Re: Can I coerce type constructors to Any?

Thanks Simon! That's reassuring. On Fri, Jan 23, 2026 at 09:08:52AM +0000, Simon Peyton Jones via ghc-devs wrote:

Yes I think coercing (f x) to (Any x) should be fine. The main Bad Thing is to change *representation*. But if the kind is just Type on both sides, I think it's fine.

On Fri, 23 Jan 2026 at 08:40, Tom Ellis via ghc-devs wrote:

...

The documentation for Any says:

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The type constructor Any is type to which you can unsafely coerce any lifted type, and back. More concretely, for a lifted type t and value x :: t, unsafeCoerce (unsafeCoerce x :: Any) :: t is equivalent to x.

https://www.stackage.org/haddock/lts-24.28/base-4.20.2.0/GHC-Base.html#t:Any

Is this also true of type constructors? For example, could I unsafeCoerce `f x` to `Any x`? (assuming f maps lifted types to lifted types)

In particular I'm interested in things like

{-# 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

Is that sound? Thanks (and additional thanks to Andreas Klebinger for having a look at this on haskell-cafe),