please elaborate on comment for base:Data.Type.Equality.(==)?

[Richard Eisenberg]

Say I have

> data Nat = Zero | Succ Nat
> data SNat :: Nat -> * where
>   SZero :: SNat Zero
>   SSucc :: SNat n -> SNat (Succ n)
> data SBool :: Bool -> * where
>   SFalse :: SBool False
>   STrue :: SBool True

Now, I want

> eq :: SNat a -> SNat b-> SBool (a == b)
> eq SZero SZero = STrue
> eq (SSucc _) SZero = SFalse
> eq SZero (SSucc _) = SFalse
> eq (SSucc c) (SSucc d) = eq c d

Does that type check?

Suppose we have

> type family EqPoly (a :: k) (b :: k) :: Bool where
>   EqPoly a a = True
>   EqPoly a b = False
> type instance a == b = EqPoly a b

(Let's forget that the instance there would overlap with any other instance.)

Now, in the last line of `eq`, we have that the type of `eq c d` is `SBool (e == f)` where (c :: SNat e), (d :: SNat f), (a ~ Succ e), and (b ~ Succ f). But, does ((e == f) ~ (a == b))? It would need to for the last line of `eq` to type-check. Alas, there is no way to proof ((e == f) ~ (a == b)), so we're hosed.

Now, suppose

> type family EqNat a b where
>   EqNat Zero Zero = True
>   EqNat (Succ n) (Succ m) = EqNat n m
>   EqNat Zero (Succ n) = False
>   EqNat (Succ n) Zero = False
> type instance a == b = EqNat a b

Here, we know that (a ~ Succ e) and (b ~ Succ f), so we compute that (a == b) ~ (EqNat (Succ e) (Succ f)) ~ (EqNat e f) ~ (e == f). Huzzah!

Thus, the second version is better.

I hope this helps!
Richard

On Feb 4, 2014, at 1:08 PM, Nicolas Frisby <nicolas.frisby at gmail.com> wrote:

> [CC'ing Richard, as I'm guessing he's the author of the comment.]
> 
> I have a question regarding the comment on the type family Data.Type.Equality.(==).
> 
>   "A poly-kinded instance [of ==] is not provided, as a recursive definition for algebraic kinds is generally more useful."
> 
> Can someone elaborate on "generally more useful".
> 
> Thank you.

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