39: Parameter objects

by Marc Feeley

Status

This SRFI is currently in final status. Here is an explanation of each status that a SRFI can hold. To provide input on this SRFI, please send email to srfi-39@nospamsrfi.schemers.org. To subscribe to the list, follow these instructions. You can access previous messages via the mailing list archive.

Abstract

This SRFI defines parameter objects, the procedure make-parameter to create parameter objects and the parameterize special form to dynamically bind parameter objects. In the dynamic environment, each parameter object is bound to a cell containing the value of the parameter. When a procedure is called the called procedure inherits the dynamic environment from the caller. The parameterize special form allows the binding of a parameter object to be changed for the dynamic extent of its body.

Rationale

The dynamic environment is the structure which allows the system to find the value returned by the R5RS procedures current-input-port and current-output-port. The R5RS procedures with-input-from-file and with-output-to-file extend the dynamic environment to produce a new dynamic environment which is in effect for the dynamic extent of the call to the thunk passed as their last argument. These procedures are essentially special purpose dynamic binding operations on hidden dynamic variables (one for current-input-port and one for current-output-port). The purpose of this SRFI is to generalize this dynamic binding mechanism (which exists in all R5RS compliant systems) to allow the user to introduce new dynamic variables and dynamically bind them.

General dynamic binding mechanisms exist in several implementations of Scheme under various names, including "fluid" variables and parameter objects. The parameter objects specified in this SRFI are compatible with the semantics of all implementations of Scheme we know which currently support parameter objects (in the sense that it is possible to implement this SRFI so that old code works the same as before). We believe Chez-Scheme was the first implementation of Scheme to have used parameter objects.

In the presence of threads, the dynamic binding mechanism does not behave the same way in all implementations of Scheme supporting dynamic binding. The issue is the relationship between the dynamic environments of the parent and child threads when a thread is created. In Scheme 48 the child gets a fresh dynamic environment where (typically but not necessarily) all the bindings are to their initial value. In MzScheme and Gambit-C the child is given a dynamic environment inherited from the parent. In this inherited dynamic environment the dynamic variables have the same values as the parent's dynamic environment. However, in MzScheme the cells bound to the dynamic variables in the child are distinct from those of the parent (i.e. an assignment of a value to a dynamic variable is not visible in the other thread). In Gambit-C the child and parent dynamic environment share the same cells (i.e. an assignment of a value to a dynamic variable is visible in the other thread). Note that in the absence of assignment to dynamic variables the MzScheme and Gambit-C approaches are equivalent.

Given that there are semantic differences in the presence of threads and that there are valid reasons for choosing each semantics, this SRFI does not specify the semantics of parameter objects in the presence of threads. It is left to the implementation and other SRFIs which extend this SRFI to specify the interaction between parameter objects and threads.

Specification

The dynamic environment is composed of two parts: the local dynamic environment and the global dynamic environment. The global dynamic environment is used to lookup parameter objects that can't be found in the local dynamic environment. When parameter objects are created, their initial binding is put in the global dynamic environment (by mutation). The local dynamic environment is only extended by the parameterize form.

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. The global dynamic environment is updated to associate the parameter object to a new cell. The initial content of the cell is the result of applying the conversion procedure to the first argument of make-parameter.

A parameter object is a procedure which accepts zero or one argument. The cell bound to a particular parameter object in the dynamic environment is accessed by calling the parameter object. When no argument is passed, the content of the cell is returned. When one argument is passed the content of the cell is updated with the result of applying the parameter object's conversion procedure to the argument.

The parameterize special form, when given a parameter object and a value, binds for the dynamic extent of its body the parameter object to a new cell. The initial content of the cell 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).

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.

Because it is possible to implement the R5RS procedures current-input-port and current-output-port as parameter objects and this offers added functionnality, it is required by this SRFI that they be implemented as parameter objects created with make-parameter.

Procedures and syntax

(make-parameter init [converter])                     ;procedure

Returns a new parameter object which is bound in the global dynamic environment to a cell 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 cell bound to this parameter object in the current dynamic environment is returned. When it is called with one argument, the content of the cell bound to this parameter object in the current dynamic environment 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)       ==>  ">"

(parameterize ((expr1 expr2) ...) <body>)             ;syntax

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 cell 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"

Implementation

The following implementation uses association lists to represent local dynamic environments. The global dynamic environment binding is stored in the parameter object itself. Since we are assuming that there is a single thread, the current local dynamic environment can be bound to a global variable, dynamic-env-local. Mutations of this variable are wrapped in a dynamic-wind so that the local dynamic environment returns to its previous value when control exits the body of the parameterize. 

    (define make-parameter
      (lambda (init . conv)
        (let ((converter
               (if (null? conv) (lambda (x) x) (car conv))))
          (let ((global-cell
                 (cons #f (converter init))))
            (letrec ((parameter
                      (lambda new-val
                        (let ((cell (dynamic-lookup parameter global-cell)))
                          (cond ((null? new-val)
                                 (cdr cell))
                                ((null? (cdr new-val))
                                 (set-cdr! cell (converter (car new-val))))
                                (else ; this case is needed for parameterize
                                 (converter (car new-val))))))))
              (set-car! global-cell parameter)
              parameter)))))

    (define-syntax parameterize
      (syntax-rules ()
        ((parameterize ((expr1 expr2) ...) body ...)
         (dynamic-bind (list expr1 ...)
                       (list expr2 ...)
                       (lambda () body ...)))))

    (define dynamic-bind
      (lambda (parameters values body)
        (let* ((old-local
                (dynamic-env-local-get))
               (new-cells
                (map (lambda (parameter value)
                       (cons parameter (parameter value #f)))
                     parameters
                     values))
               (new-local
                (append new-cells old-local)))
          (dynamic-wind
            (lambda () (dynamic-env-local-set! new-local))
            body
            (lambda () (dynamic-env-local-set! old-local))))))

    (define dynamic-lookup
      (lambda (parameter global-cell)
        (or (assq parameter (dynamic-env-local-get))
            global-cell)))

    (define dynamic-env-local '())

    (define dynamic-env-local-get
      (lambda () dynamic-env-local))

    (define dynamic-env-local-set!
      (lambda (new-env) (set! dynamic-env-local new-env)))

Copyright

Copyright (C) Marc Feeley 2002. All Rights Reserved.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Editor: Mike Sperber