What is the difference between the Reader and Input effects?

[eldritch-cookie]

looking at Effectful.Reader.Static and Effectful.Input.Static runReader has the same type as runInput with the only difference i can see is that runInput is point-free, ask and input look the same. is the only difference the HasCallStack constraint?

[tomjaguarpaw]

You mean https://hackage-content.haskell.org/package/effectful-core-2.7.1.2/docs/Effectful-Reader-Static.html for your first link.

---

One difference is that Reader has local, so it can be locally overridden. Based on the GADT defining the dynamic variants, I think that's the only difference:

• https://hackage-content.haskell.org/package/effectful-core-2.7.1.2/docs/Effectful-Input-Dynamic.html#t:Input
• https://hackage-content.haskell.org/package/effectful-core-2.7.1.2/docs/Effectful-Reader-Dynamic.html#t:Reader

[arybczak]

Input has no local, so you can't override the initial value within runInput, that's the only difference.

Note that this only applies to the static variant, because with dynamic Input you can override the value (well, the handler that gives you the value to be precise) with interpose.

[tomjaguarpaw]

That's very interesting! Perhaps the Haddock could show an example? Is it possible to, for example, use Input to read the next line of input from the terminal?

[tomjaguarpaw]

Oh OK, it can be done like this

{-# LANGUAGE GHC2021 #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE MonoLocalBinds #-}

import Effectful
import Effectful.Input.Dynamic
import Effectful.Dispatch.Dynamic

example :: IOE :> es => Eff (Input String : es) r -> Eff es r
example = interpret  $ \_ Input -> liftIO getLine

-- ghci> main
-- hello
-- world
-- hello world
main :: IO ()
main = runEff $ example $ do
  a <- input
  b <- input
  liftIO (putStrLn (a ++ " " ++ b))