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 m...@joachim-breitner.de • http://www.joachim-breitner.de/ Jabber: nome...@joachim-breitner.de • GPG-Key: 0x4743206C Debian Developer: nome...@debian.org
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