Contract Parameters

This library is experimental; compatibility may not be maintained.

This package implements contract parameters that allow for dynamically bound contracts. Contract parameters can be used to implement checks that are similar to computational contracts [Scholliers et al. 2013].

1 Example: Prohibit Contract

Suppose you want a contract that prevents calling a certain family of functions—perhaps forbidding a certain function from printing to the screen. Enforcing such a restriction requires cooperation between the function to be prohibited and its context. Contract parameters allow this cooperation by allowing the context to dynamically bind a contract.

First, create a contract parameter called prohibitable/c. What it checks depends on the context. By default, it checks any/c.

(define prohibitable/c (make-contract-parameter any/c))

Next, define a prohibit/c contract that will instantiate the prohibitable/c parameter with none/c. Thus, if you attach prohibit/c to a function, then during that function’s execution prohibitable/c will be bound to none/c.

(define prohibit/c (parameterize/c [prohibitable/c none/c]))

Using prohibitable/c in a function contract will allow you to prohibit it. In this case, we can check that will-prevent will never be called.

(define/contract (will-prevent x)

(-> (and/c integer? prohibitable/c) any)

(add1 x))

(define/contract (good-prohibit)

prohibit/c

43)

(define/contract (bad-prohibit)

prohibit/c

(will-prevent 42))

Here’s what it looks like if you try to call good-prohibit and bad-prohibit:

> (good-prohibit)

43

> (bad-prohibit)

will-prevent: contract violation;

none/c allows no values

given: 42

in: the 2nd conjunct of

the 1st argument of

(-> (and/c integer? ???) any)

contract from: (function will-prevent)

blaming: top-level

(assuming the contract is correct)

at: eval:3:0

2 Reference

Bibliography