[Haskell-cafe] Proposal to solve Haskell's MPTC dilemma

[Carlos Camarao]

Sorry to correct myself:

On Sat, May 22, 2010 at 10:24 PM, Carlos Camarao
<carlos.camarao at gmail.com>wrote:

> ...
> =====================================================================
> PS: I think that a definition of orphan/non-orphan instance definition
> for MPTCs should be different.
> Letting:  ...
>
   instance-def(I,C,M,tv) = I is an instance definition of class C,
> occurring in
>                               module M, that instantiates class variable tv
>

Should be: instance-def(I,C,M,tv,T) = I is an instance def of class C,
occurring in
                                  module M, that instanticates class
variable tv to datatype T
and replace occurrences of instance-def(I,C,M,tv) to
instance-def(I,C,M,tv,T).

And I think a correct definition of orphan/non-orphan for MPTCs should

be along the line:
> ...
> That is, an instance should be considered non-orphan if there exists
> at least one datatype to which a class type variable in such instance
> is instantiated, because other instances which instantiate such class
> type variable to such datatype would be non-orphan.
>

^^^^     (delete "non-".)
 ... type variable to such datatype would be orphan.

Cheers,

Carlos
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