Re: can/should Functor have a quantified coercible constraint?

3 Jan 2021


      Coercible is a lifted constraint wrapping the unlifted constraint, which I
remember is spelled either ~#r or some other permutation of those
characters. Last I looked, Coercible was *not* unpacked in data
constructors.

On Sun, Jan 3, 2021, 12:12 PM Oleg Grenrus  wrote:
...
I think Mag, regex-applicative etc. examples are all reparable. The main
culprit is however StateT and a like, as you pointed out. It's meaningless
to discuss Mag if we cannot even write Functor m => Functor (StateT s m).
...
Coercible constraints aren't unpacked in data constructors
Aren't they zero-width at run time? That's IMO a bug if that is not true.
- Oleg
On 3.1.2021 19.08, David Feuer wrote:
Mag uses the One it does for efficiency/compactness. Coercible constraints
aren't unpacked in data constructors, sadly. If you're looking for more
examples of slightly-invalid but useful Functors, the first place I'd check
(beyond the very-Mag-like things in lens that inspired Mag) is Roman
Cheplyaka's regex-applicative. I don't know if his lifts coercions or not
(haven't looked in a while) but it does some similarly illegitimate things
for good reasons.
On Sun, Jan 3, 2021, 12:03 PM Oleg Grenrus  wrote:
...
Prelude Control.Monad.Trans.State> :i StateT
    type role StateT nominal representational nominal
Note, `StateT` is nominal in last argument (a). Thus if (forall c d.
Coercible ...) where a Functor superclass, Functor (and thus Monad)
wouldn't be definable for StateT. That would be... unfortunate.
Until there are "higher roles" Functor cannot be Coercible1. It would
rule very simple code.
(OTOH Mag can be repaired,
https://oleg.fi/gists/posts/2019-07-31-fmap-coerce-coerce.html#functor-shoul...
).
- Oleg
On 3.1.2021 18.31, Carter Schonwald wrote:
Hey David,
could you exposit what would go wrong? a concrete proof witness or
explanation would help me a lot. other people might benefit too.
for the stateT s Maybe a, perhaps i'm still waking up this AM, so let me
try
newtype StateT s m a = StateT {runStateT :: s -> m (a, s)}
so this should expand to
'(s -> Maybe (a,s)),'
but the coerce would be on the 'a' here ... so i'm not seeing the issue?
the latter example seem to boil down to "a free appplicative/functor
Gadt" with some extra bits, though i've not worked through to seeing the
unsafety
for the latter examples, the definitions are the following :
traverseBia https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... :: (Traversable t https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica..., Biapplicative https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... p https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...)            => (a https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... -> p https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... b https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... c https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...) -> t https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... a https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... -> p https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... (t https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... b https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...) (t https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... c https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...)traverseBia https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... = inline (traverseBiaWith https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... traverse)
 --------traverseBiaWith https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... :: forall p https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... a https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... b https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... c https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... s https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... t https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica.... Biapplicative https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... p https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...  => (forall f https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... x https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica.... Applicative f https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... => (a https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... -> f https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... x https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...) -> s https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... -> f https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... (t https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... x https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...))  -> (a https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... -> p https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... b https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... c https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...) -> s https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... -> p https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... (t https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... b https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...) (t https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... c https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...)traverseBiaWith https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... trav https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... p https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... s https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... = smash https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... p https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... (trav https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... One https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... s https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...)
-------smash https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... :: forall p https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... t https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... a https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... b https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... c https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica.... Biapplicative https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... p https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...      => (a https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... -> p https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... b https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... c https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...)      -> (forall x https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica.... Mag https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... a https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... x https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... (t https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... x https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...))      -> p https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... (t https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... b https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...) (t https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... c https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...)smash https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... p https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... m https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... = go https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... m https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... m https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...  where    go https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... :: forall x https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... y https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica.... Mag https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... a https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... b https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... x https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... -> Mag https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... a https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... c https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... y https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... -> p https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... x https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... y https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...    go https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... (Pure https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... t https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...) (Pure https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... u https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...) = bipure https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... t https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... u https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...    go https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... (Map https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... f https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... x https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...) (Map https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... g https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... y https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...) = bimap f https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... g https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... (go https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... x https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... y https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...)    go https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... (Ap https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... fs https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... xs https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...) (Ap https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... gs https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... ys https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...) = go https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... fs https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... gs https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... <<*>> https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... go https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... xs https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... ys https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...
#if MIN_VERSION_base(4,10,0)    go https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... (LiftA2 https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... f https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... xs https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... ys https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...) (LiftA2 https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... g https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... zs https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... ws https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...) = biliftA2 https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... f https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... g https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... (go https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... xs https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... zs https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...) (go https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... ys https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... ws https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...)
#endif    go https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... (One https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... x https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...) (One https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... _) = p https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... x https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...    go https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... _ _ = impossibleError https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...
---- and then the magma is -- This is used to reify a traversal for 'traverseBia'. It's a somewhat-- bogus 'Functor' and 'Applicative' closely related to 'Magma' from the-- @lens@ package. Valid traversals don't use (<$), (<*), or (*>), so-- we leave them out. We offer all the rest of the Functor and Applicative-- operations to improve performance: we generally want to keep the structure-- as small as possible. We might even consider using RULES to widen lifts-- when we can:----   liftA2 f x y <*> z ==> liftA3 f x y z,---- etc., up to the pointer tagging limit. But we do need to be careful. I don't-- *think* GHC will ever inline the traversal into the go function (because that-- would duplicate work), but if it did, and if different RULES fired for the-- two copies, everything would break horribly.---- Note: if it's necessary for some reason, we *could* relax GADTs to-- ExistentialQuantification by changing the type of One to----   One :: (b -> c) -> a -> Mag a b c---- where the function will always end up being id. But we allocate a *lot*-- of One constructors, so this would definitely be bad for performance.data Mag https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... a https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... b https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... t https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... where  Pure https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... :: t https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... -> Mag https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... a https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... b https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... t https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...  Map https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... :: (x https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... -> t https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...) -> Mag https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... a https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... b https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... x https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... -> Mag https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... a https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... b https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... t https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...  Ap https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... :: Mag https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... a https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... b https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... (t https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... -> u https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...) -> Mag https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... a https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... b https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... t https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... -> Mag https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... a https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... b https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... u https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...
#if MIN_VERSION_base(4,10,0)  LiftA2 https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... :: (t https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... -> u https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... -> v https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...) -> Mag https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... a https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... b https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... t https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... -> Mag https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... a https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... b https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... u https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... -> Mag https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... a https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... b https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... v https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...
#endif  One https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... :: a https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... -> Mag https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... a https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... b https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... b https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...instance Functor (Mag https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... a https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... b https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...) where  fmap = Map https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...instance Applicative (Mag https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... a https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica... b https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...) where  pure = Pure https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...  (<*>) = Ap https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...
#if MIN_VERSION_base(4,10,0)  liftA2 = LiftA2 https://hackage.haskell.org/package/bifunctors-5.5.9/docs/src/Data.Biapplica...
#endif
On Sun, Jan 3, 2021 at 11:09 AM David Feuer 
wrote:
...
You're not being very imaginative at all. Try out, oh, `StateT s Maybe`.
Or play around with a nice fake functor like the magma used to implement
`traverseBia` in `bifunctors`—pretty sure that won't work out.
On Sun, Jan 3, 2021, 11:00 AM Carter Schonwald <
carter.schonwald@gmail.com> wrote:
...
Hey everyone!
for context, I have some code where I was seeing how far coerce lets me
go to avoid doing wrappers for certain codes,
i found i had to write the following (mapping an operation over to its
newtyped sibling)
```
-- > :t QRA.wither
--- forall a b f . Applicative f => (a -> f (Maybe b)) -> RAList a -> f
(RAList b)
---
wither :: forall a b f . (Applicative f, (forall c d .  Coercible c d
=> Coercible (f c) (f d))  ) =>
        (a -> f (Maybe b)) -> RAList a -> f (RAList b)
wither = \f la ->    coerce     $ QRA.wither f $ coerce la
```
i'd much rather be able to write
```
wither :: forall a b f . (Applicative f) =>
        (a -> f (Maybe b)) -> RAList a -> f (RAList b)
wither = \f la ->    coerce     $ QRA.wither f $ coerce la
```
this seems like it'd be best done via something like changing the
functor class definition to
```
class (forall c d .  Coercible c d => Coercible (f c) (f d))  ) =>
Functor f where ..
```
is there any specific reason why this is not feasible? I cant think of
a GADT where this wouldn't be totally safe to do (because unlike in
foldable, f is in both the domain and co-domain), but maybe i'm not being
imaginative enough?
look forward to learning what our obstacles are to making this happen
for ghc 9.2 :)
-Carter
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