Nesting of Monads

TransformersExample.js

exports.fetchPayload = null
exports.readHostNameFromSTDIN = null;

TransformersExample.purs

module TransformersExample where

import Prelude

import Control.Monad.Except.Trans (ExceptT(..))
import Control.Monad.State.Class (get)
import Control.Monad.State.Trans (StateT(..), evalStateT, put)
import Control.Monad.Trans.Class (lift)
import Data.Either (Either)
import Data.Newtype (wrap)
import Data.Traversable (for)
import Data.Tuple.Nested (type (/\), (/\))
import Effect.Aff (Aff)
import Effect.Aff.Class (liftAff)
import Effect.Exception (Error)

type URL = String
type Payload = String
type Response = { code:: Int, payload :: Payload }
type Count = Int
type HostName = String
type URLPath = String
type QueryString = String

foreign import fetchPayload :: URL -> Aff (Either Error Response)
foreign import readHostNameFromSTDIN :: Aff HostName

-- Here ExceptT is the inner Monad and the StateT is the outer monad with the state being carried through being the current URL and the total redirects count.
-- State can be updated even with an error
example1 :: StateT (URL /\ Count) (ExceptT Error Aff) _
example1 = do
  (url /\ count ) <- get
  { code, payload } <- lift $ wrap (fetchPayload url)
  -- Here let's say that `fetchPayload` gave a Error ( a Left value), the chain of computations would stop.
  -- So let's say, here that I want to reset `count` to 0 and retry from the last known URL, we can't
  -- This is because of https://github.com/purescript/purescript-transformers/blob/v4.1.0/src/Control/Monad/State/Trans.purs#L97
  if code == 301  && count < 5 then put (payload /\ (count + 1)) *> example1 else pure payload

example2 :: ExceptT Error (StateT (URLPath /\ QueryString) Aff) _
example2 = do
  finalURL <- lift $ do
    hostname <- liftAff readHostNameFromSTDIN
    (urlPath /\ qs) <- get
    pure $ "https://" <> hostname <> urlPath <> qs
  liftAff $ fetchPayload finalURL