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
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