[Haskell-beginners] Parsec 3.0 problems

Philippa Cowderoy flippa at flippac.org
Mon Apr 20 22:17:50 EDT 2009

On Mon, 2009-04-20 at 18:26 -0500, Ian Duncan wrote:
> No instance for (Stream s m Char)
>       arising from a use of `spaces' at tree.hs:18:32-37
>     Possible fix: add an instance declaration for (Stream s m Char)
>     In the first argument of `between', namely `spaces'
>     In the second argument of `(<$>)', namely
>         `(between spaces (char ':') (many1 alphaNum))'
>     In the expression:
>           Label <$> (between spaces (char ':') (many1 alphaNum))
> When I test this function in ghci, I have no problems, yet whenever I
> try to open it in a file it fails. What am I doing wrong?

Looks like you're getting bitten by the monomorphism restriction. {-#
LANGUAGE NoMonomorphismRestriction #-} at the top of the source file to
make it go away. 

The MR is a real usability problem for Parsec 3, sadly :-( You may find
it easier to work with the latest version of Parsec 2, which is still
the recommended version for most purposes.

Philippa Cowderoy <flippa at flippac.org>

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