Should coercion binders (arguments or binders in patterns) be TyVars?

[Ömer Sinan Ağacan]

> > Think of a hypothetical coercion `co :: Int ~ Double`; applying that coercion
> > as in `x |> co` to `x :: Int` would need to `fild` (load the integer in a
> > floating point register) at run-time, so you can't erase it. The fact that we
> > can for newtypes is because `coerce` is basically just the `id` function at
> > runtime.
>
> Do we even support such coercions? I think a `co :: Int ~ Double` can only be an
> unsafe coercion, which are basically no-op in runtime, so again they can be
> erased.

To elaborate, for coercions that have a meaning in runtime I think we use
primops, type coercions are not used for this purpose. E.g. for Int to Double
coercion, if you really mean boxed Ints and Doubles then you can simply treat
Int as Double in runtime (and enjoy the segfaults or worse), if you mean Int# to
Double# then you use a primop like int2Double# which does `fild` if necessary.

Ömer

Ömer Sinan Ağacan <omeragacan at gmail.com>, 6 Eki 2019 Paz, 12:55
tarihinde şunu yazdı:
>
> > I'm not sure if there's a case in GHC (yet, because newtype coercions are
> > zero-cost), but coercions in general (as introduced for example in Types and
> > Programming Languages) can carry computational content and thus can't be
> > erased.
>
> But they're already erased! They don't have a runtime representation and the
> unarise pass eliminates coercion values and binders. See the last paragraph in
> my original email.
>
> > Think of a hypothetical coercion `co :: Int ~ Double`; applying that coercion
> > as in `x |> co` to `x :: Int` would need to `fild` (load the integer in a
> > floating point register) at run-time, so you can't erase it. The fact that we
> > can for newtypes is because `coerce` is basically just the `id` function at
> > runtime.
>
> Do we even support such coercions? I think a `co :: Int ~ Double` can only be an
> unsafe coercion, which are basically no-op in runtime, so again they can be
> erased.
>
> Let me know if I'm mistaken.
>
> Ömer
>
> Sebastian Graf <sgraf1337 at gmail.com>, 6 Eki 2019 Paz, 12:50 tarihinde
> şunu yazdı:
> >
> > Hi Ömer,
> >
> > I'm not sure if there's a case in GHC (yet, because newtype coercions are zero-cost), but coercions in general (as introduced for example in Types and Programming Languages) can carry computational content and thus can't be erased.
> >
> > Think of a hypothetical coercion `co :: Int ~ Double`; applying that coercion as in `x |> co` to `x :: Int` would need to `fild` (load the integer in a floating point register) at run-time, so you can't erase it. The fact that we can for newtypes is because `coerce` is basically just the `id` function at runtime.
> >
> > Cheers,
> > Sebastian
> >
> > Am So., 6. Okt. 2019 um 10:28 Uhr schrieb Ömer Sinan Ağacan <omeragacan at gmail.com>:
> >>
> >> Hi,
> >>
> >> I just realized that coercion binders are currently Ids and not TyVars (unlike
> >> other type arguments). This means that we don't drop coercion binders in
> >> CoreToStg. Example:
> >>
> >>     {-# LANGUAGE ScopedTypeVariables, TypeOperators, PolyKinds, GADTs,
> >>        TypeApplications, MagicHash #-}
> >>
> >>     module UnsafeCoerce where
> >>
> >>     import Data.Type.Equality ((:~:)(..))
> >>     import GHC.Prim
> >>     import GHC.Types
> >>
> >>     unsafeEqualityProof :: forall k (a :: k) (b :: k) . a :~: b
> >>     unsafeEqualityProof = error "unsafeEqualityProof evaluated"
> >>
> >>     unsafeCoerce :: forall a b . a -> b
> >>     unsafeCoerce x = case unsafeEqualityProof @_ @a @b of Refl -> x
> >>
> >> If I build this with -ddump-stg this is what I get for `unsafeCoerce`:
> >>
> >>     UnsafeCoerce.unsafeCoerce :: forall a b. a -> b
> >>     [GblId, Arity=1, Unf=OtherCon []] =
> >>         [] \r [x_s2jn]
> >>             case UnsafeCoerce.unsafeEqualityProof of {
> >>               Data.Type.Equality.Refl co_a2fd -> x_s2jn;
> >>             };
> >>
> >> See the binder in `Refl` pattern.
> >>
> >> Unarise drops this binder because it's a "void" argument (doesn't have a runtime
> >> representation), but still it's a bit weird that we drop types but not coercions
> >> in CoreToStg.
> >>
> >> Is this intentional?
> >>
> >> Ömer
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