Julian Fleischer wrote:
Hello,
i'm playin' around with GHCs Haskell and some extensions. I'm already aware of that functional
dependencies are "very very tricky", but there is something I don't understand about
there implementation in GHC. I've constructed my own TypeClass "Num" providing a
signature for (+), having multiple params a, b and c. I'm than declaring a (flexible) Instance for
Prelude.Num, simply using (Prelude.+) for the definition of my (+) - and it does not work as I
expect it to.
First, this is the code:
{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies,
TypeSynonymInstances, FlexibleInstances #-}
import qualified Prelude
class Num a b c | a b -> c where
(+) :: a -> b -> c
instance (Prelude.Num x) => Num x x x where
(+) = (Prelude.+)
now if I load it into GHCi and type "3 + 4" i get a whole bunch of
error-messages.
I do understand that
(3::Prelude.Int) + (4::Prelude.Int)
works, since I've explicitly declared 3 and 4 to be Prelude.Int and there is a
functional dependency stating that (+) :: a b determines the results type c, by
the Instance declaration cleary c will be the same as a and b.
Now, if I type
3 + 4
it does not work, and i really don't understand why. If i ask GHCi for 3's type ($ :t 3) it
will answer "3 :: (Prelude.Num t) => t". But, if 3 and 4 are Prelude.Nums and
there is an instanfe Num x x x for x of Prelude.Num - than why can't GHC deduce from the
definitions that 3 and 4, both Prelude.Nums, can be used with (+) since there is an instance
for Prelude.Num and my class Num - and the result will of course be something of Prelude.Num?
My guess would be, that while 3 and 4 are both of a type instantiating
Prelude.Num (your terminology "are Prelude.Nums" is quite confusing --
Prelude.Num is not a type but a type class), they need not be of the
same type (e.g., 3 could be of type Integer, and 4 of type Double).
Jochem
--
Jochem Berndsen | joc...@functor.nl
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