The Either type

Sometimes it would be nice if you could add an error message or something to Nothing. That’s why we have the Either type. The Either type takes two type arguments. The type Either a b has two constructors: Left and Right. Both take an argument, Left an argument of type a and Right an argument of type b.

Here’s a simple example: a readInt function that only knows a couple of numbers and returns a descriptive error for the rest. Note the Haskell convention of using Left for errors and Right for success.

readInt :: String -> Either String Int
readInt "0" = Right 0
readInt "1" = Right 1
readInt s = Left ("Unsupported string: " ++ s)

Sidenote: the constructors of Either are called Left and Right because they refer to the left and right type arguments of Either. Note how in Either a b, a is the left argument and b is the right argument. Thus Left contains a value of type a and likewise Right of type b. The convention of using Right for success is probably simply because right also means correct. No offense is intended to left-handed people.

Here’s another example: pattern matching an Either. Just like with Maybe, there are two patterns for an Either, one for each constructor.

iWantAString :: Either Int String -> String
iWantAString (Right str)   = str
iWantAString (Left number) = show number

As you recall, Haskell lists can only contain elements of the same type. You can’t have a value like [1,"foo",2]. However, you can use a type like Either to represent lists that can contain two different types of values. For example we could track the number of people on a lecture, with a possibility of adding an explanation if a value is missing:

lectureParticipants :: [Either String Int]
lectureParticipants = [Right 10, Right 13, Left "easter vacation", Right 17, Left "lecturer was sick", Right 3]

Exercises:

All exercises can be found in Set2a and Set2b. Please pay attention in the title of the exercise in which file the exercises of this section can be found.

Exercises from 2a:

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