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Article contents:== Outdated egg! This is an egg for CHICKEN 3, the unsupported old release. You're almost certainly looking for [[/eggref/4/contracts|the CHICKEN 4 version of this egg]], if it exists. If it does not exist, there may be equivalent functionality provided by another egg; have a look at the [[https://wiki.call-cc.org/chicken-projects/egg-index-4.html|egg index]]. Otherwise, please consider porting this egg to the current version of CHICKEN. [[tags: egg]] == contracts This extension provides basic support for contract-oriented programming. Definitions can be grouped into a ''contract'' which specifies what preconditions should be met when an exported procedure is called from outside the group, as well as what post-conditions and invariants have to be fulfilled when a call to an exported procedure returns. === contracted [syntax] (contracted CLAUSE ...) Defines a group of definitions that share a contract. Each {{CLAUSE}} should be one of the following: === define/contract [syntax] (define/contract (NAME ARGUMENT ...) [-> RESULT] BODY ...) Defines a single function with a contract. {{ARGUMENT}} may be eithe a symbol (equivalent to {{(any? SYMBOL)}}) or a list of the form {{(PROCEDURE SYMBOL)}} which specifies the variable name and precondition predicate. The optional {{RESULT}} designates a predicate on the result as a post-condition. ==== export [contract clause] (export ((NAME ARGUMENT ...) [-> RESULT ...]) ...) Exports the procedure {{NAME}} from the contract group. {{ARGUMENT}} should evaluate to a procedure that is called on the argument at the given position and which must succeed (i.e. return a value distinct from {{#f}}) for the precondition to hold. If the argument predicate returns {{#f}} an exception is signalled by invoking {{contract-violation}} (see below). {{RESULT}} should evaluate to a procedure that must return true for the result at the designated position when the invocation of {{NAME}} returns. If no {{-> RESULT ...}} is given, the procedure is expected to return a single unspecified result. ==== export* [contract clause] (export* ((NAME ARGUMENT ...) [-> RESULT ...]) ...) Identical to {{export}} but also exports the procedure {{NAME}} when used during compilation by generating a {{(declare (export NAME))}} form in the expansion of the contract body. ==== invariant [contract clause] (invariant EXPRESSION) Defines an invariant: {{EXPRESSION}} must return true after any call to an exported procedure or a contract violation is signalled. ==== definition [contract clause] (define ...) Defines an exported or non-exported entity. Note that all definitions have non-toplevel semantics. If no definition of an exported procedure is found a warning will be signalled (since macros may expand into definitions, the resulting code may still be valid). ==== form [contract clause] EXPRESSION An arbitrary expression in the contract body. Depending on the macro-expander used this may (default low-level macros) or may not ([[http://www.call-with-current-continuation.org/eggs/syntax-case.html|syntax-case]], [[syntactic-closures]]) be followed by more definitions. === contract-violation [procedure] (contract-violation CONTRACT LOCATION MESSAGE FORM [ARGUMENT ...]) Signals a contract violation condition of the kinds {{(exn contract-violation)}}. The {{contract-violation}} condition has the property {{NAME}} which contains the {{CONTRACT}} argument as value. {{CONTRACT}} should be a string or symbol that names the contract, {{LOCATION}} a symbol designating the procedure where the contract violation occurred (or {{#f}}). {{MESSAGE}} should be a string and {{FORM}} the expression that produced the contract violation (or some hint at the failed expression). Any extra {{ARGUMENT}}s are stored as the {{arguments}} condition property in the condition. === Notes Argument lists in {{export}} clauses may contain DSSSL lambda-list markers ({{#!optional}}, {{#!rest}} and {{#!key}}) - in fact rest-argument predicates can only be specified using {{#!rest}} since dotted list syntax would be ambiguous. Keyword arguments should be specified as {{(SYMBOL PREDICATE)}}. Calls to exported definitions go through an intermediate wrapper procedure and are never in tail-position. By defining the feature {{disable-contracts}} (in the interpreter either by evaluating {{(register-feature! 'disable-contracts)}} or via the {{-D}} {{csi}} option, or in the compiler by using the {{-D}} or {{-feature}} options), pre- and postconditions and invariants are not checked. === Example <enscript highlight="scheme"> ;;;; stack.scm (use srfi-1 contracts) (contracted (export ((stack-push #!rest (list-of number?))) ((stack-pop) -> number?) ((stack-size) -> integer?) ((stack-empty) -> boolean?) ) (define *the-stack* '()) (define check-stack (let ((numlist? (list-of number?))) (lambda () (numlist? *the-stack*)) ) ) (invariant (check-stack)) (define (stack-push . ns) (set! *the-stack* (append-reverse ns *the-stack*)) ) (define (stack-pop) (assert (not (null? *the-stack*))) (let-values (((x more) (car+cdr *the-stack*))) (set! *the-stack* more) x) ) (define (stack-size) (length *the-stack*)) (define (stack-empty) (null? *the-stack*)) ) </enscript> === Author [[/users/felix winkelmann|felix winkelmann]] === License BSD === History ; 0.2 : Added {{define/contract}} ; 0.1 : Initial version Description of your changes:

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