Also worth remembering here is that ghc will often unpack such things if it can determine it can do so (but only with optimization iirc), so there is no guarantee that the two will be representationally equivalent even without pragmas. On Mon, Feb 25, 2019 at 10:17 AM Oleg Grenrus wrote:

Two data types are `Coercible` (as in Data.Coerce) if their representation in memory is the same: they are representiationally equivalent You ask for looser structural isomorphism, consider e.g.

data B b = B {-# UNPACK #-} !Int b

and

(Int, b)

The values of these types have quite different representation/memory layout.

- Oleg

On 25 Feb 2019, at 16.38, Georgi Lyubenov wrote:

Greetings!

Is there any reason behind/what is the reason behind ``` data A a b = A a b ``` and ``` (a, b) ```

not being coercible?

And in general all n-ary constructors not being coercible to one another?

Thanks in advance!

======= Georgi

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