GHC rejects use of mulit-param type class.
In the following example, I instantiate a few small typeclasses. The last instantiation provided does not compile, despite it being virtually identical the prior instantiation.
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE MultiParamTypeClasses #-}
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class Collection collection handle | collection -> handle, handle -> collection where
getHandles :: collection -> [handle]
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data HandleX = HandleX Int
data CollectionX = CollectionX [HandleX]
instance Collection CollectionX HandleX where
getHandles (CollectionX handles) = handles
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newtype HandleY = HandleY HandleX
newtype CollectionY = CollectionY CollectionX
instance Collection CollectionY HandleY where
getHandles (CollectionY collectionX) = map HandleY $ getHandles collectionX
-- Good, this compiles!
--------------------------------------------------------------------------------
newtype HandleZ = HandleZ HandleY
newtype CollectionZ = CollectionZ CollectionY
instance Collection CollectionZ HandleZ where
getHandle (CollectionZ collectionY) = map HandleZ $ getHandles collectionY
-- Error: `getHandle' is not a (visible) method of class `Collection'
Trac metadata
Trac field | Value |
---|---|
Version | 8.0.2 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler (Type checker) |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |