Functional dependencies can get GHC to print "UnkSkol" (#15355) · Issues · Glasgow Haskell Compiler / GHC

Functional dependencies can get GHC to print "UnkSkol"

When I say

class C a b | a -> b where
  foo :: a -> b

instance C Int Bool where
  foo = (>0)

blah :: C Int Double => Int -> _a
blah = foo

I get

    • Couldn't match type ‘Double’ with ‘Bool’
        arising from a functional dependency between constraints:
          ‘C Int Bool’ arising from a use of ‘foo’ at Scratch.hs:51:8-10
          ‘C Int Double’ arising from UnkSkol at Scratch.hs:51:1-10
    • In the expression: foo
      In an equation for ‘blah’: blah = foo

There are actually two problems:

If we replace _a with Double, GHC accepts the program. I'm not sure it should. But that's a larger problem than...
... when it prints the error, it says UnkSkol. It probably shouldn't.

This ticket is about the second problem, because the first one require more fundep love than is usually on offer around here.

Erroring here is reasonable enoug

Trac metadata

Trac field	Value
Version	8.4.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture