Question about type families
Is there a way to make the following code working? {-# LANGUAGE TypeFamilies #-} data family Foo a data instance (Num a) => Foo a = A a deriving Show data instance (Fractional a) => Foo a = B a deriving Show I want to have different constructors for 'Foo a' depending on a class of 'a'. Note also, that in the example above I also meant constructor A to be available for (Fractional a) => Foo, since in that case 'a' has Num too. How can I achieve it, maybe not with TypeFamilies? Current error is Conflicting family instance declarations: data instance Foo a -- Defined at 1.hs:7:33-35 data instance Foo a -- Defined at 1.hs:5:33-35
On Tue, Sep 13, 2011 at 4:58 PM, Grigory Sarnitskiy
Is there a way to make the following code working?
{-# LANGUAGE TypeFamilies #-}
data family Foo a
data instance (Num a) => Foo a = A a deriving Show
data instance (Fractional a) => Foo a = B a deriving Show
I want to have different constructors for 'Foo a' depending on a class of 'a'. Note also, that in the example above I also meant constructor A to be available for (Fractional a) => Foo, since in that case 'a' has Num too. How can I achieve it, maybe not with TypeFamilies? Current error is
Conflicting family instance declarations: data instance Foo a -- Defined at 1.hs:7:33-35 data instance Foo a -- Defined at 1.hs:5:33-35
Directly, with current GHC? Doubly, maybe triply impossible. Type and data families aren't allowed to overlap, and there's no way to dispatch over whether a type is or is not a member of a class. (You can require that it be a member, but you can't say "if not, do this other thing"). Also, you can't give type and data families superclass contexts the way you can classes. I haven't actually encountered this before, but I think that what you've written here is datatype contexts for the various instances of the data family, which means you can only construct a 'Foo a' if the 'a' is a member of the class - but it in no way affects which instance is chosen. Datatype contexts are considered a misfeature, besides.[1] Anyway. What's your wider goal? [1] http://hackage.haskell.org/trac/haskell-prime/wiki/NoDatatypeContexts
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On Tue, Sep 13, 2011 at 4:58 PM, Grigory Sarnitskiy
Is there a way to make the following code working?
{-# LANGUAGE TypeFamilies #-}
data family Foo a
data instance (Num a) => Foo a = A a deriving Show
data instance (Fractional a) => Foo a = B a deriving Show
I want to have different constructors for 'Foo a' depending on a class of 'a'. Note also, that in the example above I also meant constructor A to be available for (Fractional a) => Foo, since in that case 'a' has Num too. How can I achieve it, maybe not with TypeFamilies?
Thinking further... just guessing, but maybe you want a GADT? data Foo a where FooN :: Num a => a -> Foo a FooF :: Fractional a => a -> Foo a You use FooF for Fractionals, and either one for Nums. 'a' is required to be an instance of the class, and the instance is made available when pattern matching on the constructor (unlike with datatype contexts). But, as I said, automating the choice based on whether 'a' is or is not an instance of Fractional is impossible.* You have to specify it explicitly. * You can probably do it with Template Haskell, but you probably don't want to. -- Work is punishment for failing to procrastinate effectively.
participants (2)
-
Grigory Sarnitskiy -
Gábor Lehel