Exposing newtype coercions to Haskell

Joachim Breitner mail at joachim-breitner.de
Wed Jul 3 10:01:10 CEST 2013


Am Dienstag, den 02.07.2013, 16:28 +0000 schrieb Simon Peyton-Jones:
> | I also noticed a problem with my logic for creating the NT-lifting
> | function.  Suppose
> |   data C a = MkC (Foo a)
> | Just having the constructors of C in scope is not sufficient
> | to safely provide
> |         NT a b -> NT (C a) (C b)
> | as the parameters of the constructor might wrap a in another type
> | constructor, eg
> |         data Foo a = Foo (Set a)
> | 
> | then we certainly don’t want the user to be able to write
> |         deriving fooNT :: NT a b -> NT (Foo a) (Foo b)
> Dually, suppose Foo was an *abstract* type, where we can't see the constructors of Foo.  But the programmer as exported fooNT :: NT a b -> NT (Foo a) (Foo b).  Then we *do* want to be able to derive
> 	cNT :: NT a b -> NT (C a) (C b)
> Instead maybe we say
> 	deriving cNT :: NT a b -> NT (C a) (C b) using( fooNT )
> listing the imported witnesses that we use.  Or maybe we say simply
> 	deriving cNT :: NT a b -> NT (C a) (C b) 
> and magically use any NT-typed things that are in scope.

Is this really the compiler’s job here? After all, the programmer would
be able to write

        deriving cNT' :: NT (Foo a) (Foo b) -> NT (C a) (C b)
        cNT :: NT a b -> NT (C a) (C b)
        cNT = cNT' . fooNT

and expose just cNT to his users, so no expressiveness is lost by not
providing automatic support here.

> This clearly deserves treatment on the wiki page.


> The criterion "could you write it by hand?" remains a good one.



Joachim “nomeata” Breitner
  mail at joachim-breitner.dehttp://www.joachim-breitner.de/
  Jabber: nomeata at joachim-breitner.de  • GPG-Key: 0x4743206C
  Debian Developer: nomeata at debian.org
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