[Haskell-cafe] partial kinds and instances
matthew.pocock at ncl.ac.uk
Mon Apr 16 09:15:42 EDT 2007
I saw that it's possible to peal off kinds from the right-hand side when
instantiating classes. Is it possible to peel them off from the left-hand
side? Or in any order?
I have been told in #haskell by people who seems to know that Very Bad Things
happen if you do this without also making the kind system more clever.
Is there a quick fix that will give me the same effect, without introducing a
newtype or re-writing the original type declaration to introduce the extra
types in a different order?
Illustrative code below.
data MyType e = Singleton e
data MyOtherType k v
| Node k v
data YetAnotherType k v
| OtherNode k v
type RevYAT v k = YetAnotherType k v
class Iterable ite where
iterate :: ite e -> [e]
instance Iterable MyType where
iterate (Singleton e) = [e]
instance Iterable (MyOtherType k) where
iterate ite =  --broken but illustrative
instance Iterable (RevYAT v) where
iterate it =  --broken again
Type synonym `RevMOM' should have 2 arguments, but has been given 1
In the instance declaration for `Iterable (RevYAT v)'
More information about the Haskell-Cafe