Frederik Eaton writes:
You need to swap the arguments to TCons...
data TCons l a = TCons !l a
Then:
instance Functor (TCons (TCons HNil a)) where fmap f (TCons (TCons HNil x) y) = TCons (TCons HNil (f x)) y)
How does one solve this problem in general, i.e. when the arguments to a type are in the wrong order for an instance that one wants to declare?
In general? You either make a newtype or change the definition of the original type.
Someone on the haskell IRC channel mentioned that if you could derive instances for partially applied type synonyms then one could just make a dummy synonym with the arguments in the right order, but that doesn't appear to be premitted. The other question is why isn't it permitted.
As I understand it, the reason you can't do instances for partially
applied type synonyms is because it makes instance selection ambiguous.
For example:
instance Functor ((,) a) where fmap f (x,y) = (x, f y)
type RevPair a b = (b,a)
instance Functor (RevPair a) where fmap f (x,y) = (f x, y)
If I write 'fmap f (x,y)', do I get '(f x, y)' or '(x, f y)'?
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David Menendez