[Haskell-cafe] in-equality type constraint?

[Paul L]

HList certainly provides an alternative. But given the use of
UndecidableInstances and OverlappingInstances, I was hoping that type
families could come a little cleaner. Or does it not matter?

On Fri, Jul 16, 2010 at 8:32 PM, Steffen Schuldenzucker
<sschuldenzucker at uni-bonn.de> wrote:
> On 07/17/2010 01:08 AM, Paul L wrote:
>> Does anybody know why the type families only supports equality test
>> like a ~ b, but not its negation?
>>
>
> This has annoyed me, too. However, HList provides something quite similar,
> namely the TypeEq[1] fundep-ed class which will answer type-equality with a
> type-level boolean. (this is actually more powerful than a simple constraint,
> because it allows us to introduce type-level conditionals)
>
> To turn it into a predicate, you can use something like
>
> (disclaimer: untested)
>
>> class C a b c where  -- ...
>>
>> -- for some reason, we can provide an instance C a b [c] *except* for
>> -- a ~ c.
>> instance (TypeEq a c x, x ~ HFalse) => a b [c] where  -- ...
>
> Best regards,
>
> Steffen
>
> [1]
> http://hackage.haskell.org/packages/archive/HList/0.2.3/doc/html/Data-HList-FakePrelude.html#t%3ATypeEq
> (Note that for it to work over all types, you have to import one of the
> Data.HList.TypeEqGeneric{1,2} modules)
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-- 
Regards,
Paul Liu

Yale Haskell Group
http://www.haskell.org/yale