RE: question about parsing integers and floats with Parsec

16 Aug 2002


      Hi -

	After a bit of monkeying around, it seems the following parser works
to detect both integers and floats, both positive and negative.  I'm sure
you wizards have many better ways of doing this.  Anyway here it is for your
amusement:

import Parsec
import qualified ParsecToken as P
import ParsecLanguage (emptyDef)

lexer = P.makeTokenParser emptyDef
integer = P.integer lexer
float = P.float lexer

genericgrab :: Parser (Either Integer Double)
genericgrab = try ( do { n <- char '-'
                          ; f <- float
                          ; return (Right (-f))
                          }
                  )
             <|> try ( do { f <- float
                          ; return (Right f)
                          }
                     )
             <|> try ( do { f <- integer
                       ;  return (Left f)
                       }
                     )

-andrew
...
-----Original Message-----
From: Harris, Andrew [mailto:Andrew.Harris@jhuapl.edu]
Sent: Thursday, August 15, 2002 6:55 PM
To: 'haskell-cafe@haskell.org'
Subject: question about parsing integers and floats with Parsec
Hi -
This isn't a pure "Haskell" question, but I'm trying to use the
Parsec library to parse out space separated numbers, which 
could be integers
or floats and either positive or negative.  I was using the 
"naturalOrFloat"
lexeme parser from the ParsecToken module, until I realized 
that it doesn't
seem to handle negative integers.  I've been poking with 
trying to pair the
integer and float lexeme parsers with try() blocks, but I 
ain't no parsing
expert and am not making good progress.
Any help/hints would be appreciated!
thanks,
-andrew
---
Andrew Harris        andrew.harris@jhuapl.edu
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