by Richard Kelsey
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This SRFI describes a configuration language to be used for specifying the set of Scheme features or extensions required to run a program. In addition to a list of required features, a program may also contain Scheme code to be used only when a particular feature or combination of features is available.
The configuration language is entirely distinct from Scheme; it is neither embedded in Scheme nor includes Scheme as a subset.
The use of a separate configuration language makes it easy for both human and machine readers to determine the requirements of a program. It also avoids ambiguities arising from having to expand macros to determine which features, and thus which macros, are used in the program.
See SRFI 0 for a rationale for the need for some kind of configuration control.
<program> --> (program <program clause>+) <program clause> --> (requires <feature identifier>+) | (files <filename>*) | (code <Scheme expression, definition, or syntax definition>*) | (feature-cond <feature-cond clause>+) | (feature-cond <feature-cond clause>* (else <program clause>+)) <feature-cond clause> --> (<feature requirement> <program clause>+) <feature requirement> --> <feature identifier> | (and <feature requirement>*) | (or <feature requirement>*) | (not <feature requirement>) <feature identifier> --> a symbol which is the name of a SRFI
The configuration language is distinct from Scheme. Given a set of
available features a <program> can be converted into a
sequence of Scheme commands, definitions, and syntax definitions.
This conversion does not require expanding macros or doing any other
processing of Scheme code.
An implementation of the configuration language will need to provide
some method for executing a program. For example, it might have a
(LOAD-PROGRAM <filename>) function or a compiler that
compiles a program into an executable file.
The meanings of the different <program> clauses are
given below. The ordering of the clauses in a <program>
determines the order of the forms in the resultant Scheme program.
In processing the REQUIRES and
FEATURE-COND clauses in a <program>,
an implementation should be consistent with some fixed set of present
and absent features. An implementation may analyze a <program>
before choosing a set of features to use in processing it, but it
should not use different sets of features for different clauses in the
same <program>.
(requires <feature-identifier>+)(files <filename>*)(code <body>)(feature-cond <feature-cond clause>+)
The meaning of a FEATURE-COND clause is that of the
<program-clause>s in the first <feature-cond clause> whose
<implementation-requirement> is satisfied by the implementation.
If an ELSE clause is present it is used if and only if no preceding
clause is satisfied; a FEATURE-COND with an
ELSE clause is always satisfied.
If no clause can be satisfied the <program> cannot be evaluated in
the implementation.
The meaning of the <implementation requirement>s is as follows:
<feature identifier> | satisfied if the feature is present |
(and) | always satisfied |
(and x ...) | satisfied if every
X is satisfied |
(or) | never satisfied |
(or x ...) | satisfied if any
X is satisfied |
(not x) | satisfied if X is not satisfied |
Two implementations are provided here. The first is a
PROCESS-PROGRAM function that converts a
<program>, represented as an S-expression, and a
list of feature identifiers and returns the list expressions,
definitions, and syntax definitions that are the source for the
<program> in the presence of those features. The
function returns #F if the program cannot be run using
the features provided.
This is not a complete implementation of the configuration language; it needs
an (implementation-dependent) method for evaluating the forms returned by
PROCESS-PROGRAM.
(define (process-program program features)
(call-with-current-continuation
(lambda (exit) ; We exit early when an unsatisfiable clause is found.
; Process each clause in turn
(define (process-clauses clauses)
(if (null? clauses)
'()
(append (process-clause (car clauses))
(process-clauses (cdr clauses)))))
; Dispatch on the type of the clause.
(define (process-clause clause)
(case (car clause)
((requires)
(if (all-satisfied? (cdr clause))
'()
(exit #f)))
((code)
(cdr clause))
((files)
(read-files (cdr clause)))
((feature-cond)
(process-cond-clauses (cdr clause)))))
; Loop through CLAUSES finding the first that is satisfied.
(define (process-cond-clauses clauses)
(cond ((null? clauses)
(exit #f))
((or (and (eq? (caar clauses) 'else)
(null? (cdr clauses)))
(satisfied? (caar clauses)))
(process-clauses (cdar clauses)))
(else
(process-cond-clauses (cdr clauses)))))
; Compound requirements are handled recursively, simple ones are tested.
(define (satisfied? requirement)
(if (pair? requirement)
(case (car requirement)
((and)
(all-satisfied? (cdr requirement)))
((or)
(any-satisfied? (cdr requirement)))
((not)
(not (satisfied? (cadr requirement)))))
(memq requirement features)))
; True if every requirement in LIST is satisfied.
(define (all-satisfied? list)
(if (null? list)
#t
(and (satisfied? (car list))
(all-satisfied? (cdr list)))))
; True if any requirement in LIST is satisfied.
(define (any-satisfied? list)
(if (null? list)
#f
(or (satisfied? (car list))
(any-satisfied? (cdr list)))))
; Start by doing the whole program.
(process-clauses (cdr program)))))
; Returns a list of the forms in the named files.
(define (read-files filenames)
(if (null? filenames)
'()
(append (call-with-input-file (car filenames)
(lambda (in)
(let label ()
(let ((next (read in)))
(if (eof-object? next)
'()
(cons next (label)))))))
(read-files (cdr filenames)))))
The second implementation is a PROGRAM macro that implements
the configuration language in terms of the COND-EXPAND
syntax of SRFI 0.
Note that this implementation requires that LOAD use the current
evaluation environment.
(define-syntax program
(syntax-rules (requires files code feature-cond)
((program)
(begin))
((program (requires feature-id ...)
more ...)
(begin (cond-expand ((and feature-id ...) 'okay))
(program more ...)))
((program (files filename ...)
more ...)
(begin (load filename) ...
(program more ...)))
((program (code stuff ...)
more ...)
(begin stuff ...
(program more ...)))
((program (feature-cond (requirement stuff ...) ...)
more ...)
(begin (cond-expand (requirement (program stuff ...)) ...)
(program more ...)))))
Copyright (C) Richard Kelsey (1999). All Rights Reserved.
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