Hi, Am Sonntag, den 18.08.2013, 15:24 +0000 schrieb Simon Peyton-Jones:
* Make NT a wired-in Class. We don't have any of these at the moment, but there should be no difficulty in creating one, along the lines of the wired-in TyCons in TysWiredIn. Its data con should have the type NT :: (a ~R# b) -> NT a b
* Also make ntCast a wired-in It (see example in MkId), with an unfolding like ntCast :: NT a b => a -> b ntCast = /\ab. \d:NT a b. \x:a. case d of NT (g : a ~R# b) -> x |> g
thanks; the Core of ntCast looks very much like what I have at the moment (although I still use an intermediate datatype; but that can easily be changed later).
* In the base module, define NT as a data type, and ntCast as a function:
data NT a b where NT :: NT a a
ntCast :: NT a b -> a -> b ntCast NT x = x
These definitions will never get imported, because the definitions are wired-in. And indeed they are slightly wrong: we are declaring NT as a data type not a class, and the type of NT will be NT :: (a ~T# b) -> NT a b
But they may still be used if, for example, we say (map ntCast xs). All that matters is that their execution behaviour is correct, which it will be.
Is there any reason to prefer such “slightly wrong” definitions which are overwritten with built-in stuff over simply putting the type constructors and functions into GHC.Prim?
* We are only going to allow GHC-derived instances of NT.
Just to be sure: Preventing manual instances is currently not possible and such a feature would have to be added to GHC, right?
The instance for NT a a may be super-special: as Iavor mentions we can check for that specially in matchInst. All the others are generated by a 'deriving' clause.
For some reason I’m still trying hard to make NT as un-special as possible, including a regular "NT a a" instance. Let’s see how far I get with that. I hope to have some code that I can show soon.
Happy to discuss. There is probably too much to do, and get settled, for it to be in 7.8 anyway.
I have absolutely no rush for 7.8, so that is not an issue from my side. Greetings, Joachim -- Joachim “nomeata” Breitner mail@joachim-breitner.de • http://www.joachim-breitner.de/ Jabber: nomeata@joachim-breitner.de • GPG-Key: 0x4743206C Debian Developer: nomeata@debian.org