Exposing newtype coercions to Haskell
simonpj at microsoft.com
Tue Jul 2 18:28:49 CEST 2013
| 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.
This clearly deserves treatment on the wiki page.
The criterion "could you write it by hand?" remains a good one.
More information about the Glasgow-haskell-users