Newtype wrappers

[Ian Lynagh]

On Mon, Jan 14, 2013 at 09:03:38PM +0200, Roman Cheplyaka wrote:
> * Simon Peyton-Jones <simonpj at microsoft.com> [2013-01-14 18:09:50+0000]
> > Friends
> > 
> > I'd like to propose a way to "promote" newtypes over their enclosing type.  Here's the writeup
> >           http://hackage.haskell.org/trac/ghc/wiki/NewtypeWrappers
> > 
> > Any comments?
> 
> Why not just have a pseudo-function 'coerce'?
> 
> By pseudo-function I mean something that can be used anywhere (or almost
> anywhere?) where a function can, but is a keyword and doesn't have a
> type. (It'd be similar to ($) as implemented by GHC, I figure.)
> 
> The static semantics would be to compute the "inner" and "outer" types
> to the extent possible, and then behave as if the function was defined
> as a wrapper or unwrapper function for those types. In case when it is
> ambiguous, an error is issued, and the standard tricks can be used to
> refine the type (including annotation coerce itself with a type).

It would be even better if we implemented a syntax for type arguments.
Then, if type application was written "f @ t", you would be able to (or
perhaps required to) write

    coerce @ from_type @ to_type expr


Thanks
Ian