[Haskell-cafe] Re: Parsec is being weird at me

[ChrisK]

Andrew Coppin wrote:
> Anybody want to explain to me why this doesn't work?
> 
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>  / _ \ /\  /\/ __(_)
> / /_\// /_/ / /  | |      GHC Interactive, version 6.6.1, for Haskell 98.
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> 
> Loading package base ... linking ... done.
> Prelude> :m Text.ParserCombinators.Parsec
> Prelude Text.ParserCombinators.Parsec> parseTest (endBy anyToken (char
> '#')) "abc#"
> Loading package parsec-2.0 ... linking ... done.
> parse error at (line 1, column 1):
> unexpected "b"
> expecting "#"

anyToken is singular: it accepts a single token, in this case 'a'.

Then endBy expects (char '#') to match and reads 'b' instead and gives the error
message.

So using (many anyToken) gets further:

> Prelude Text.ParserCombinators.Parsec> parseTest (endBy (many anyToken) (char '#')) "abc#"
> Loading package parsec-2.0 ... linking ... done.
> parse error at (line 1, column 1):
> unexpected end of input
> expecting "#"
> Prelude Text.ParserCombinators.Parsec> parseTe

Here (many anyToken) reads all of "abc#" and then endBy wants to read (char '#')
and get the end of input instead.

So the working version of endBy is thus:

> Prelude Text.ParserCombinators.Parsec> parseTest (endBy (many (noneOf "#")) (char '#')) "abc#"
> ["abc"]

Or you may need to not use endBy...