Hey all, I was playing around with type families, and I have a strange problem. Suppose we have an alternative to an Either datatype:
data (:|:) a b = Inl a | Inr b
and a class Ix:
class Ix i where type IxMap i :: * -> * empty :: IxMap i [Int]
Now I want to give an instance for (a :|: b):
instance (Ix l, Ix r) => Ix (l :|: r) where type IxMap (l :|: r) = BiApp (IxMap l) (IxMap r) empty = BiApp empty empty
BiApp is defined as following:
data BiApp a b c = BiApp (a c) (b c)
However, it looks like the recursive calls to empty can't be unified, I get the following error message: Couldn't match expected type `IxMap l' against inferred type `IxMap i' Expected type: IxMap (l :|: r) [Int] Inferred type: BiApp (IxMap i) (IxMap i1) [Int] In the expression: BiApp empty empty In the definition of `empty': empty = BiApp empty empty In the inferred type, there should be IxMap l instead of IxMap i, does anybody know what I'm doing wrong? Thanks, -chris
On Sat, Aug 23, 2008 at 7:55 AM, Chris Eidhof
Hey all,
I was playing around with type families, and I have a strange problem.
Suppose we have an alternative to an Either datatype:
data (:|:) a b = Inl a | Inr b
and a class Ix:
class Ix i where type IxMap i :: * -> * empty :: IxMap i [Int]
Now I want to give an instance for (a :|: b):
instance (Ix l, Ix r) => Ix (l :|: r) where type IxMap (l :|: r) = BiApp (IxMap l) (IxMap r) empty = BiApp empty empty
BiApp is defined as following:
data BiApp a b c = BiApp (a c) (b c)
However, it looks like the recursive calls to empty can't be unified, I get the following error message:
Couldn't match expected type `IxMap l' against inferred type `IxMap i' Expected type: IxMap (l :|: r) [Int] Inferred type: BiApp (IxMap i) (IxMap i1) [Int] In the expression: BiApp empty empty In the definition of `empty': empty = BiApp empty empty
In the inferred type, there should be IxMap l instead of IxMap i, does anybody know what I'm doing wrong?
Thanks,
-chris
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Hi, I'm not very familiar with type families, but shouldn't BiApp be defined as
data BiApp a b c = BiApp (a b) (a c)
since you're applying it as BiApp (IxMap l) (IxMap r)? Alex
Chris,
In the inferred type, there should be IxMap l instead of IxMap i, does anybody know what I'm doing wrong?
Your calls to empty are just ambiguous. Let's say I want to get a hold of an empty map for A :|: B for some types A and B. And let's say that you've instance for A hanging around that specifies type IxMap A = C. Now our call to empty for A :|: B delegates to empty to get the left map. Clearly you expect it to call to the instance for A, but any type D with IxMap D = C would do. Does that make sense? Cheers, Stefan On Aug 23, 2008, at 4:55 PM, Chris Eidhof wrote:
Hey all,
I was playing around with type families, and I have a strange problem.
Suppose we have an alternative to an Either datatype:
data (:|:) a b = Inl a | Inr b
and a class Ix:
class Ix i where type IxMap i :: * -> * empty :: IxMap i [Int]
Now I want to give an instance for (a :|: b):
instance (Ix l, Ix r) => Ix (l :|: r) where type IxMap (l :|: r) = BiApp (IxMap l) (IxMap r) empty = BiApp empty empty
BiApp is defined as following:
data BiApp a b c = BiApp (a c) (b c)
However, it looks like the recursive calls to empty can't be unified, I get the following error message:
Couldn't match expected type `IxMap l' against inferred type `IxMap i' Expected type: IxMap (l :|: r) [Int] Inferred type: BiApp (IxMap i) (IxMap i1) [Int] In the expression: BiApp empty empty In the definition of `empty': empty = BiApp empty empty
In the inferred type, there should be IxMap l instead of IxMap i, does anybody know what I'm doing wrong?
Thanks,
-chris
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
On Sun, Aug 24, 2008 at 1:44 AM, Stefan Holdermans
{-# LANGUAGE TypeFamilies, TypeOperators, ScopedTypeVariables #-} module Ix where
The first solution still uses type families, but "empty" takes a parameter so that the which instance to use can be chosen unambiguously.
class Ix i where type IxMap i :: * -> * empty :: i -> IxMap i [Int]
-- uses ScopedTypeVariables instance (Ix left, Ix right) => Ix (left :|: right) where type IxMap (left :|: right) = BiApp (IxMap left) (IxMap right) empty _ = BiApp (empty (undefined :: left)) (empty (undefined :: right))
The second solution uses data families instead, because no such ambiguity can exist.
class IxD i where data IxMapD i :: * -> * emptyD :: IxMapD i [Int]
instance (IxD left, IxD right) => IxD (left :|: right) where data IxMapD (left :|: right) a = BiAppD (IxMapD left a) (IxMapD right a) emptyD = BiAppD emptyD emptyD
-- ryan
data (:|:) a b = Inl a | Inr b data BiApp a b c = BiApp (a c) (b c)
participants (4)
-
Alexander Dunlap -
Chris Eidhof -
Ryan Ingram -
Stefan Holdermans