by Martin Gasbichler
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SRFI 22 defines a multi line comment that may only appear at the beginning of a file.
SRFI 10 proposes the notation
"#" <discriminator> <other-char>*
for further SRFIs that introduce values which may be read and written.
This SRFI extends R5RS by possibly nested, multi-line
comments. Multi-line comments start with
#| and end
Multi-line comments are common to many programming languages. They provide a convenient mean for adding blocks of text inside a program and support development by fading out incomplete code or alternative implementations.
|# was chosen
because it is already widely used (Chez Scheme, Chicken, Gambit-C,
Kawa, MIT Scheme, MzScheme and RScheme) and since it fits smoothly
with R5RS. Nested comments are an important feature for
incrementally commenting out code.
This SRFI extends the specification of comments -- R5RS section 2.2 -- as follows:
#| indicates the start of a
multi-line comment. The multi-line comment continues until
|# appears. If the closing
omitted, an error is signaled. Multi-line comments are visible to
Scheme as a single whitespace. Multi-line comments may be nested:
#| within the comment starts a new multi-line
comment, which has to be terminated by a matching
This SRFI furthermore extends the production for
<comment> in the specification of lexical
structure -- R5RS
section 7.1.1 -- to:
<comment> ---> ; <all subsequent characters up to a line break> | #| <comment-text> (<comment> <comment-text>)* |# <comment-text> ---> <character sequence not containing #| or |#>
The SRFI does not extend the specification of
<delimiter> from R5RS
section 7.1.1. It is therefore required to separate tokens
which require implicit termination (identifiers, numbers,
characters, and dot) from multi-line comments by a
<delimiter> from R5RS.
Rationale An extension of the
#| would be
incompatible with existing implementations which allow
# as legal character within identifiers.
The following procedure
skip-comment deletes a
leading multi-line comment from the current input port. Its
optional argument specifies whether the caller has already removed
leading characters of the comment from the current input port. The
start means that the caller has removed
read-sharp means that it has removed a sharp
read-bar indicates that the caller has
(define (skip-comment! . maybe-start-state) (let lp ((state (if (null? maybe-start-state) 'start (car maybe-start-state))) (nested-level 0)) (define (next-char) (let ((c (read-char))) (if (eof-object? c) (error "EOF inside block comment -- #| missing a closing |#") c))) (case state ((start) (case (next-char) ((#\|) (lp 'read-bar nested-level)) ((#\#) (lp 'read-sharp nested-level)) (else (lp 'start nested-level)))) ((read-bar) (case (next-char) ((#\#) (if (> nested-level 1) (lp 'start (- nested-level 1)))) (else (lp 'start nested-level)))) ((read-sharp) (case (next-char) ((#\|) (lp 'start (+ nested-level 1))) (else (lp 'start nested-level)))))))
A SRFI 22 script to remove nested multi-line comments is
The script will read a Scheme program containing nested
multi-line comments from standard input and emit the same
programs with the comments removed to standard output. The
script mimics the Scheme 48 reader and uses the
error procedure from SRFI 23.
A number of Scheme implemenations already support this SRFI:
Chez Scheme, Chicken, Gambit-C, Kawa, MIT Scheme, MzScheme and
RScheme. Bigloo supports multi-line comments via
!#. Among the major Scheme
implementations, Scheme 48 and Guile do not support this SRFI
Copyright (C) Martin Gasbichler (2002). All Rights Reserved.
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