30: Nested Multi-line Comments

by Martin Gasbichler

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-30@nospamsrfi.schemers.org. To subscribe to the list, follow these instructions. You can access previous messages via the mailing list archive.

• Draft: 2002-04-12--2002-06-10
• Revised: 2002-06-05
• Final: 2002-08-07

Related SRFIs

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.

Abstract

This SRFI extends R5RS by possibly nested, multi-line comments. Multi-line comments start with #| and end with |#.

Rationale

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.

The syntax #| ... |# 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.

Specification

This SRFI extends the specification of comments -- R5RS section 2.2 -- as follows:

The sequence #| indicates the start of a multi-line comment. The multi-line comment continues until |# appears. If the closing |# is omitted, an error is signaled. Multi-line comments are visible to Scheme as a single whitespace. Multi-line comments may be nested: Every #| 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 |#>

    

<delimiter> and #

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 <delimiter> to #| would be incompatible with existing implementations which allow # as legal character within identifiers.

Implementation

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 symbol start means that the caller has removed nothing, read-sharp means that it has removed a sharp and finally read-bar indicates that the caller has removed #|.

(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 available at https://srfi.schemers.org/srfi-30/remove-srfi30-comments-script.scm. 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 yet.

Copyright

Copyright (C) Martin Gasbichler (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.