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learnxinyminutes-docs/CHICKEN.html.markdown
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---
language: "CHICKEN"
filename: CHICKEN.scm
contributors:
- ["Diwakar Wagle", "https://github.com/deewakar"]
---
CHICKEN is an implementation of Scheme programming language that can
compile Scheme programs to C code as well as interpret them. CHICKEN
supports R5RS and R7RS (work in progress) standards and many extensions.
```scheme
;; #!/usr/bin/env csi -s
;; Run the CHICKEN REPL in the commandline as follows :
;; $ csi
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; 0. Syntax
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Single line comments start with a semicolon
#| Block comments
can span multiple lines and...
#| can be nested
|#
|#
;; S-expression comments are used to comment out expressions
#; (display "nothing") ; discard this expression
;; CHICKEN has two fundamental pieces of syntax: Atoms and S-expressions
;; an atom is something that evaluates to itself
;; all builtin data types viz. numbers, chars, booleans, strings etc. are atoms
;; Furthermore an atom can be a symbol, an identifier, a keyword, a procedure
;; or the empty list (also called null)
'athing ;; => athing
'+ ;; => +
+ ;; => <procedure C_plus>
;; S-expressions (short for symbolic expressions) consists of one or more atoms
(quote +) ;; => + ; another way of writing '+
(+ 1 2 3) ;; => 6 ; this S-expression evaluates to a function call
'(+ 1 2 3) ;; => (+ 1 2 3) ; evaluates to a list
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; 1. Primitive Datatypes and Operators
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Numbers
99999999999999999999 ;; integers
#b1010 ;; binary ; => 10
#o10 ;; octal ; => 8
#x8ded ;; hexadecimal ; => 36333
3.14 ;; real
6.02e+23
3/4 ;; rational
;;Characters and Strings
#\A ;; A char
"Hello, World!" ;; strings are fixed-length arrays of characters
;; Booleans
#t ;; true
#f ;; false
;; Function call is written as (f x y z ...)
;; where f is a function and x,y,z, ... are arguments
(print "Hello, World!") ;; => Hello, World!
;; formatted output
(printf "Hello, ~a.\n" "World") ;; => Hello, World.
;; print commandline arguments
(map print (command-line-arguments))
(list 'foo 'bar 'baz) ;; => (foo bar baz)
(string-append "pine" "apple") ;; => "pineapple"
(string-ref "tapioca" 3) ;; => #\i;; character 'i' is at index 3
(string->list "CHICKEN") ;; => (#\C #\H #\I #\C #\K #\E #\N)
(string-intersperse '("1" "2") ":") ;; => "1:2"
(string-split "1:2:3" ":") ;; => ("1" "2" "3")
;; Predicates are special functions that return boolean values
(atom? #t) ;; => #t
(symbol? #t) ;; => #f
(symbol? '+) ;; => #t
(procedure? +) ;; => #t
(pair? '(1 2)) ;; => #t
(pair? '(1 2 . 3)) ;; => #t
(pair? '()) ;; => #f
(list? '()) ;; => #t
;; Some arithmetic operations
(+ 1 1) ;; => 2
(- 8 1) ;; => 7
(* 10 2) ;; => 20
(expt 2 3) ;; => 8
(remainder 5 2) ;; => 1
(/ 35 5) ;; => 7
(/ 1 3) ;; => 0.333333333333333
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; 2. Variables
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; You can create variables with define
;; A variable name can use any character except: ()[]{}",'`;#\
(define myvar 5)
myvar ;; => 5
;; Alias to a procedure
(define ** expt)
(** 2 3) ;; => 8
;; Accessing an undefined variable raises an exception
s ;; => Error: unbound variable: s
;; Local binding
(let ((me "Bob"))
(print me)) ;; => Bob
(print me) ;; => Error: unbound variable: me
;; Assign a new value to previously defined variable
(set! myvar 10)
myvar ;; => 10
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; 3. Collections
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Pairs
;; 'cons' constructs pairs,
;; 'car' extracts the first element, 'cdr' extracts the rest of the elements
(cons 'subject 'verb) ;; => '(subject . verb)
(car (cons 'subject 'verb)) ;; => subject
(cdr (cons 'subject 'verb)) ;; => verb
;; Lists
;; cons creates a new list if the second item is a list
(cons 0 '()) ;; => (0)
(cons 1 (cons 2 (cons 3 '()))) ;; => (1 2 3)
;; 'list' is a convenience variadic constructor for lists
(list 1 2 3) ;; => (1 2 3)
;; Use 'append' to append lists together
(append '(1 2) '(3 4)) ;; => (1 2 3 4)
;; Some basic operations on lists
(map add1 '(1 2 3)) ;; => (2 3 4)
(reverse '(1 3 4 7)) ;; => (7 4 3 1)
(sort '(11 22 33 44) >) ;; => (44 33 22 11)
(define days '(SUN MON FRI))
(list-ref days 1) ;; => MON
(set! (list-ref days 1) 'TUE)
days ;; => (SUN TUE FRI)
;; Vectors
;; Vectors are heterogeneous structures whose elements are indexed by integers
;; A Vector typically occupies less space than a list of the same length
;; Random access of an element in a vector is faster than in a list
#(1 2 3) ;; => #(1 2 3) ;; literal syntax
(vector 'a 'b 'c) ;; => #(a b c)
(vector? #(1 2 3)) ;; => #t
(vector-length #(1 (2) "a")) ;; => 3
(vector-ref #(1 (2) (3 3)) 2);; => (3 3)
(define vec #(1 2 3))
(vector-set! vec 2 4)
vec ;; => #(1 2 4)
;; Vectors can be created from lists and vice-verca
(vector->list #(1 2 4)) ;; => '(1 2 4)
(list->vector '(a b c)) ;; => #(a b c)
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; 4. Functions
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Use 'lambda' to create functions.
;; A function always returns the value of its last expression
(lambda () "Hello World") ;; => #<procedure (?)>
;; Use extra parens around function definition to execute
((lambda () "Hello World")) ;; => Hello World ;; argument list is empty
;; A function with an argument
((lambda (x) (* x x)) 3) ;; => 9
;; A function with two arguments
((lambda (x y) (* x y)) 2 3) ;; => 6
;; assign a function to a variable
(define sqr (lambda (x) (* x x)))
sqr ;; => #<procedure (sqr x)>
(sqr 3) ;; => 9
;; We can shorten this using the function definition syntactic sugar
(define (sqr x) (* x x))
(sqr 3) ;; => 9
;; We can redefine existing procedures
(foldl cons '() '(1 2 3 4 5)) ;; => (((((() . 1) . 2) . 3) . 4) . 5)
(define (foldl func accu alist)
(if (null? alist)
accu
(foldl func (func (car alist) accu) (cdr alist))))
(foldl cons '() '(1 2 3 4 5)) ;; => (5 4 3 2 1)
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; 5. Equality
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; For numbers use '='
(= 3 3.0) ;; => #t
(= 2 1) ;; => #f
;; 'eq?' returns #t if two arguments refer to the same object in memory
;; In other words, it's a simple pointer comparison.
(eq? '() '()) ;; => #t ;; there's only one empty list in memory
(eq? (list 3) (list 3)) ;; => #f ;; not the same object
(eq? 'yes 'yes) ;; => #t
(eq? 3 3) ;; => #t ;; don't do this even if it works in this case
(eq? 3 3.0) ;; => #f ;; it's better to use '=' for number comparisons
(eq? "Hello" "Hello") ;; => #f
;; 'eqv?' is same as 'eq?' all datatypes except numbers and characters
(eqv? 3 3.0) ;; => #f
(eqv? (expt 2 3) (expt 2 3)) ;; => #t
(eqv? 'yes 'yes) ;; => #t
;; 'equal?' recursively compares the contents of pairs, vectors, and strings,
;; applying eqv? on other objects such as numbers and symbols.
;; A rule of thumb is that objects are generally equal? if they print the same.
(equal? '(1 2 3) '(1 2 3)) ;; => #t
(equal? #(a b c) #(a b c)) ;; => #t
(equal? 'a 'a) ;; => #t
(equal? "abc" "abc") ;; => #t
;; In Summary:
;; eq? tests if objects are identical
;; eqv? tests if objects are operationally equivalent
;; equal? tests if objects have same structure and contents
;; Comparing strings for equality
(string=? "Hello" "Hello") ;; => #t
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; 6. Control Flow
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Conditionals
(if #t ;; test expression
"True" ;; then expression
"False") ;; else expression
;; => "True"
(if (> 3 2)
"yes"
"no") ;; => "yes"
;; In conditionals, all values that are not '#f' are treated as true.
;; 0, '(), #() "" , are all true values
(if 0
"0 is not false"
"0 is false") ;; => "0 is not false"
;; 'cond' chains a series of tests and returns as soon as it encounters a true condition
;; 'cond' can be used to simulate 'if/elseif/else' statements
(cond ((> 2 2) "not true so don't return this")
((< 2 5) "true, so return this")
(else "returning default")) ;; => "true, so return this"
;; A case expression is evaluated as follows:
;; The key is evaluated and compared with each datum in sense of 'eqv?',
;; The corresponding clause in the matching datum is evaluated and returned as result
(case (* 2 3) ;; the key is 6
((2 3 5 7) 'prime) ;; datum 1
((1 4 6 8) 'composite)) ;; datum 2; matched!
;; => composite
;; case with else clause
(case (car '(c d))
((a e i o u) 'vowel)
((w y) 'semivowel)
(else 'consonant)) ;; => consonant
;; Boolean expressions
;; 'and' returns the first expression that evaluates to #f
;; otherwise, it returns the result of the last expression
(and #t #f (= 2 2.0)) ;; => #f
(and (< 2 5) (> 2 0) "0 < 2 < 5") ;; => "0 < 2 < 5"
;; 'or' returns the first expression that evaluates to #t
;; otherwise the result of the last expression is returned
(or #f #t #f) ;; => #t
(or #f #f #f) ;; => #f
;; 'when' is like 'if' without the else expression
(when (positive? 5) "I'm positive") ;; => "I'm positive"
;; 'unless' is equivalent to (when (not <test>) <expr>)
(unless (null? '(1 2 3)) "not null") ;; => "not null"
;; Loops
;; loops can be created with the help of tail-recursions
(define (loop count)
(unless (= count 0)
(print "hello")
(loop (sub1 count))))
(loop 4) ;; => hello, hello ...
;; Or with a named let
(let loop ((i 0) (limit 5))
(when (< i limit)
(printf "i = ~a\n" i)
(loop (add1 i) limit))) ;; => i = 0, i = 1....
;; 'do' is another iteration construct
;; It initializes a set of variables and updates them in each iteration
;; A final expression is evaluated after the exit condition is met
(do ((x 0 (add1 x ))) ;; initialize x = 0 and add 1 in each iteration
((= x 10) (print "done")) ;; exit condition and final expression
(print x)) ;; command to execute in each step
;; => 0,1,2,3....9,done
;; Iteration over lists
(for-each (lambda (a) (print (* a a)))
'(3 5 7)) ;; => 9, 25, 49
;; 'map' is like for-each but returns a list
(map add1 '(11 22 33)) ;; => (12 23 34)
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; 7. Extensions
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; The CHICKEN core is very minimal, but additional features are provided by library extensions known as Eggs.
;; You can install Eggs with 'chicken-install <eggname>' command.
;; 'numbers' egg provides support for full numeric tower.
(require-extension numbers)
;; complex numbers
3+4i ;; => 3+2i
;; Supports fractions without falling back to inexact flonums
1/3 ;; => 1/3
;; provides support for large integers through bignums
(expt 9 20) ;; => 12157665459056928801
;; And other 'extended' functions
(log 10 (exp 1)) ;; => 2.30258509299405
(numerator 2/3) ;; => 2
;; 'utf8' provides unicode support
(require-extension utf8)
"\u03BBx:(\u03BC\u0251.\u0251\u2192\u0251).xx" ;; => "λx:(μɑ.ɑ→ɑ).xx"
;; 'posix' provides file I/O and lots of other services for unix-like operating systems
;; Some of the functions are not available in Windows system,
;; See http://wiki.call-cc.org/man/4/Unit%20posix for more details
;; Open a file to append, open "write only" and create file if it does not exist
(define outfn (file-open "chicken-hen.txt" (+ open/append open/wronly open/creat)))
;; write some text to the file
(file-write outfn "Did chicken came before hen?")
;; close the file
(file-close outfn)
;; Open the file "read only"
(define infn (file-open "chicken-hen.txt" open/rdonly))
;; read some text from the file
(file-read infn 30) ;; => ("Did chicken came before hen? ", 28)
(file-close infn)
;; CHICKEN also supports SRFI (Scheme Requests For Implementation) extensions
;; See 'http://srfi.schemers.org/srfi-implementers.html" to see srfi's supported by CHICKEN
(require-extension srfi-1) ;; list library
(filter odd? '(1 2 3 4 5 6 7)) ;; => (1 3 5 7)
(count even? '(1 2 3 4 5)) ;; => 2
(take '(12 24 36 48 60) 3) ;; => (12 24 36)
(drop '(12 24 36 48 60) 2) ;; => (36 48 60)
(circular-list 'z 'q) ;; => z q z q ...
(require-extension srfi-13) ;; string library
(string-reverse "pan") ;; => "nap"
(string-index "Turkey" #\k) ;; => 3
(string-every char-upper-case? "CHICKEN") ;; => #t
(string-join '("foo" "bar" "baz") ":") ;; => "foo:bar:baz"
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; 8. Macros
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; A 'for .. in ..' iteration like python, for lists
(define-syntax for
(syntax-rules (in)
((for elem in alist body ...)
(for-each (lambda (elem) body ...) alist))))
(for x in '(2 4 8 16)
(print x)) ;; => 2, 4, 8, 16
(for chr in (string->list "PENCHANT")
(print chr)) ;; => P, E, N, C, H, A, N, T
;; While loop
(define-syntax while
(syntax-rules ()
((while cond body ...)
(let loop ()
(when cond
body ...
(loop))))))
(let ((str "PENCHANT") (i 0))
(while (< i (string-length str)) ;; while (condition)
(print (string-ref str i)) ;; body
(set! i (add1 i))))
;; => P, E, N, C, H, A, N, T
;; Advanced Syntax-Rules Primer -> http://petrofsky.org/src/primer.txt
;; Macro system in chicken -> http://lists.gnu.org/archive/html/chicken-users/2008-04/msg00013.html
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; 9. Modules
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Also See http://wiki.call-cc.org/man/4/Modules
;; The 'test' module exports a value named 'hello' and a macro named 'greet'
(module test (hello greet)
(import scheme)
(define-syntax greet
(syntax-rules ()
((_ whom)
(begin
(display "Hello, ")
(display whom)
(display " !\n") ) ) ) )
(define (hello)
(greet "world") ) )
;; we can define our modules in a separate file (say test.scm) and load them to the interpreter with
;; (load "test.scm")
;; import the module
(import test)
(hello) ;; => Hello, world !
(greet "schemers") ;; => Hello, schemers !
;; We can compile the module files in to shared libraries by using following command,
;; csc -s test.scm
;; (load "test.so")
;; Functors
;; Functors are high level modules that can be parameterized by other modules
;; Following functor requires another module named 'M' that provides a function called 'multiply'
;; The functor itself exports a generic function 'square'
(functor (squaring-functor (M (multiply))) (square)
(import scheme M)
(define (square x) (multiply x x)))
;; Module 'nums' can be passed as a parameter to 'squaring-functor'
(module nums (multiply)
(import scheme) ;; predefined modules
(define (multiply x y) (* x y)))
;; the final module can be imported and used in our program
(module number-squarer = (squaring-functor nums))
(import number-squarer)
(square 3) ;; => 9
;; We can instantiate the functor for other inputs
;; Here's another example module that can be passed to squaring-functor
(module stars (multiply)
(import chicken scheme) ;; chicken module for the 'use' keyword
(use srfi-1) ;; we can use external libraries in our module
(define (multiply x y)
(list-tabulate x (lambda _ (list-tabulate y (lambda _ '*))))))
(module star-squarer = (squaring-functor stars))
(import star-squarer)
(square 3) ;; => ((* * *)(* * *)(* * *))
```
## Further Reading
* [CHICKEN User's Manual](http://wiki.call-cc.org/man/4/The%20User%27s%20Manual).
* [R5RS standards](http://www.schemers.org/Documents/Standards/R5RS)
## Extra Info
* [For programmers of other languages](http://wiki.call-cc.org/chicken-for-programmers-of-other-languages)
* [Compare CHICKEN syntax with other languages](http://plr.sourceforge.net/cgi-bin/plr/launch.py)
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