Hrm. It's a shame that supporting this map/coerce RULE causes such pain.

This makes me wonder: can we get rid of this RULE? Eric Mertens pointed out a trick [1] that's used in the profunctors library to make mapping coerce over certain Profunctors more efficient. To adapt this trick for Functor, we'd need to add another class method:

    class Functor f where
        fmap :: (a -> b) -> f a -> f b
        (<#>) :: Coercible a b => (a -> b) -> f a -> f b
        (<#>) = \f -> \p -> p `seq` fmap f p

Now, when implementing Functor instances, if we are working with a datatype whose role is representational or phantom, we can make (<#>) really fast:

    data List a = Nil | Cons a (List a)
    instance Functor List where
        fmap = ...
        (<#>) = coerce

Now, instead of relying on (map MkNewtype Nil) to rewrite to Nil, we can just use (MkNewtype <#> Nil)! No map/coerce RULE necessary :)

OK, I realize that suggesting that we remove the RULE is perhaps a touch too far. But it does sting that we have to pay hefty compilation penalties because of its existence...

Ryan S.
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[1] http://hackage.haskell.org/package/profunctors-5.2/docs/Data-Profunctor-Unsafe.html#v:-35-.

On Wed, May 31, 2017 at 7:25 PM, Richard Eisenberg <rae@cs.brynmawr.edu> wrote:

> On May 31, 2017, at 5:21 PM, Ryan Scott <ryan.gl.scott@gmail.com> wrote:
> Does you know what might be going on here?

I think so, but I don't know how to fix it.

The commit you found (thank you!) makes simple_opt_expr (the "simple optimizer", run directly after desugaring, even with -O0) a little more selective in what `case` expressions it throws away. Previous to that commit, the optimizer would throw away a `case error "deferred type error" of _ -> ...` which is terrible. It seems that you have discovered that we are now too timid in throwing away unhelpful cases. It would be interesting to know what the newly-retained cases look like, so that we might throw them away.

But there remains a mystery: Why do we need this code at all? Reading Note [Getting the map/coerce RULE to work] closely, it seems we need to simplify

  forall a b (co :: a ~R# b).
    let dict = MkCoercible @* @a @b co in
    case Coercible_SCSel @* @a @b dict of
      _ [Dead] -> map @a @b (\(x :: a) -> case dict of
         MkCoercible (co :: a ~R# b) -> x |> co) = let dict = ... in ...

to

  forall a b (co :: a ~R# b).
    map @a @b (\(x :: a) -> x |> co) = \(x :: [a]) -> x |> [co]

Part of doing so is to drop the `case Coercible_SCSel ...`, which gets in the way. The mystery is why this needs special code -- shouldn't the eliminate-case-of-known-constructor do the trick? This would require unfolding Coercible_SCSel. Does that happen? It would seem not... but maybe it should, which would remove the special-case code that I changed in that commit, and quite likely would simplify much more code besides.

So: Is Coercible_SCSel unfolded during simple_opt? If not, what wonderful or terrible things happen if we do? If so, why does case-of-known-constructor not work here? My guess is that answering these questions may solve the original problem, but this guess could be wrong.

Richard