Aff-catchError.purs

module Main where

import qualified Debug.Trace as T
import Control.Alt
import Control.Monad.Aff
import Control.Monad.Eff
import Control.Monad.Eff.Exception
import Control.Monad.Error.Class

import Data.Either

main :: forall e. Eff (trace :: T.Trace | e) Unit
main = launchAff $ do 
         (throwError (error "Foo!")) `catchError` (const $ trace "bork!")

trace :: forall e a. (Show a) => a -> Aff (trace :: T.Trace | e) a
trace a = do
  liftEff' $ T.trace (show a)
  return a

{-
Output:

Compiling Main
Error in module Main:
Error in value declaration main:
Error at src/Main.purs line 3, column 1 - line 4, column 1:
  No instance found for Control.Monad.Error.Class.MonadError u48 (Control.Monad.Aff.Aff (trace :: Debug.Trace.Trace | e27))
See https://github.com/purescript/purescript/wiki/Error-Code-NoInstanceFound for more information, or to contribute content related to this error.
-}