Error Monad.

category-theory/error-monad.hs

@@ -0,0 +1,14 @@
+import Control.Monad.Error
+
+-- definition of the error monad on top of Either
+-- instance (Error e) => Monad (Either e) where  
+--    return x = Right x   
+--    Right x >>= f = f x  
+--    Left err >>= f = Left err  
+--    fail msg = Left (strMsg msg)  -- the strMsg function is defined in the Error class
+
+a1 = Left "boom" >>= \x -> return (x+1)  
+a2 = Right 100 >>= \x -> Left "no way!"
+
+-- very important to define the type at the end!!!
+b1 = Right 3 >>= \x -> return (x + 100) :: Either String Int