[Haskell-cafe] Typeclass question
nostrademons at gmail.com
Sun Dec 3 12:26:30 EST 2006
I've got what's probably a beginner's question, but I'm out of ideas
for solving it. It looks like it tripped me up in "Write Yourself a
Scheme..." too, since the code there seems like it's arranged so I
never ran into it...
I've got a couple functions:
binop :: (AmbrosiaData -> AmbrosiaData -> AmbrosiaM AmbrosiaData) ->
binop operator = do
second <- popStack
first <- popStack
result <- operator first second
numNumOp :: Num a => (a -> a -> a) -> AmbrosiaData -> AmbrosiaData ->
numNumOp op (Number val1) (Number val2) = return $ Number $ op val1 val2
numNumOp op (Float val1) (Float val2) = return $ Float $ op val1 val2
numNumOp op _ _ = throwError . TypeMismatch $ "Number"
the intention being that I can define primitive operations for an
interpreter with ("+", binop $ numNumOp (+)). In the Number case,
val1 & val2 are Integers, while in the Float case, they're Floats.
However, when I try to compile, I get:
Cannot unify the type-signature variable `a'
with the type `Integer'
Expected type: Integer
Inferred type: a
In the application `op val1 val2'
In the second argument of `($)', namely `op val1 val2'
This makes no sense to me, since Integer is an instance of Num and op
is defined as type Num a => a -> a -> a. Shouldn't the typechecker be
able to instantiate 'a' with Integer and give the resulting type
Integer -> Integer -> Integer, and then pass the final Integer to
Number? Is my understanding of typeclasses off somehow?
Any help would be appreciated.
More information about the Haskell-Cafe