Hello list, I have a rather trivial question, but one that I can't seem to get my head around. In the following example function (using Text.ParserCombinators.Parsec) I'm getting an error without the type declaration, but it compiles just fine with it:
parseNumber' :: Parser LispVal parseNumber' = do x <- many1 digit return $ Number . read $ x
The error message when compiling without the type declaration is the following: No instance for (Text.Parsec.Prim.Stream s m Char) arising from a use of `digit' at Scheme.hs:33:30-34 Possible fix: add an instance declaration for (Text.Parsec.Prim.Stream s m Char) In the first argument of `many1', namely `digit' In a stmt of a 'do' expression: x <- many1 digit In the expression: do { x <- many1 digit; return $ Number . read $ x } The function is an example in "Write yourself a Scheme in 48 hours." I'd consider myself fairly accustomed to programming with monads; and still, I can't seem to find out *why* exactly GHC ( 6.12.1 ) cannot compile this without the type signature. What does it try to build here? Why does the inferencing fail? My best guess is that it somehow doesn't get that 'x' is supposed to be a String value ([Char], not Char.) What really confuses me, though, is that adding the instance declaration makes the whole thing work! The type declaration for LispVal looks like this:
data LispVal = Number Integer
So it should be pretty clear to GHC that, due to `Number . read` demanding a [Char], I actually want x to be a [Char]. I'm assuming here that I'm just in the wrong Monad, starting with `do`. Somehow, GHC assumes it's not Parser LispVal, but something more complex, possibly Text.Parsec.Prim.Stream s m LispVal or so? Thank you everybody for you help :-) This list is a truly amazing place with a lot of helpful people! Aleks