18 Parameter utilities

(require (planet untyped/unlib/parameter))

Convenience forms for working with parameters.

(make-guard pred type-message) → (any -> any)

pred : (any -> boolean?)

type-message : string?

Creates a procedure that may be used as a parameter’s guard procedure. The guard only allows values for which pred returns #t. If an invalid value is supplied, the guard raises exn:fail:contract with an error message based on the supplied type-message.

make-guard has been superseded by the parameter/c contract in PLT 4.

Examples:

> (define param       (make-parameter #f (make-guard integer+false? "(U integer #f)")))

> (param 1)

> (param #f)

> (param #t)

Expected (U integer #f), received #t

(define-parameter id initial-value guard-proc with-form-id)

Convenience form that expands into two definitions:

• id is bound to a parameter with the supplied initial-value and guard-proc;

• with-form-id is bound to a syntax of the form:

  (with-form-id expr

  body ...)

  that expands to:

    (parameterize ([id expr]) body ...)

Examples:

> (define-parameter foo       #f       (make-guard integer+false? "(U integer #f)")       with-foo)

> (with-foo 10 (foo))

10

> (with-foo "bar" 10)

Expected (U integer #f), received "bar"