by Marc Feeley
This copy of the SRFI 39 specification document is distributed as part of the Racket package srfi-doc.
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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.
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.
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.
(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"
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 (C) Marc Feeley 2002. All Rights Reserved.
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