Re: newtype coercion wrapping status

Hi, Am Samstag, den 07.09.2013, 12:16 -0400 schrieb Richard Eisenberg:

On Sep 6, 2013, at 12:47 PM, Simon Peyton-Jones wrote: I've always lived under the impression that changing the word "newtype" to "data" should have exactly 0 effect on the compile-time behavior of Haskell programs.

I agree, that is why I raised the point.

Yet, you're proposing that

...

module Foo (T) where -- MkT is *not* exported

newtype T a = MkT [a]

is different from the same with "newtype" replaced with "data". To wit, in the "data" version, the instance (Coercible a b => Coercible (T a) (T b)) would exist, but this would not be derivable in the "newtype" version. Right?

In the current code, the instance Coercible a b => Coercible (T a) (T b) is available for both data and newtypes, if T’s type argument has Representational role, but independent of any constructor presence. See the note at https://github.com/nomeata/ghc/compare/ntclass-clean#L9R1902 for a concise and complete list of the conditions for a Coercible instance.

A related issue is that, even with roles, I don't think GeneralizedNewtypeDeriving (GND) is in the Safe Haskell subset because it can break abstraction barriers -- you can use GND even when a newtype's constructor is not in scope. The above proposal for coercions will have the same problem.

What to do? Map should certainly not export its constructor(s). Yet, we want (Coercible a b => Coercible (Map x a) (Map x b)). It seems that the writer of Map would have to explicitly export this instance. This goes at odds with the idea of "there aren't any instances of Coercible, really", but otherwise I have a hard time seeing how this would all work.

If I understood Simon’s last suggestion correctly than exporting a type constructor with a non-Nominal role means “I am fine if you cast this argument”. If this is not desired (e.g. maybe Ptr a is an example here), then the library author has to annotate the type argument as Nominal. Greetings, Joachim -- Joachim Breitner e-Mail: mail@joachim-breitner.de Homepage: http://www.joachim-breitner.de ICQ#: 74513189 Jabber-ID: nomeata@joachim-breitner.de