" ")
664  ;; NOT from srfi-13 but chicken's extras unit:
665  (string-split "this is very cool" " ")
666  ("this" "is" "very" "cool")
668=== Regular expressions
670Just like in Ruby, there's a Regex data type, but in CHICKEN there is
671no special syntax for it:
675  irb(main):001:0> /(.)(.)(\d+)(\d)/.match("THX1138.").to_a
676  => ["HX1138", "H", "X", "113", "8"]
680  #;1> (use regex)
681  ; loading library regex ...
682  #;2> (string-search "(.)(.)(\\d+)(\\d)" "THX1138.")
683  ("HX1138" "H" "X" "113" "8")
685The {{string-search}} procedure automatically transforms
686the first string to a regexp.  You can also do that yourself:
688  #;3> (string-search (regexp "(.)(.)(\\d+)(\\d)") "THX1138.")
689  ("HX1138" "H" "X" "113" "8")
691The advantage of doing this is that when you need to match several
692strings you can use the same regexp so it doesn't have to precompile
693the regexp every time you call {{string-search}}.
695=== Hashes
697The final datatype we use a lot in Ruby is the Hash.  In CHICKEN there
698are two datatypes you could use instead of the Ruby Hash;
699''association lists'' (or ''alists'' for short) or ''hash tables''.
701==== Association Lists
703Association lists are the simpler Hash like structure in chicken.
704Effectively, alists are standard lists of ''pairs,'' where the
705first item in the pair is the key and the second item is the value.
706Consequently, alists have a nice literal form:
708<enscript highlight=scheme>
709'((foo  1) (bar 42) (baz 101))
712To lookup a value in the alist, use {{assoc}}. For example to check
713if the pair {{(bar 42)}} is in the alist:
715  #;1> (assoc 'bar '((foo  1) (bar 42) (baz 101)))
716  (bar 42)
718If the pair is not in the list, you would get the boolean false ({{#f}}).
719If you need more stringent checks, you can also use {{assq}} or {{assv}},
720learning more about these procedures is an exercise for the reader.
722Alists may simplistic, and inquisitive readers may notice that lookup is
723{{O(n)}} time. However, they are convenient and adding new items is a constant
724time operation. You may find they work in may places that you might use a
725small Hash in Ruby.
727==== Hash tables
729For more complex hashing operations, CHICKEN supplies true hash tables.
731  #;11> (define h (make-hash-table))
732  #;12> h
733  #<hash-table>
734  #;13> (hash-table-set! h 'foo 12)
735  12
736  #;14> (hash-table-set! h 'bar 101)
737  101
738  #;15> (hash-table-ref h 'bar)
739  101
740  #;16> (hash-table-delete! h 'bar)
741  #t
742  #;17> (hash-table-ref h 'bar)
743  Error: (hash-table-ref) hash-table does not contain key
744  bar
746Hash tables are more powerful overall, but do not offer convenient literal
747notation. If you need to convert from a hash table an alist you can use
748{{hash-table->alist}}. The {{alist->hash-table}} procedure converts in the
749opposite direction. For a complete list of supported procedures, check the
750[[/manual/Unit srfi-69|hash table section]] in the manual.
753=== Booleans
755Scheme has a boolean type where {{#f}} is false and {{#t}} is true.
756Its handling of truthiness is a lot like Ruby's; anything that is not
757{{#f}} is treated as being ''true'':
759  #;1> (if #f
760           (print "WTF, it's true")
761           (print "It's not true"))
762  It's not true
763  #;2> (if #t
764           (print "Yes, it's really true")
765           (print "No, it's not true"))
766  Yes, it's really true
767  #;3> (if "Some random other value than #f"
768           (print "Yes, this is also true")
769           (print "No, it's not true"))
770  Yes, this is also true
772Ruby's {{nil}} does not have a direct equivalent in Scheme.  In the
773situations where a ''not present'' value is supposed to be returned,
774usually {{#f}} is used:
776  #;1> (use srfi-1)
777  ; loading library srfi-1 ...
778  #;2> (find even? '(3 1 4 1 5 9))
779  4
780  #;3> (find even? '(1 3 7 9))
781  #f
783In cases where a procedure really has no sensible thing to return, we
784use the special ''void'' value, returned by the {{void}} procedure:
786  #;1> (define (say-hello)
787         (print "Hello")
788         (void))
789  #;2> (say-hello)
790  Hello
791  #;3>
793As we see, the interpreter understands that there is no proper value
794to return so it displays the prompt immediately without showing the
795result value.  The example is a little contrived, because in real code
796we wouldn't explicitly call {{(void)}} because {{print}} already
797returns the {{void}} value.
799== Examples
801Now we have the tools to make programs, let's look at a few larger
802programs to better appreciate how one would program in CHICKEN.
804  TODO
806== CHICKEN and the Real World
808Programming is about more than having a pretty language, so let's look
809at what CHICKEN has to offer for real construction work.
811=== Eggs
813Eggs are to chicken what ''gems'' are to Ruby: installable extensions
814like libraries and programs.  The list of [[eggs]] is where you should
815look first when you are going to implement something big. You can
816install an egg almost like you install a gem, as follows:
818  $ chicken-install matchable
820This downloads and installs the egg with the name "matchable".  This
821egg has no dependencies, but if it did it would have downloaded and
822installed them as well.
825== Meta programming
827A hot topic in the Ruby community is meta programming and DSLs
828(Domain specific languages).  These ideas originated from Lisp, which
829means you can just keep on trucking in CHICKEN!
831=== Data is code and code is data
833The most fundamental concept of Lisp is that code is data.  As we've
834seen, procedure calls look like lists.  We've also seen that we can
835quote lists to "block" Scheme from interpreting a list as a procedure
836call.  We can also turn this around on its head and force a list to
837be evaluated as code:
839  #;1> (define calculate '(+ 1 2 3 4))
840  #;2> calculate
841  (+ 1 2 3 4)
842  #;3> (eval calculate)
843  10
844  #;4>
846"Big deal", you might say, "Ruby also has eval".  But the difference
847is what matters: In Ruby you have to construct strings to be evaluated,
848which means you need to be able to parse strings if you want to change
849a Ruby-program-stored-as-string.  In Scheme we can simply hack the
850list.  ''The program is stored in a parsed state'', so to speak.
852If we would want to change the operator from + to *, we can simply
853do that:
855  #;1> (eval (cons '* (cdr calculate)))
856  24
858This is much more robust than any regexp hacking or ad-hoc parsing on
859strings you want to evaluate.
861=== Macros
863One of the coolest concepts, but also the one most easily abused is
864''macros''.  Because Scheme stores code as data, you can change the
865code on-the-fly as described above.  You can do that at run-time on
866random data through eval, but you can also do it at compile-time on
867your program, which is usually the best time to do it.
869Scheme macros ''rewrite'' your code during compile time. They can
870range from simple to complex, with some macros defining entire
871"sublanguages" embedded in Scheme.
873Some people call Rails' {{acts_as_foo}} methods "macros". This
874description is not wrong, as these methods do ''rewrite'' your
875classes in a similar way to Scheme macros, but they are not quite
876as powerful.
878Here is a simple example of a task that is easy in Scheme, but
879much, much harder using Ruby's eval. Say you were debugging a
880program and found yourself printing out variables at certain
881points in the execution, along with the name of the variable
882so you could tell what you were looking at.
884<enscript highlight=scheme>
885  (print "myvar: " myvar)
888You decide that repeatedly typing the variable name twice (once to
889indicate which variable, once to get the value) is a waste of time.
890Using a macro, you can quickly and easily abstract away the common
891syntax into one place.
893<enscript highlight=scheme>
894  (define-syntax ez-debug
895    (syntax-rules ()
896      ((_ var)
897       (print 'var ": " var))))
899  (define myvar '(this is a list))
901  (ez-debug myvar)
904This simply wouldn't be possible with a regular procedure. By the time
905a procedure is called, syntactic information like variables names has
906been optimized away.
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