Describe the bug
In my code, I have implemented the following utility which looks similar those of lens:
import Control.Functor.Linear
zoom ::
(Functor m) =>
Optics.Optic_ (Kleisli (Compose ((,) t) (FUN 'One t))) s s t t ->
StateT t m a %1 ->
StateT s m a
{-# INLINE zoom #-}
zoom l (StateT f) = StateT \ !s0 ->
Optic.reifyLens l s0 & \(!t, !back) ->
fmap back <$> f t
infix 4 %=
(%=) ::
(Applicative m) =>
Optics.Optic_ (Kleisli (Compose ((,) t) (FUN 'One t'))) s s t t' ->
(t %1 -> t') %1 ->
StateT s m ()
{-# INLINE (%=) #-}
l %= f = StateT \s0 ->
Optic.reifyLens l s0 & \(!t, !back) ->
pure ((), back $! f t)That is, zoom zoomes internal state with linear lens, and (%=) linearly modifies internal state by lens.
Those combinators should work with Lenses made with Control.Optics.Linear.Lens.lens, but they don't due to the lack of Functor (FUN 'One) instances (both for Data.Functor.Linear and Control.Functor.Linear).
To Reproduce
Suppose we have following:
data Foo where Foo :: Int %1 -> Foo
fooIntL :: Opt.Lens' Foo Int
fooIntL = lens \(Foo i) -> (i, Foo)
stepBy :: Int -> State Foo ()
stepBy i = fooIntL %= (+ i)
This throws a type error:
• No instance for (Control.Functor.Linear.Internal.Class.Functor
(FUN 'One Int))
arising from a use of ‘fooIntL’Expected behavior
This must not throw a type error.
Indeed, the error disappears once we have added the following:
instance D.Functor (FUN 'One t) where
fmap = (.)
instance Functor (FUN 'One t) where
fmap = (.)
Environment
- OS name + version: macOS Ventura 13.4.1
- Version of the code: linear-base 0.3.1 (+ GHC 9.4)
Describe the bug
In my code, I have implemented the following utility which looks similar those of
lens:That is,
zoomzoomes internal state with linear lens, and(%=)linearly modifies internal state by lens.Those combinators should work with
Lenses made withControl.Optics.Linear.Lens.lens, but they don't due to the lack ofFunctor (FUN 'One)instances (both forData.Functor.LinearandControl.Functor.Linear).To Reproduce
Suppose we have following:
This throws a type error:
Expected behavior
This must not throw a type error.
Indeed, the error disappears once we have added the following:
Environment