Narrowing existing effects

[wolverian]

Is it idiomatic to want to narrow down the possible actions in an effect? In my specific case, I'd like to expose a FileSystem effect that only allows createDirectory calls. I can write the effect entirely myself, or I can arrange this with re-exports of Effectful.FileSystem, but I have a few questions:

• Just re-exporting Effectful.FileSystem.createDirectory allows the user to import Effectful.FileSystem separately and add other actions, e.g. removeDirectory. Is there a way to make this safer?
• What about reinterpretation and algebraic effects? Take the FileSystem effect in the example from Effectful.Dispatch.Dynamic. I might want to expose a read-only version of this effect; how should I do that?

[ocharles]

One option you have is to forget about effectful at all here and just pass the function in:

module M where

onlyCreateDirectory :: (FilePath -> Eff es ()) -> Eff es ()
onlyCreateDirectory createDirectory = ...

module N where

import M qualified
import Effectful.FileSystem

controller :: FileSystem :> es => Eff es ()
controller = M.onlyCreateDirectory createDirectory

[wolverian]

Thanks! This inverts my desired relationship between M and N: I want N to not have access to Effectful.FileSystem.

[dark-ether]

what would be the problem in creating a separate effect and using an existing effect in its handler with reinterpret?

[wolverian]

Verbosity and the opportunity to make mistakes, but perhaps it's the best approach.