Parameters in The Racket Reference also documents parameterize.
The parameterize form associates a new value with a parameter during the evaluation of body expressions:
The term “parameter” is sometimes used to refer to the arguments of a function, but “parameter” in Racket has the more specific meaning described here.
For example, the error-print-width parameter controls how many characters of a value are printed in an error message:
car: contract violation
expected: pair?
given: 10...
car: contract violation
expected: pair?
given: 1000000...
More generally, parameters implement a kind of dynamic binding. The make-parameter function takes any value and returns a new parameter that is initialized to the given value. Applying the parameter as a function returns its current value:
In a parameterize form, each parameter-expr must produce a parameter. During the evaluation of the bodys, each specified parameter is given the result of the corresponding value-expr. When control leaves the parameterize form—either through a normal return, an exception, or some other escape—the parameter reverts to its earlier value:
"there"
> (location)"here"
'("in a house" "with a mouse" "in a house")
car: contract violation
expected: pair?
given: "in a box"
> (location)"here"
The parameterize form is not a binding form like let; each use of location above refers directly to the original definition. A parameterize form adjusts the value of a parameter during the whole time that the parameterize body is evaluated, even for uses of the parameter that are textually outside of the parameterize body:
> (would-you-could-you?)#f
> (parameterize ([location "on a bus"]) (would-you-could-you?))#t
If a use of a parameter is textually inside the body of a parameterize but not evaluated before the parameterize form produces a value, then the use does not see the value installed by the parameterize form:
The current binding of a parameter can be adjusted imperatively by calling the parameter as a function with a value. If a parameterize has adjusted the value of the parameter, then directly applying the parameter procedure affects only the value associated with the active parameterize:
> (define (try-again! where) (location where)) > (location)"here"
'("on a train" "in a boat")
> (location)"here"
Using parameterize is generally preferable to updating a parameter value imperatively—for much the same reasons that binding a fresh variable with let is preferable to using set! (see Assignment: set!).
It may seem that variables and set! can solve many of the same problems that parameters solve. For example, lokation could be defined as a string, and set! could be used to adjust its value:
> (define lokation "here") > (set! lokation "on a bus") > (would-ya-could-ya?)#t
Parameters, however, offer several crucial advantages over set!:
The parameterize form helps automatically reset the value of a parameter when control escapes due to an exception. Adding exception handlers and other forms to rewind a set! is relatively tedious.
Parameters work nicely with tail calls (see Tail Recursion). The last body in a parameterize form is in tail position with respect to the parameterize form.
Parameters work properly with threads (see Threads). The parameterize form adjusts the value of a parameter only for evaluation in the current thread, which avoids race conditions with other threads.
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