[Haskell-cafe] Parsec Problem

Bayley, Alistair Alistair_Bayley at ldn.invesco.com
Tue Jul 27 04:05:14 EDT 2004

> I copied this example exactly from the page
> http://www.cs.uu.nl/people/daan/download/parsec/parsec.html
> price   = lexeme (do{ ds1 <- many1 digit
> ...
> However attempting to compile it gives the error message
> ...
> I wish I knew what that meant.  If someone could explain it 
> and tell me what's wrong, I'd appreciate it.

The documentation for Parsec is a bit out-of-date; some things still work as
described, and some things have changed. You'll have to browse the current
libraries at the same time as reading the Parsec documentation to determine
if a particular feature is still working as described, or if there's a newer
(better?) way of doing the same thing.

If you want to learn how to use the libraries, I recommend writing some very
simple character/string parsers first, to see how the combinators work.

Back to your parser... A lexeme parser (as described in the Parsec document)
is one that consumes any trailing whitespace after the token has been read,
so as to leave the next non-whitespace token ready on the input stream. The
easiest way to fix the price parser is simply to add
Text.ParserCombinators.Parsec.Char.spaces after the last digits have been

price = do
  { ds1 <- many1 digit
  ; char '.'
  ; ds2 <- count 2 digit
  ; spaces  -- eat trailing whitespace
  ; return (convert 0 (ds1 ++ ds2))
  <?> "price"
    convert n []     = n
    convert n (d:ds) = convert (10*n + digitToInt d) ds
(The lexeme function is now a record field accessor in
Text.ParserCombinators.Parsec.Token. There is a data type called
TokenParser, of which lexeme is one field. There's probably a way to write
the price parser using TokenParser and the lexeme function/field, but I
haven't figured this out yet.)


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