[Haskell] A puzzle and an annoying feature

[Keean Schupke]

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.

    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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>
>
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