Edward Kmett has explained that this isn't sufficient when things go higher order. His suggested improvement is liftCoercion :: Maybe (Coercion a b -> Coercion (f a) (f b))
David FeuerWell-Typed, LLP -------- Original message --------From: Ryan Scott <ryan.gl.sc...@gmail.com> Date: 6/6/17 1:41 PM (GMT-05:00) To: Richard Eisenberg <r...@cs.brynmawr.edu> Cc: GHC developers <ghc-devs@haskell.org>, Eric Mertens <emert...@gmail.com> Subject: Re: Hunting down a compilation performance regression involving type families 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. ----- [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 <r...@cs.brynmawr.edu> wrote: > > > On May 31, 2017, at 5:21 PM, Ryan Scott <ryan.gl.sc...@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 > >
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