nishio:godfat caasi$ stack runghc ./trans5.hs
Run from outside a project, using implicit global project config
Using resolver: lts-5.10 from implicit global project's config file: /Users/caasi/.stack/global-project/stack.yaml

trans5.hs:2:1: Warning:
    Module ‘Control.Monad.Error’ is deprecated:
      Use Control.Monad.Except instead

trans5.hs:20:46: Warning:
    In the use of type constructor or class ‘ErrorT’
    (imported from Control.Monad.Error, but defined in Control.Monad.Trans.Error):
    Deprecated: "Use Control.Monad.Trans.Except instead"

trans5.hs:42:20: Warning:
    In the use of ‘runErrorT’
    (imported from Control.Monad.Error, but defined in Control.Monad.Trans.Error):
    Deprecated: "Use Control.Monad.Trans.Except instead"
Right (Num 0,1)
Looking for x...
Right (Num 1,2)
Adding 1 and 1
Right (Num 2,1)
Looking for x...
Adding 1 and 2
Right (Num 3,2)
Looking for y...
Left "Var y not found."
Looking for x...
Left "Var x not found."