Hello everyone, I have the simple applicative parser combinator: parseLabel = Label <$> (between spaces (char ':') (many1 alphaNum)) This gives me the error: No instance for (Stream s m Char) arising from a use of `spaces' at tree.hs:18:32-37 Possible fix: add an instance declaration for (Stream s m Char) In the first argument of `between', namely `spaces' In the second argument of `(<$>)', namely `(between spaces (char ':') (many1 alphaNum))' In the expression: Label <$> (between spaces (char ':') (many1 alphaNum)) When I test this function in ghci, I have no problems, yet whenever I try to open it in a file it fails. What am I doing wrong? For those who are curious, Label is a data constructor in the following data type: data Loc = Label String | Addr Int deriving Show Thanks! Ian Duncan
On Mon, 2009-04-20 at 18:26 -0500, Ian Duncan wrote:
No instance for (Stream s m Char) arising from a use of `spaces' at tree.hs:18:32-37 Possible fix: add an instance declaration for (Stream s m Char) In the first argument of `between', namely `spaces' In the second argument of `(<$>)', 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
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Ian Duncan -
Philippa Cowderoy