RE: Kinds of type synonym arguments

newtype T = MkT Int# Provided T :: # (i.e. unlifted), I don’t think this would be too hard. That is, you can give a new name (via newtype) to an unlifted type like Int#, Float#, Double# etc. Worth a wiki page and a ticket. Simon From: ghc-devs [mailto:ghc-devs-bounces@haskell.org] On Behalf Of Edward Kmett Sent: 21 December 2015 09:10 To: Ömer Sinan Ağacan Cc: ghc-devs Subject: Re: Kinds of type synonym arguments I brought up the subject of allowing newtypes in kind # (or even in any kind that ends in * or # after a chain of ->'s to get more powerful Coercible instances) at ICFP this year and Simon seemed to think it'd be a pretty straightforward modification to the typechecker. I confess, he's likely waiting for me to actually sit down and give the idea a nice writeup. ;) This would be good for many things, especially when it comes to improving the type safety of various custom c-- tricks. -Edward On Sun, Dec 20, 2015 at 2:14 PM, Ömer Sinan Ağacan mailto:omeragacan@gmail.com> wrote: I have another related question: What about allowing primitive types in newtypes? λ:4> newtype Blah1 = Blah1 Int λ:5> newtype Blah2 = Blah2 Int# <interactive>:5:23: error: • Expecting a lifted type, but ‘Int#’ is unlifted • In the type ‘Int#’ In the definition of data constructor ‘Blah2’ In the newtype declaration for ‘Blah2’ Ideally second definition should be OK, and kind of Blah2 should be #. Is this too hard to do? 2015-12-16 17:22 GMT-05:00 Richard Eisenberg mailto:eir@cis.upenn.edu>:

On Dec 16, 2015, at 2:06 PM, Ömer Sinan Ağacan mailto:omeragacan@gmail.com> wrote:

...

In any case, this is not that big deal. When I read the code I thought this should be a trivial change but apparently it's not.

No, it's not. Your example (`f :: (Int#, b) -> b`) still has an unboxed thing in a boxed tuple. Boxed tuples simply can't (currently) hold unboxed things. And changing that is far from trivial. It's not the polymorphism that's the problem -- it's the unboxed thing in a boxed tuple.

Richard

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