Module /fusion/parameter

Dynamically-scoped values.

A parameter is a single-value procedure whose result is dynamically scoped. A parameter's value is set via parameterize and the parameter retains the value for the dynamic extent of the parameterized body, and then reverts to its prior value.

Parameters are created via make_parameter, which also gives it an initial default value. The parameter's value is retrieved by applying it (remember, parameters are procedures):

(define p (make_parameter 19))

p    =>  {{{procedure 'p'}}}
(p)  =>  19

To change the parameter's value for the extent of some evaluation, the parameterize syntax form takes pairs of parameters and values (similar to let) and body. The parameters will have the given values while the body is evaluated, and will revert to their earlier values when the body exits:

(define (use_p)
  (+ (p)
     (parameterize [(p 7)]
       (p))))               // 7

(use_p)                     => 26

(parameterize [(p 13)]
  (use_p))                  => 20

(use_p)                     => 26
(p)                         => 19

It's important to understand that the parameter itself is statically bound and lexically scoped just like any other binding, while the parameter's value has dynamic extent.

Another way to look at it: parameterize associates the parameter and the value on the call stack, and the association is visible until either the stack frame is abandoned (by exiting the parameterize body) or a dynamically-nested parameterize "shadows" it with another value. Java developers will recognize that this is very similar to how a Java catch clause sets an exception handler with dynamic extent.