Composing and zooming several effects

[expipiplus1]

I found it quite common when writing MTL style code to implement whatever functionality I needed on top of existing types. For example to have a program which can throw errors and record warning, I might nest ExceptT and WriterT in a newtype and hide the internal details that way.

I couldn't seem to find such functionality in effectful, but I'm thinking of doing something like this and am wondering if I'm repeating work done elsewhere, of if there's a cleaner way to do this.

The error/warning example

data ErrorsAndWarnings (e :: Type) :: Effect
type instance DispatchOf (ErrorsAndWarnings e) = Static NoSideEffects
data instance StaticRep (ErrorsAndWarnings e) = ErrorsAndWarnings (StaticRep (Error e)) (StaticRep (Writer [e]))

runErrorsAndWarnings :: forall e es a. Eff (ErrorsAndWarnings e : es) a -> Eff es (Either e a, [e])
runErrorsAndWarnings m = runWriter . runErrorNoCallStack $ do
  w <- getStaticRep
  e <- getStaticRep
  raise . raise $ evalStaticRep (ErrorsAndWarnings e w) m

warn :: forall e es. (ErrorsAndWarnings e :> es) => e -> Eff es ()
warn e = stateStaticRepM \(ErrorsAndWarnings a b) ->
  second (a `ErrorsAndWarnings`) <$> runStaticRep b (tell [e])

fail :: forall e es a. (ErrorsAndWarnings e :> es) => e -> Eff es a
fail e = stateStaticRepM \(ErrorsAndWarnings a b) ->
  second (`ErrorsAndWarnings` b) <$> runStaticRep a (throwError e)

[arybczak]

I might nest ExceptT and WriterT in a newtype and hide the internal details that way.

Yeah, reinterpret is for this. Have a look at the introduction section of the Effectful.Dispatch.Dynamic. runFileSystemPure uses a private State effect not visible from outside.