[Haskell-cafe] Re: Soundness of forall mixed with Coercible

12 Dec 2025

      Yeah, I think this is my intuition which I was trying to get at with my 
"isn't this Void" comment - it seems to me that in all cases where you 
have a `forall a. f a` that would be the same thing as having an `f 
Void`, i.e. the `a` is never relevant or alternatively the f a inside a 
Forall will never actually contain an a.

Is there still something else that I'm missing?

What's the concrete use case for Forall?

On 12/12/25 16:03, Zemyla wrote:
...
Wouldn't the only inhabited value of Forall [] be []?
On Fri, Dec 12, 2025, 02:18 Adam Gundry  wrote:
Hi Tom,
You may be interested in a ticket where a similar issue was discussed
    before: https://gitlab.haskell.org/ghc/ghc/-/issues/20048
Given `forall a . Coercible (f a) (g a)` it seems intuitively obvious
    that it should be sound to derive `Coercible f g`, but Core's typing
    rules don't currently allow it. Since Core is proof irrelevant it is
    probably safe just to add it, but actually proving soundness would
    take
    a bit of work.
That said, you are asking about the subtly different
     Coercible (forall a . f a) (forall a . g a)
which is derivable from `Coercible f g`, but in fact I think is also
    derivable from `forall a. Coercible (f a) (g a)` in today's Core type
    system. The difficulty seems to be that GHC's constraint solver for
    equality/Coercible constraints is incomplete in the presence of
    quantified constraints. So unsafeCoerce seems defensible in this case.
Cheers,
Adam
On 11/12/2025 17:58, Tom Ellis wrote:
    > Oh, I forgot to give the definition of Forall, so I may as well
    define
    > it in its full glory, imports, extensions and all:
    >
    > {-# LANGUAGE QuantifiedConstraints #-}
    >
    > import Data.Coerce (Coercible)
    > import Data.Type.Coercion (Coercion (Coercion))
    > import GHC.Exts (Any)
    > import Unsafe.Coerce (unsafeCoerce)
    >
    > newtype Forall f = MkForall (forall a. f a)
    >
    > forallCoercible ::
    >    forall f g.
    >    (forall a. Coercible (f a) (g a)) =>
    >    Coercion (Forall f) (Forall g)
    > forallCoercible =
    >    unsafeCoerce (Coercion @(f Any) @(g Any))
    >
    > On Thu, Dec 11, 2025 at 05:47:25PM +0000, Tom Ellis wrote:
    >>      forallCoercible ::
    >>        forall f g.
    >>        (forall a. Coercible (f a) (g a)) =>
    >>        Coercion (Forall f) (Forall g)
    >>      forallCoercible =
    >>        unsafeCoerce (Coercion @(f Any) @(g Any))
-- 
    Adam Gundry, Haskell Consultant
    Well-Typed LLP, https://www.well-typed.com/
Registered in England & Wales, OC335890
    27 Old Gloucester Street, London WC1N 3AX, England
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[Haskell-cafe] Re: Soundness of forall mixed with Coercible

Georgi Lyubenov