Possible to automatically determine typeclass membership?
Is it possible in Haskell + GHC extensions to use reflection techniques to determine typeclass membership? I'm thinking of things like the following: Idea 1:
data MaybeEq a = NoEq a | Eq a => HasEq a checkEq :: (some typeclass constraint) => a -> MaybeEq a
(such that checkEq x returns HasEq x if and only if x has an Eq instance.) Idea 2:
data HTrue data HFalse
class MaybeEq a b | a -> b
instance Eq a => MaybeEq a HTrue instance (otherwise) => MaybeEq a HFalse
-- ryan
Hello,
Is it possible in Haskell + GHC extensions to use reflection techniques to determine typeclass membership? I'm thinking of things like the following:
I think the short answer is not in general; i.e. I don't think there is any way to access the members of an arbitrary typeclass (but I'd love to be proved wrong). However, you could always explicitly list the members of a typeclass you are interested in (this is similar to your Idea 2): {-# OPTIONS_GHC -fglasgow-exts -fallow-undecidable-instances -fallow-overlapping-instances #-} class InEq a b | a -> b where inEq :: a -> Bool instance TypeCast b HFalse => InEq a b where inEq _ = False instance InEq Int HTrue where inEq _ = True instance InEq a b => InEq [a] b where inEq _ = inEq (undefined :: a) data HTrue data HFalse class TypeCast a b | a -> b, b-> a where typeCast :: a -> b class TypeCast' t a b | t a -> b, t b -> a where typeCast' :: t -> a -> b class TypeCast'' t a b | t a -> b, t b -> a where typeCast'' :: t -> a -> b instance TypeCast' () a b => TypeCast a b where typeCast x = typeCast' () x instance TypeCast'' t a b => TypeCast' t a b where typeCast' = typeCast'' instance TypeCast'' () a a where typeCast'' _ x = x You can also do an arguably nicer, more flexible version of the previous by recreating some of the typeclass machinery yourself; Oleg has several examples of this such as: //okmij.org/ftp/Haskell/poly2.txt -Jeff
Hopefully, accessing the instance environment from Tempalte Haskell
will be possible in next GHC's release:
http://hackage.haskell.org/trac/ghc/ticket/1835
2008/3/31 jeff p
Hello,
Is it possible in Haskell + GHC extensions to use reflection techniques to determine typeclass membership? I'm thinking of things like the following:
I think the short answer is not in general; i.e. I don't think there is any way to access the members of an arbitrary typeclass (but I'd love to be proved wrong).
However, you could always explicitly list the members of a typeclass you are interested in (this is similar to your Idea 2):
{-# OPTIONS_GHC -fglasgow-exts -fallow-undecidable-instances -fallow-overlapping-instances #-}
class InEq a b | a -> b where inEq :: a -> Bool instance TypeCast b HFalse => InEq a b where inEq _ = False instance InEq Int HTrue where inEq _ = True instance InEq a b => InEq [a] b where inEq _ = inEq (undefined :: a)
data HTrue data HFalse
class TypeCast a b | a -> b, b-> a where typeCast :: a -> b class TypeCast' t a b | t a -> b, t b -> a where typeCast' :: t -> a -> b class TypeCast'' t a b | t a -> b, t b -> a where typeCast'' :: t -> a -> b instance TypeCast' () a b => TypeCast a b where typeCast x = typeCast' () x instance TypeCast'' t a b => TypeCast' t a b where typeCast' = typeCast'' instance TypeCast'' () a a where typeCast'' _ x = x
You can also do an arguably nicer, more flexible version of the previous by recreating some of the typeclass machinery yourself; Oleg has several examples of this such as: //okmij.org/ftp/Haskell/poly2.txt
-Jeff
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participants (3)
-
Alfonso Acosta -
jeff p -
Ryan Ingram