On December 2, 2010 04:57:45 Conor McBride wrote:
Seems a bit peculiar to me, given that Pointed f basically asserts the inhabitation of f (). Why don't we just have an Inhabited class?
class Inhabited x where something :: x
Then we can build Pointed once for all
instance Inhabited (f ()) => Pointed f where pure x = fmap (uconst x) something where uconst :: x -> () -> x uconst = const <snip> And we can all have fun writing Djinn in type class prolog (didn't Oleg do that already?) until somebody points out that every type is inhabited by bottom.
Doesn't this pretty much bring us back to my earlier email with pure x = fmap (const x) undefined which has obviously problems if fmap tries to take apart the undefined in any non-lazy way (e.g., as it will have to for something like the list monad)? Returning to Poinrted=>Functor vs Functor=>Pointed, it really does seem like there may be something to saying functoriality seems a lot closer to building on pointedness then pointedness is to building on functoriality. Wren said On November 30, 2010 00:32:18 wren ng thornton wrote:
Not all functors are pointed therefore Pointed=>Functor is invalid.
but, as I think your email argued, this may be equally pedantic to not all pointed things are functors as so Functor=>Pointed must also be invalid? Personally, I think something like an embedded DSL might have reason to want to let embed values (i.e., be pointed) but not lift functions to functions over embed values (i.e., be functorial). I think these should be separate. Cheers! -Tyson