[Haskell-cafe] type families and type signatures
Ganesh Sittampalam
ganesh at earth.li
Wed Apr 9 01:02:04 EDT 2008
On Wed, 9 Apr 2008, Manuel M T Chakravarty wrote:
> Sittampalam, Ganesh:
>> No, I meant can't it derive that equality when matching (Id a) against
>> (Id b)? As you say, it can't derive (a ~ b) at that point, but (Id a ~
>> Id b) is known, surely?
>
> No, it is not know. Why do you think it is?
Well, if the types of foo and foo' were forall a . a -> a and forall b . b
-> b, I would expect the type-checker to unify a and b in the argument
position and then discover that this equality made the result position
unify too. So why can't the same happen but with Id a and Id b instead?
> The problem is really with foo and its signature, not with any use of foo.
> The function foo is (due to its type) unusable. Can't you change foo?
Here's a cut-down version of my real code. The type family Apply is very
important because it allows me to write class instances for things that
might be its first parameter, like Id and Comp SqlExpr Maybe, without
paying the syntactic overhead of forcing client code to use Id/unId and
Comp/unComp. It also squishes nested Maybes which is important to my
application (since SQL doesn't have them).
castNum is the simplest example of a general problem - the whole point is
to allow clients to write code that is overloaded over the first parameter
to Apply using primitives like castNum. I'm not really sure how I could
get away from the ambiguity problem, given that desire.
Cheers,
Ganesh
{-# LANGUAGE TypeFamilies, GADTs, UndecidableInstances,
NoMonomorphismRestriction #-}
newtype Id a = Id { unId :: a }
newtype Comp f g x = Comp { unComp :: f (g x) }
type family Apply (f :: * -> *) a
type instance Apply Id a = a
type instance Apply (Comp f g) a = Apply f (Apply g a)
type instance Apply SqlExpr a = SqlExpr a
type instance Apply Maybe Int = Maybe Int
type instance Apply Maybe Double = Maybe Double
type instance Apply Maybe (Maybe a) = Apply Maybe a
class DoubleToInt s where
castNum :: Apply s Double -> Apply s Int
instance DoubleToInt Id where
castNum = round
instance DoubleToInt SqlExpr where
castNum = SECastNum
data SqlExpr a where
SECastNum :: SqlExpr Double -> SqlExpr Int
castNum' :: (DoubleToInt s) => Apply s Double -> Apply s Int
castNum' = castNum
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