A more general cond clause
Taylor Campbell
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This SRFI proposes an extension to the cond syntax to allow a more
general clause, one that allows binding the results of tests as in the
=> clauses and user-defined meaning of the success & failure of tests.
The present set of cond clauses is based on simple boolean testing. It
is prohibitively inexpressive in that the condition part of a cond
clause that uses => may pass only a single value to the receiver, and
it enforces a semantics whereby #f implies failure of the condition.
Programmers frequently use different tokens to imply failure, such as
in R5RS's I/O readers which return a distinguished 'EOF object' to
denote failure, and a successful condition may produce more than one
useful value. This simple extension allows any meaning of 'failure' to
be assigned on a per-clause basis, and it also allows the condition to
return multiple values to be passed to the receiver.
The <cond clause> production in the formal syntax of Scheme as written
by R5RS in section 7.1.3 is extended with a new option:
<cond clause> ---> ... | (<generator> <guard> => <receiver>)
where <generator>, <guard>, & <receiver> are all <expression>s.
Clauses of this form have the following semantics: <generator> is
evaluated. It may return arbitrarily many values. <Guard> is applied
to an argument list containing the values in order that <generator>
returned. If <guard> returns a true value for that argument list,
<receiver> is applied with an equivalent argument list. If <guard>
returns a false value, however, the clause is abandoned and the next
one is tried.
This port->char-list procedure accepts an input port and returns a list
of all the characters it produces until the end.
(define (port->char-list port) (cond ((read-char port) char? => (lambda (c) (cons c (port->char-list port)))) (else '())))
Consider now a hypothetical table-entry procedure that accepts two
arguments, a table (perhaps a hash table) and a key to an entry that
may be in the table; it returns two values: a boolean that denotes
whether or not an entry with the given key was in the table and, if it
was, the value associated with the key. Also, a hypothetical proj0
combinator (projection of argument 0) returns its 0th argument and
ignores all others. One might conditionally branch to a certain body
of code if the table contains the desired entry like so with the new
type of cond clause:
(cond ... ((table-entry <table> <key>) proj0 => (lambda (present? value) ...[VALUE is bound to the value of the entry]...)) ...)
The entirety of a syntax transformer for the new cond syntax is given
here. It uses an auxiliary macro, cond/maybe-more, to simplify the
construction of if expressions with or without more cond clauses. The
code is in the public domain.
(define-syntax cond (syntax-rules (=> ELSE) ((COND (ELSE else1 else2 ...)) ;; The (IF #T (BEGIN ...)) wrapper ensures that there may be no ;; internal definitions in the body of the clause. R5RS mandates ;; this in text (by referring to each subform of the clauses as ;; <expression>) but not in its reference implementation of COND, ;; which just expands to (BEGIN ...) with no (IF #T ...) wrapper. (IF #T (BEGIN else1 else2 ...))) ((COND (test => receiver) more-clause ...) (LET ((T test)) (COND/MAYBE-MORE T (receiver T) more-clause ...))) ((COND (generator guard => receiver) more-clause ...) (CALL-WITH-VALUES (LAMBDA () generator) (LAMBDA T (COND/MAYBE-MORE (APPLY guard T) (APPLY receiver T) more-clause ...)))) ((COND (test) more-clause ...) (LET ((T test)) (COND/MAYBE-MORE T T more-clause ...))) ((COND (test body1 body2 ...) more-clause ...) (COND/MAYBE-MORE test (BEGIN body1 body2 ...) more-clause ...)))) (define-syntax cond/maybe-more (syntax-rules () ((COND/MAYBE-MORE test consequent) (IF test consequent)) ((COND/MAYBE-MORE test consequent clause ...) (IF test consequent (COND clause ...)))))
Copyright (C) 2004 Taylor Campbell. All rights reserved.
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