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?
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