[Haskell-cafe] Type equality proof

[Martijn van Steenbergen]

Conor McBride wrote:
>> instance Category (:=:) where
>>    id = Refl
>>    Refl . Refl = Refl
> 
> That and the identity-on-objects functor to sets and
> functions.

Not sure what you mean by this, Conor. Can you please express this in 
Haskell code?

Thanks,

Martijn.