[Haskell-cafe] Haskell spec vs FlexibleInstances

[Karl Voelker]

The spec doesn't allow multi-parameter type classes at all, so I don't see how you could run into this problem while writing extensionless Haskell.

I guess it's still a valid question whether the MultiParamTypeClasses extension should allow repeated type parameters.

-Karl

On Feb 18, 2014, at 12:53 PM, AntC <anthony_clayden at clear.net.nz> wrote:

>> Doug McIlroy <doug <at> cs.dartmouth.edu> writes:
>> 
>> What potential evil motivates  the prohibition of repeated type
>> parameters in an instance declaration?
> 
> Hi Doug, 
> 
> I guess the first reason would be to catch accidental duplicate names of 
> typevars.
> 
> But as to "potential evils", the only point of such an instance:
> 
>    instance C a a where ...
> 
> would be to have another instance:
> 
>    instance C a b where ...
> 
> So we're into overlapping instances. (And discriminating on type equality -
> - which only works if the types are grounded. See the HList paper, section 
> 9 'By chance or design?')
> 
> Overlapping instances like that will probably need FunDeps. So the more 
> recent (and probably more blessed) approach is to use the Closed type 
> families/branched instances doo-hicky upcoming with GHC 7.8.
> http://www.haskell.org/haskellwiki/GHC/Indexed_types#Type_instance_declarat
> ions
> 
> AntC
> 
> 
> 
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