Keean Schupke wrote:
No, closed classes are different, here we are talking about lazy overlap resolution, so if at _call_ time only one instance fits we choose it. Closing a class is different.
A "closed class" directive however is an explicit specification that makes the intention of the designer explicit in the program. Since it would solve the puzzle in a rather elegant and explicit way, I thought that it was interesting to mention. All the best, -- Daan.
Keean.
Daan Leijen wrote:
Lennart Augustsson wrote:
[snip] So in what sense is this really ambiguous?
I think it would be quite reasonable to allow the Puzzle module to compile, resolving `a' to be Bool. I.e., if there is only one instance that can satisfy a constraint and there is no possibility of adding instances outside the compiled module, I think resolving the overloading makes sense.
You may be interested in a recent paper by Bastiaan Heeren and Juriaan Hage [1] about type class directives. The "closed" directive in that paper is more or less implied by your example. Here is a quote from the paper:
"The main advantage of a closed type class is that we know the fixed set of instances. Using this knowledge, we can influence the type inference process. As discussed in the introduction to Section 2, we can reject definitions early on (in case the set of instances for a certain type class is empty) or improve a type variable to a certain type (in case the set of instances is a singleton)."
All the best, -- Daan Leijen.
[1] http://www.cs.uu.nl/~bastiaan/papers.html#typeclassdirectives (to appear in PADL 2005)
-- Lennart _______________________________________________ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
_______________________________________________ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
_______________________________________________ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell