Parameter objects

A parameter object is a procedure that is bound to a location, and may optionally have a conversion procedure. The procedure accepts zero or one argument. When the procedure is called with zero arguments, the content of the location is returned. On a call with one argument the content of the location is updated with the result of applying the parameter object’s conversion procedure to the argument.

Parameter objects are created with the make-parameter procedure which takes one or two arguments. The second argument is a one argument conversion procedure. If only one argument is passed to make-parameter the identity function is used as a conversion procedure. A new location is created and asociated with the parameter object. The initial content of the location is the result of applying the conversion procedure to the first argument of make-parameter.

Note that the conversion procedure can be used for guaranteeing the type of the parameter object’s binding and/or to perform some conversion of the value.

The parameterize special form, when given a parameter object and a value, binds the parameter object to a new location for the dynamic extent of its body. The initial content of the location is the result of applying the parameter object’s conversion procedure to the value. The parameterize special form behaves analogously to let when binding more than one parameter object (that is the order of evaluation is unspecified and the new bindings are only visible in the body of the parameterize special form).

When a new thread is created using future or runnable then the child thread shares locations with its parents. This sharing is broken by a parameterize that changes the location, and restored when the parameterize exits.

Note that parameterize and fluid-let have similar binding and sharing behavior. The difference is that fluid-let modifies locations accessed by name, while make-parameter and parameterize create anonymous locations accessed by calling a parameter procedure.

The R5RS procedures current-input-port and current-output-port are parameter objects.

Procedure: make-parameter init [converter]

Returns a new parameter object which is bound in the global dynamic environment to a location containing the value returned by the call (converter init). If the conversion procedure converter is not specified the identity function is used instead.

The parameter object is a procedure which accepts zero or one argument. When it is called with no argument, the content of the location bound to this parameter object in the current dynamic environment is returned. When it is called with one argument, the content of the location is set to the result of the call (converter arg), where arg is the argument passed to the parameter object, and an unspecified value is returned.

(define radix
  (make-parameter 10))

(define write-shared
  (make-parameter
    #f
    (lambda (x)
      (if (boolean? x)
          x
          (error "only booleans are accepted by write-shared")))))

(radix)           ⇒  10
(radix 2)
(radix)           ⇒  2
(write-shared 0)  gives an error

(define prompt
  (make-parameter
    123
    (lambda (x)
      (if (string? x)
          x
          (with-output-to-string (lambda () (write x)))))))

(prompt)       ⇒  "123"
(prompt ">")
(prompt)       ⇒  ">"

Syntax: parameterize ((expr1 expr2) ...) body

The expressions expr1 and expr2 are evaluated in an unspecified order. The value of the expr1 expressions must be parameter objects. For each expr1 expression and in an unspecified order, the local dynamic environment is extended with a binding of the parameter object expr1 to a new location whose content is the result of the call (converter val), where val is the value of expr2 and converter is the conversion procedure of the parameter object. The resulting dynamic environment is then used for the evaluation of body (which refers to the R5RS grammar nonterminal of that name). The result(s) of the parameterize form are the result(s) of the body.

(radix)                                              ⇒  2
(parameterize ((radix 16)) (radix))                  ⇒  16
(radix)                                              ⇒  2

(define (f n) (number->string n (radix)))

(f 10)                                               ⇒  "1010"
(parameterize ((radix 8)) (f 10))                    ⇒  "12"
(parameterize ((radix 8) (prompt (f 10))) (prompt))  ⇒  "1010"