| Tell me how this make sense:
| 1. I enter the definition for f.
| 2. I ask ghc for the type of f and get an answer.
| 3. I take the answer and tell ghc this is the type of f, and ghc
| tells me I'm wrong.
| Somewhere in this sequence something is going wrong.
I agree! Indeed I wrote:
| It doesn't get much simpler than that! With the type sig, GHC can't see that
the (C a b) provided can
| satisfy the (C a b1) which arises from the call to op. However, without the
constraint, GHC simply
| abstracts over the constrains arising in the RHS, namely (C a b1), and hence
infers the type
| f :: C a b1 => a -> a
| It is extremely undesirable that the inferred type does not work as a type
signature, but I don't see
| how to fix it
If you have an idea for an inference algorithm that would typecheck this
program, I'd be glad to hear it. Just to summarise, the difficulty is this:
I have a dictionary of type (C a b1)
I need a dictionary of type (C a b2)
There is no functional dependency between C's parameters
Simon
PS: the complete program is this:
class C a b where
op :: a -> a
f :: C a b => a -> a
f x = op x
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