Hi devs,
with recent iterations of GHC.TypeLits (HEAD) I am struggling to get
something simple working. I have
> data Number nat = KnownNat nat => Number !(Proxy nat)
and want to write
> addNumbers :: Number a -> Number b -> Maybe (Number (a + b))
Unfortunately I cannot find a way to create the necessary KnownNat (a
+ b) constraint.
Declaring the function thus
> addNumbers :: KnownNat (a + b) => Number a -> Number b -> Maybe (Number (a +
> b))
only dodges the problem around.
Also I am wondering where the ability to perform a type equality check
went. I.e. I cannot find the relevant functionality to obtain
> sameNumber :: Number a -> Number b -> Maybe (a :~: b)
I guess there should be some TestEquality instance (for Proxy Nat?, is
this possible at all), but I cannot find it. Same applies for Symbols.
Any hints?
Thanks and cheers,
Gabor
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