+1 This would be very easy to implement, too.
But I suggest a different name. Ground? Terminating? NormalForm? Irreducible? ValueType? I don't love any of these, but I love Sane less. On Jan 24, 2016, at 4:24 PM, David Feuer <david.fe...@gmail.com> wrote: > Since type families can be stuck, it's sometimes useful to restrict > things to sane types. At present, the most convenient way I can see to > do this in general is with Typeable: > > type family Foo x where > Foo 'True = Int > > class Typeable (Foo x) => Bar x where > blah :: proxy x -> Foo x > > This will prevent anyone from producing the bogus instance > > instance Bar 'False where > blah _ = undefined > > Unfortunately, the Typeable constraint carries runtime overhead. One > possible way around this, I think, is with a class that does just > sanity checking and nothing more: > > class Sane (a :: k) > instance Sane Int > instance Sane Char > instance Sane 'False > instance Sane 'True > instance Sane '[] > instance Sane '(:) > instance Sane (->) > instance Sane 'Just > instance Sane 'Nothing > instance (Sane f, Sane x) => Sane (f x) > > To really do its job properly, Sane would need to have instances for > all sane types and no more. An example of an insane instance of Sane > would be > > instance Sane (a :: MyKind) > > which would include stuck types of kind MyKind. > > Would it be useful to add such an automatic-only class to GHC? > > David > _______________________________________________ > Libraries mailing list > librar...@haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
