Hi, 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.
Added.
> The criterion "could you write it by hand?" remains a good one.
Agreed.
Greetings,
Joachim
--
Joachim “nomeata” Breitner
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