[Haskell-cafe] Could not deduce ... using functional dependencies with GHC7

16 Mar 2011

      Hello, when moving to GHC7 a body of code that I'm not immensely
familiar with, I got an error compiling with GHC 7.0.2. Can somebody
kindly explain to me what it means and how to get around it?
This is in the scion code base so is using some GHC types.

I have a class with a functional dependency (I think :-p):
class Search id a | a -> id where
  search :: (SrcSpan -> Bool) -> SrcSpan -> a -> SearchResults id

And some instances, notably:
instance Search Id Id where
  search _ _ i = only (FoundId i)

And the error occurs on another instance:
instance (Search id id) => Search id (Sig id) where
  search p s (IdSig i)       = search p s i
  ... other cases here, that work

The error is:

Could not deduce (id ~ Id)
    from the context (Search id id)
      bound by the instance declaration
      at lib\Scion\Inspect\Find.hs:477:10-45
      `id' is a rigid type variable bound by
           the instance declaration at lib\Scion\Inspect\Find.hs:477:18
    When using functional dependencies to combine
      Search Id Id,
        arising from the dependency `a -> id'
        in the instance declaration at lib\Scion\Inspect\Find.hs:183:10
      Search id Id,
        arising from a use of `search'
        at lib\Scion\Inspect\Find.hs:479:32-37
    In the expression: search p s i
    In an equation for `search': search p s (IdSig i) = search p s i

If I replace the call to search with the actual implementation of
search for Id, it compiles, so at some level the code "makes sense".
I much prefer when error messages end with "Possible fix:..."!

Thanks!

-- 
JP Moresmau
http://jpmoresmau.blogspot.com/