Re: Can I coerce type constructors to Any?

On Thu, Jan 22, 2026 at 11:20:28PM +0100, Andreas Klebinger via Haskell-Cafe wrote:
> > (I don't think what Andreas said can *literally* be correct, because
> surely it's safe to unsafeCoerce from a to b when `Coercible a b` is in
> scope?)
> 
> Yes I was perhaps a bit too restrictive. The docs in fact explicitly mention
> this case among some others. I don't see how it could go wrong here.
> In fact the docs mention this case in particular! (https://hackage-content.haskell.org/package/base-4.22.0.0/docs/Unsafe-Coerce.html#v:unsafeCoerce):
> 
> > (superseded) Casting between two types which have exactly the same
> structure: between a newtype of T and T, or between types which differ only
> in "phantom" type parameters. It is now preferred to use |coerce <https://hackage-content.haskell.org/package/base-4.22.0.0/docs/Data-Coerce.html#v:coerce>|
> from |Data.Coerce|, which is type-safe.
> 
> But really there is no reason to use dirty tricks like unsafeCoerce in this
> situation to begin with when `coerce` is right there!

Yes, agreed.  My objection was a bit of a technicality, but since I'm
in unfamiliar and dangerous territory technicalities are all I have to
hold on to.

> -----------------
> 
> The concrete example sadly doesn't seem to compile.

Ah, you're right. Here's a version which does compile.

{-# LANGUAGE QuantifiedConstraints #-}

import GHC.Exts (Any)
import Data.Kind (Type)
import Unsafe.Coerce (unsafeCoerce)

data C (f :: Type -> Type -> Type) where
  MkC :: (forall a. Monad (f a)) => C f

newtype CD (f :: Type -> Type -> Type) = MkCD (C Any)

putCD :: C f -> CD f
putCD c = MkCD (unsafeCoerce c)

getCD :: CD f -> C f
getCD (MkCD cd) = unsafeCoerce cd


> And type level edge cases are really not my speciality so maybe
> someone else can make more definite statements there.

OK, thanks very much for taking a look!

Tom
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