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 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 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 _______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries