Calico Scheme

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Revision as of 20:32, 21 July 2013 by Doug Blank (Talk | contribs) (choose/fail)

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Here we will provide documentation for using Calico Scheme.

Calico Scheme is a new implementation of a Scheme-based language for Calico. It implements many core Scheme functions, but also adds some functionality to bring it into line with the other modern languages like Python and Ruby.

Example Code

  • Examples - these come with Calico in Calico/examples/scheme/

Scheme Extensions

  1. Exceptions - implements try/catch/finally
  2. Modules and namespaces - implements namespaces for imports and foreign objects
  3. DLLs - import libraries written for the CLR
  4. Interop - ways for Scheme to interop with other Calico languages

Exceptions

(raise <exp>)
(try <exp>...)
(try <exp>... (catch <sym> <exp>...))
(try <exp>... (finally <exp>...))
(try <exp>... (catch <sym> <exp>...) (finally <exp>...))

There are four main forms of try:

  1. try alone: has no effect---doesn't catch exceptions
  2. try with a catch-clause: if exception in try-body, catch-clause will catch and provide return value, otherwise body of try provides. If exception in catch-clause will simple raise it.
  3. try with a finally-clause: finally-clause will run if exception in try-body or not. If there is an exception in the finally, it will raise it.
  4. try with a catch-clause and finally-clause: if an exception in try-body, the catch-clause will run, followed by the finally-clause. If an exception is raised in the catch-clause too, then finally-clause will run, and then raise the catch-exception; if there is an exception raised in the

finally-clause, then it will raise.

The <sym> of the catch-clause will be bound to the expression raised.

Examples

==> 

> (try x)
(uncaught exception: "unbound variable x")
==> "(try x (catch e e))"
"unbound variable x"
==> "(try x (catch e (raise e)))"
(uncaught exception: "unbound variable x")
==> "(try x (finally 'hi))"
hi 
(uncaught exception: "unbound variable x")
==> "(try x (catch e 1 2 3 4 (finally 'hi))"
hi 
4

Modules

Modules provide an easy method for structuring hierarchies of libraries and code.

Import

(import <string-exp>)
(import <string-exp> '<symbol-exp>)

Examples

==> (import "my-file.ss")

my-file.ss is a Scheme program file, which itself could have imports.

==> (import "my-file.ss" 'my-stuff)

Loads the file, and puts it in the namespace "my-stuff" accessible through the lookup interface below.

import uses the local environment, while load uses the toplevel-environment.

Lookup

module.name
module.module.name
==> (import "my-file.ss" 'my-stuff)
==> my-stuff.x
5

Directory Listing

==> (dir)
(- % * / + < <= = =? > >= abort abs and append apply assq assv atom? boolean? caaaar caaadr caaar caadar 
caaddr caadr caar cadaar cadadr cadar caddar cadddr caddr cadr call/cc call-with-current-continuation 
car case cases cd cdaaar cdaadr cdaar cdadar cdaddr cdadr cdar cddaar cddadr cddar cdddar cddddr cdddr 
cddr cdr char? char=? char-alphabetic? char-numeric? char-whitespace? cond cons current-directory 
current-environment current-time cut debug define-datatype dir display eq? equal? eqv? error eval 
eval-ast even? exit float for-each format get get-member globals import int iter? length let let* letrec 
list list? list->string list->vector list-head list-ref list-tail load make-set make-vector map member 
memq memv newline not null? number? number->string odd? or pair? parse parse-string print printf 
procedure? property quotient range rational read-string record-case remainder require reset-toplevel-env 
reverse safe-print set-car! set-cdr! sort sqrt string string? string<? string=? string->list string->number 
string->symbol string-append string-length string-ref string-split substring symbol symbol? symbol->string 
typeof unparse unparse-procedure use-lexical-address use-tracing using vector vector? vector->list 
vector-ref vector-set! void zero?)
==> (dir my-stuff)
(x y z)

You can also use dir on an object:

(using "Myro")
(dir Myro)

define-syntax

define-syntax is used to change the semantics of Scheme in a manner not possible with regular functions. For example, imagine that you wanted to time a particular function call. To time a function, you can do:

(let ((start (current-time)))
  (fact 5)
  (- (current-time) start))

If you tried to define a function time such that you could call it like:

(time (fact 5))

then, unfortunately, you would evaluate (fact 5) before you could do anything in the function time. You could call it like:

(time fact 5)

but that looks a bit strange. Perhaps a more natural way would be to just change the semantics of Scheme to allow (time (fact 5)). Scheme makes that easy with define-sytnax:

(define-syntax time 
  [(time ?exp) (let ((start (current-time)))
		 ?exp
		 (- (current-time) start))])

Now, you can call it like:

(time (fact 5))

and you get the correct answer.

define-syntax takes a list of two items: a template, and a response. If the template matches, then you evaluate the response. In this example, (time ?exp) matches, so the system will record the start time, evaluate the ?exp, and then return the time minus the start time.

Calico Scheme uses this simple, but powerful pattern matcher to implement define-case. Here is a more complex example: for.

(define-syntax for
  [(for ?exp times do . ?bodies)
   (for-repeat ?exp (lambda () . ?bodies))]
  [(for ?var in ?exp do . ?bodies)
   (for-iterate1 ?exp (lambda (?var) . ?bodies))]
  [(for ?var at (?i) in ?exp do . ?bodies)
   (for-iterate2 0 ?exp (lambda (?var ?i) . ?bodies))]
  [(for ?var at (?i ?j . ?rest) in ?exp do . ?bodies)
   (for ?var at (?i) in ?exp do
     (for ?var at (?j . ?rest) in ?var do . ?bodies))])

In this example, define-syntax creates a for function with 4 forms:

(for 4 times do (function ...))

(for x in '(1 2 3) do (function ...))

(for x at (0) in '(1 2 3) do (function ...))

(for x at (0 1 2) in (range 10) do (function ...))

Calico Libraries

You can use any of the Calico libraries:

scheme> (using "DLLName")
scheme> (DLLName.Class arg1 arg2)

For example, you can use Scheme to do art or control robots:

(using "Myro")
(Myro.init "sim")
(Myro.joystick)

Interop

There is a special object in the environment, calico. It has access to a variety of Calico functions. See Calico: calico object for more details.

Use the define! to put a variable in the global Calico namespace.

scheme> (define! x 8)
python> x
8

Wrap a function for use by other Calico languages:

scheme> (func (lambda (a b) (+ a b)))

Combine for cross-language interoperation:

 scheme> (define fact (lambda (n) (if (= n 1) 1 (* n (fact (- n 1))))))
 scheme> (define! factorial (func fact))
 python> factorial(5)
 120

Be careful not to wrap the func around the part that is called recursively, or you will destroy the tail-call optimization.

Iterators

Strings, vectors, and lists all work with map and for-each.

==> (map display "123")
123(void void void)
==> (for-each (lambda (v) (printf "~a\n" v)) (range 3))
1
2
3

choose/fail

Calico Scheme also contains a non-deterministic search with back-tracking. To use this, you select choice-points using the keyword "choose" with arguments, set requirements using the keyword "require", and fail using "(choose)". For example, to automatically find two numbers that sum to seven:

(define sum-to-seven
  (lambda ()
    (let ((num1 (choose 0 1 2 3 4 5 6 7 8 9))
          (num2 (choose 0 1 2 3 4 5 6 7 8 9)))
      (require (= (+ num1 num2) 7))
      (printf "The numbers are ~s ~s\n" num1 num2))))

Then, you can let Scheme do the searching for you:

scheme>>> (sum-to-seven)
The numbers are 0 7
Done

If you don't like that result, you can force Scheme back to any choice-points to make a different choice:

scheme>>> (choose)
The numbers are 1 6
Done

This can continue until there are no more choices left.

See menu -> File -> Examples -> Scheme -> choose-examples.ss for more examples.

scheme>>> (choose)
The numbers are 7 0
Done

cut

The (cut) operation will succeed, but cannot be back-tracked afterwards.

Help/Doc System

When you define a variable, you can optionally add a doc-string:

==> (define x "This variable holds the sum" 0)
==> (define y 0)
==> (define function "This computes the polynomial..." (lambda (x) ...))

You can lookup the doc-string with:

==> (help 'x)
This variable holds the sum

Misc

typeof will give you the .NET type of a value:

==> (typeof 1)
System.Int32
==> (typeof 238762372632732736)
Microsoft.Scripting.Math.BigInteger
==> (typeof 1/5)                
Rational
==> (cd "/path/to/folder")
==> (cd)
"/path/to/folder"
==> (import "file.ss" 'F)
==> (set! F.x 45)
==> (current-environment)
#<environment>
==> (remainder 4 5)
==> (quotient 6 4)
==> (dir (current-environment))
==> (make-set '(1 1 2 3 4 5))
==> (using "Myro")
==> (dir Myro.robot)

Language Interop

Converting from Scheme to Python:

scheme> (define pylist (calico.Evaluate "lambda *args: list(args)" "python"))
Ok
scheme> (define pytuple (calico.Evaluate "lambda *args: args" "python"))
Ok
scheme> (pylist 1 2 3)
[1, 2, 3]
scheme> (pytuple 1 2 3)
(1, 2, 3)

Converting from Python to Scheme

scheme> (define pylist2list
           (lambda (args)
              (map (lambda (i) i) args)))
Ok
scheme> (pylist2list (pylist 1 2 3))
(1 2 3)
scheme> (pylist2list (pytuple 1 2 3))
(1 2 3)

References

Calico

  1. CalicoDevelopment - plans and details for Calico development
  2. http://www.scheme.com/tspl4/ - The Scheme Programming Language, 4th edition

For Developers