PROPOSAL: Make Data.Type.Bool.If polykinded

[Merijn Verstraaten]

Hi Richard,

It would appear that you’re right! I had expected GHC to infer the kind Constraint for () from the second argument, rather than reporting an error, but adding an explicit annotation works. I would still say that warrants a clearer error message from GHC, but at least it works :)

Cheers,
Merijn

On 02 Sep 2014, at 01:20 , Richard Eisenberg <eir at cis.upenn.edu> wrote:
> Hi Merijn,
> 
> I believe that `If` is already the way you want:
> 
>  λ> :k If
>  If :: Bool -> k -> k -> k
> 
> The problem in your code is that GHC is a little... er... unprincipled about the kind of `()`. It basically assumes that `()` is of kind `*` unless it is very, absolutely, abundantly obvious that it should have kind `Constraint`. Your code is not abundantly obvious enough in this regard. If you say `If cond (() :: Constraint) (...) => ...`, I believe your code will work.
> 
> Does this in fact work for you?
> 
> Richard
> 
> On Sep 1, 2014, at 11:41 PM, Merijn Verstraaten <merijn at inconsistent.nl> wrote:
> 
>> Ola!
>> 
>> Currently we have:
>> 
>> type family If cond tru fls where
>>   If True tru fls = tru
>>   If False tru fls = fls
>> 
>> Unfortunately, this appears to turned into a monomorphicly kinded type family by GHC, which means it’s impossible to do
>> 
>> foo :: If cond () (“Condition does not hold” ~ “”) => Foo -> Bar
>> 
>> or similar fancy constraints.
>> 
>> I hereby propose altering If to:
>> 
>> type family If (cond :: Bool) (tru :: k) (fls :: k) :: k where
>>  If True tru fls = tru
>>  If False tru fls = fls
>> 
>> Allowing it to work with kinds other than *.
>> 
>> Discussion period: 2 weeks? Seems like a minor change.
>> 
>> Cheers,
>> Merijn
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>> Libraries at haskell.org
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