Idea: Coercible trait

[DemiMarie]

GHC implements a two-paramenter type class called Coercible and a corresponding safe function coerce, with the following types:

-- Instances of this class exist only if a and b
-- have the same runtime representations AND
-- all constructors in A and B are in scope
class Coercible a b

-- | compiler built-in – not really implemented this way
coerce :: Coercible a b => a -> b
coerce = unsafeCoerce

This allows the following:

import Data.Coerce

newtype X a = X a

changeList :: [X a] -> [a]
changeList = coerce

Without coerce, this would require unsafeCoerce (GHC's version of mem::transmute) or an O(n) mapping over the list.

In Rust, the same problem can arise. It would be nice to support something like this.

[glaebhoerl]

It has been proposed multiple times (originally by me): #270

[Centril]

Closing as duplicate.