4.13 Dynamic Binding: parameterize

Parameters in Reference: Racket also documents parameterize.

The parameterize form associates a new value with a parameter during the evaluation of body expressions:

(parameterize ([parameter-expr value-expr] ...)   body ...+)

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:

> (parameterize ([error-print-width 5])       (car (expt 10 1024)))

car: expects argument of type <pair>; given 10...

> (parameterize ([error-print-width 10])       (car (expt 10 1024)))

car: expects argument of type <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:

> (define location (make-parameter "here"))

> (location)

"here"

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:

> (parameterize ([location "there"])       (location))

"there"

> (location)

"here"

> (parameterize ([location "in a house"])       (list (location)             (parameterize ([location "with a mouse"])               (location))             (location)))

'("in a house" "with a mouse" "in a house")

> (parameterize ([location "in a box"])       (car (location)))

car: expects argument of type <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:

> (define (would-you-could-you?)       (and (not (equal? (location) "here"))            (not (equal? (location) "there"))))

> (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:

> (let ([get (parameterize ([location "with a fox"])                  (lambda () (location)))])       (get))

"here"

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"

> (parameterize ([location "on a train"])       (list (location)             (begin (try-again! "in a boat")                    (location))))

'("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")

> (define (would-ya-could-ya?)       (and (not (equal? lokation "here"))            (not (equal? lokation "there"))))

> (set! location "on a bus")

> (would-ya-could-ya?)

#f

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.