Zooming in MonadState

[nikita-volkov]

I have a function in which I'm trying to use zoom like so:

replaceArtistCascadingly 
  :: Artist -> Artist -> Acid.Update ProcessedCatalogue ()
replaceArtistCascadingly old new = do
  zoom albumsL $ do
    undefined

Acid.Update is an instance of MonadState. However I get the following error:

  No instance for (Functor
                       (Control.Lens.Internal.Zoom.Zoomed
                          (Acid.Update ProcessedCatalogue) [Album]))
      arising from a use of `albumsL'

But if I change the type signature to

Artist -> Artist -> State ProcessedCatalogue ()

the function compiles just fine.

[glguy]

You need to provide the 'type instance' for 'Zoomed (Acid.Update s)' and not just a MonadState instance

[ekmett]

zoom can't (currently) work polymorphically over MonadState. MonadState isn't sufficient for us to be able to swap out the type of state in your monad.

As a somewhat idle aside before I forget:

One option to make this less painful for users is we could make a monad transformer to the supplied monad that switches out the state for the nested context rather than swapping the state type. However, that has a consequence. Right now the correctness of applying zoom to a Traversal hinges on the fact that it can't write to other parts of the state than what is zoomed. That would be lost by the transformer version.

[neongreen]

@ekmett What do you mean by “MonadState isn't sufficient for us to be able to swap out the type of state in your monad”? I thought it meant that you can change the type of the state, but it looks like you can't:

> flip execState (1 :: Int, 2) $ zoom _1 $ id %= show

<interactive>:4:45:
    Couldn't match type ‘Int’ with ‘[Char]’
    arising from a functional dependency between:
      constraint ‘MonadState String (StateT Int Identity)’
        arising from a use of ‘%=’
      instance ‘MonadState s (StateT s m)’ at <no location info>
    In the second argument of ‘($)’, namely ‘id %= show’
    In the second argument of ‘($)’, namely ‘zoom _1 $ id %= show’
    In the expression:
      flip execState (1 :: Int, 2) $ zoom _1 $ id %= show

Or did you just mean that with MonadState we'd only be able to use lenses of the type Lens' s s?

[neongreen]

On further reflection, yeah, of course it's impossible. Nevermind.