On 2/21/08, Jeff φ wrote:

Thanks again. I'm not familiar with type equality constraints. I could not find information on this in the GHC users guide. Is it documented somewhere?

Section 7.3 here talks about equality constraints. http://www.haskell.org/haskellwiki/GHC/Indexed_types There's an extremely good paper talking about the low-level mechanisms for equality constraints in GHC Core here: http://research.microsoft.com/%7Esimonpj/papers/ext%2Df/ GHC Core -is- System Fc as described in that paper; the type equality constraints amount to a "coercion" function that is passed around the core language to convert between the types and which is statically guaranteed to be the identity function.

It is interesting that you were able to drop the IArray constraint from the instances of SmartArraySelector.

During compilation from Haskell to Core, the compiler implicitly adds calls to any available conversion functions it knows about as required to convert between types. This is why the IArray declaration can be dropped; the compiler looks at the coercions currently available, finds (a ~ UArray), and inserts a call that converts the existing "IArray UArray Bool" declaration into an "IArray a Bool" declaration. Determining that such a path exists is a search problem and much of the work with associated types and type equality constraints involves proving that the search is decidable. -- ryan