Petr,
If I want to make it a functor in the last type variable (b), I can
just define
instance Functor (X a) where
fmap f (X a b) = X a (f b)
But how do I write it if I want X to be a functor in its first type
variable?
Short answer: you can't. Easiest way to workaround is to define a
newtype wrapper around your original datatype:
newtype X' b a = X' {unX' :: X a b}
instance Functor (X' b) where
fmap g (X' (X a b)) = X' (X b (g a))
Cheers,
Stefan
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