Exposing newtype coercions to Haskell
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.de • http://www.joachim-breitner.de/
Jabber: nomeata at joachim-breitner.de • GPG-Key: 0x4743206C
Debian Developer: nomeata at debian.org
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