Using Either

Similar to Maybe, another data type that is used often in Haskell is Either. While Maybe decides to map something or nothing, Either goes with two types and keeps either of them. In this recipe, we will construct a safe division using the Either data type and will see how we can represent the error messages in a better way.

Getting ready

Use the following command to create a new project called using-either using the simple template:

    stack new using-either simple

Change into the newly created project directory.

How to do it...

1. Open src/Main.hs. 
2. Import the Data.Either module:

        import Data.Either

3. Define safe division, handling the division by zero case:

        safeDiv :: Either String Int -> Either String Int -> Either  
        String Int
        safeDiv (Left e) _ = Left e  -- Any Left _ is an error, we 
        produce the same
        safeDiv _ (Left e) = Left e  -- error as a result.
        safeDiv (Right i) (Right j) | j == 0 = Left "Illegal Operation:  
      ...