Hi,
I got a definition like this in my parser library:
data ParRes a = Error String Int Int Source |
Success a State
data Parser a = Parser (State -> ParRes a)
unParser (Parser a) = a
and the monad looks like this
instance Monad Parser where
p >>= f = Parser (\state -> case ((unParser p) state) of
all@(Error _ _ _ _) -> all
Success v newState -> (unParser (f v)) newState
)
Haskell complains with
Type checking
ERROR "Parser1.hs" (line 28): Declared type too general
*** Expression : (>>=)
*** Declared type : Parser a -> (a -> Parser b) -> Parser b
*** Inferred type : Parser a -> (a -> Parser a) -> Parser a
because it assumes that the 'all' binding which is of type 'ParRes a'
is not equivalent with 'ParRes b' although the Error constructor
does not contain the parameter a at all. This works fine:
instance Monad
Parser where
p >>= f = Parser (\state -> case ((unParser p) state) of
Error msg x y s -> Error msg x y s
Success v newState -> (unParser (f v)) newState
)
Is this the right behavior or rather a simplification in the type checker?
Yours sincerely,
Axel Simon.
