[plt-scheme] Re: Parsing not ignoring \n

On Fri, Jun 19, 2009 at 11:18 AM, Paulo J. Matos <pocmatos at gmail.com> wrote:

> Still on this, is it not possible to re-insert a char in an input-port?
> This way, once the lexer recognizes a one-line comment I could
> re-insert the \n into the port and keep lexing...
>

If you are reading from either a file or string port, then you can use
file-position to rewind the port to the correct position so the next lex
will start from the correct place.

http://docs.plt-scheme.org/reference/port-buffers.html#(def._((quote._~23~25kernel)._file-position))

Another approach is to use lexer to only handle the start of the comment,
e.g., // and then write your own custom reader to read until it hits newline
(but preserving it), and then hand the control back to the lexer.

(define (custom-comment-reader in)
  (read-until-new-line ...)) ;; defined elsewhere

(define eb-lexer
  (lexer
   ("//" (custom-comment-reader input-port)
         ...)
   ...))

Cheers,
yc
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