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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.
+
+;; 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
+(import 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/5/Module%20(chicken%20file%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
+(import 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 ...
+
+(import 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/5/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](https://wiki.call-cc.org/manual).
+* [R5RS standards](http://www.schemers.org/Documents/Standards/R5RS)
+
+
+## Extra Info
+
+* [For programmers of other languages](https://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)