[Haskell-cafe] instance Monad Either?

[Tillmann Rendel]

Eric wrote:
> According to this 
> <http://www.randomhacks.net/articles/2007/03/10/haskell-8-ways-to-report-errors> 
> Either is an instance of class Monad, but when I try to use the do 
> notation I get a compiler error. What's going on?

Try to import Control.Monad.Error to get a Monad instance for Either. 
Actually, the instance is for (Error a => Either a), So it's more like 
the 5. aproach on that page. But since there is an instance Error String 
you can use (Either String) as a Monad (or a MonadError):

   import Control.Monad.Error

   test1 :: Either String Int
   test1 = do
     [x, y, z] <- return [3]
     return 42

   test2 :: MonadError e m => m Int
   test2 = do
     [x, y, z] <- return [3]
     return 42


*Main> test
Left "Pattern match failure in do expression at test.hs:5:2-10"

*Main> test2 :: Either String Int
Left "Pattern match failure in do expression at test.hs:11:2-10"

*Main> test2 :: IO Int
*** Exception: user error (Pattern match failure in do expression at 
test.hs:11:2-10)

   Tillmann