PureScript モナド変換子 MaybeT 例

MaybeTExample.purs

module MaybeTExample
  ( main
  ) where

import Control.Monad.Maybe.Trans (MaybeT(..), lift, runMaybeT)
import Data.Maybe (Maybe(..))
import Effect (Effect)
import Effect.Class.Console (log)
import Prelude (Unit, discard, map, pure, unit, void)

-- https://pursuit.purescript.org/packages/purescript-transformers/4.1.0/docs/Control.Monad.Maybe.Trans#t:MaybeT
-- newtype MaybeT m a = MaybeT (m (Maybe a))
-- https://pursuit.purescript.org/packages/purescript-transformers/4.1.0/docs/Control.Monad.Maybe.Trans#v:runMaybeT
-- runMaybeT :: ∀ m a. MaybeT m a → m (Maybe a)
main :: Effect Unit
main =
  let
    expr1 :: Unit
    expr1 = unit
    expr1' :: MaybeT Effect Unit
    expr1' = pure expr1

    expr2 :: Maybe Unit
    expr2 = Just unit
    expr2' :: MaybeT Effect Unit
    expr2' = MaybeT (pure expr2)

    expr3 :: Effect Unit
    expr3 = log "expr3"
    expr3' :: MaybeT Effect Unit
    expr3' = lift expr3 -- or MaybeT (map Just expr3)

    expr4 :: Effect (Maybe Unit)
    expr4 = map Just (log "expr4")
    expr4' :: MaybeT Effect Unit
    expr4' = MaybeT expr4

    block :: MaybeT Effect Unit
    block = do
      expr1'
      expr2'
      expr3'
      expr4'

    main' :: Effect (Maybe Unit)
    main' = runMaybeT block
  in
    void main'