As has been mentioned in the comments, poking around with :kind and :kind! in GHCi is usually how I go about doing it, but it also surprisingly matters where you place the functions, and what looks like it should be the same, isn’t always.
For instance, I was trying to make a dependently typed functor equivalent, for a personal project, which looked like
class IFunctor f where
ifmap :: (a -> b) -> f n a -> f n b
and I was writing the instance for
data IEither a n b where
ILeft :: a -> IEither a Z b
IRight :: b -> IEither a (S n) b
It should be fairly simple, I thought, just ignore f for the left case, apply it in the right.
I tried
instance IFunctor (IEither a) where
ifmap _ l@(ILeft _) = l
ifmap f (IRight r) = IRight $ f r
but for the specialized version of ifmap in this case being ifmap :: (b -> c) -> IEither a Z b -> IEither a Z c, Haskell inferred the type of l to be IEither a Z b on the LHS, which, makes sense, but then refused to produce b ~ c.
So, I had to unwrap l, get the value of type a, then rewrap it to get the IEither a Z c.
This isn’t just the case with dependent types, but also with rank-n types.
For instance, I was trying to convert isomorphisms of a proper form into natural transformations, which should be fairly easy, I thought.
Apparently, I had to put the deconstructors in a where clause of the function, because otherwise type inference didn’t work properly.