Purescript Eff Monad: using non-native computational effects

Question

I want to be able to write

x :: Eff (reader :: Reader Int, maybe :: Maybe) Int
x = do
  config <- ask -- configuration from (Reader Int) monad
  just config -- using (Maybe) Monad

runX :: Int
runX = runPure (runMaybe doIfNothing (runReader 6 x)) -- outputs: 6

using the Eff Monad

Is this possible to do using Eff?

If not how can we make it work not using Eff?

Phil Freeman · Accepted Answer · 2017-04-12 19:22:16Z

1

You can use the MaybeT and ReaderT monad transformers on top of Eff, but you cannot match the two in the way you wrote above:

import Prelude
import Data.Maybe
import Control.Monad.Eff
import Control.Monad.Eff.Class
import Control.Monad.Eff.Console
import Control.Monad.Maybe.Trans
import Control.Monad.Reader.Trans

x :: ReaderT Int (MaybeT (Eff (console :: CONSOLE))) Int
x = do
  liftEff (log "Asking...")
  config <- ask
  pure config

main :: Eff (console :: CONSOLE) (Maybe Int)
main = runMaybeT (runReaderT x 6)

answered Apr 12, 2017 at 19:22

Phil Freeman

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1 Comment

user47376 Over a year ago

I don't like monad transformers, Iv'e seen some talks where something like Purescript's Eff monad was used with monads like Reader, but I don't know if it's possible to do in Purescript, maybe we need to define a new monad similar to Eff.

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