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@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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