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Either.md

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Either

The Either type is very similar to the Maybe type, in that it is often used to represent the notion of failure in some way. The difference between the two is that an error represented with an Either can hold some value (perhaps an exception or error message), while Maybe can only indicate that absence of some value.

While the Either type is often used to represent potential errors, there is nothing restricting it to this purpose. It is therefore perhaps more appropriate to simply think of Either as a representation of two possible types of values, sometimes referred to as the disjoint union, or coproduct of two types.

Construction

The Either type consists of two constructors, Left :: a -> Either a b and Right :: b -> Either a b. When an Either type is used to represent the possibility of an error, Left is typically used to hold the error value and Right holds the "successful" value (as a mnemonic, you can think of Right as being the right value).

It is worth highlighting that the types of the value stored in an Left does not have to be the same type as that in the Right of the same Either. This is the reason why it is documented as having two type parameters Either a b, where a represents the type contained within the Left and b for the value contained in the Right.

Interaction

In order to satisfy a number of laws relating to the implemented interfaces, only a single type parameter can be used as the argument of the various functions. For this reason, Either is right-biased, meaning methods such as map, ap and chain will only call the given function for a Right instance while a Left will simply pass the instance straight through, ignoring the provided function.

Aside from the transformation methods mentioned above, Either.either can also be used to extract the value from an Either type, which takes a function to handle the type of value within the Left and another function to handle the type contained in the Right. Only one of these functions will be called depending on the constructor containing the value.

Reference

Constructors

Either.Left

:: a -> Either a b

Constructs a Left instance of Either. This is often used to represent a failure.

Either.Right

:: b -> Either a b

Constructs a Right instance of Either. This is often used to represent a success where there is a possibility of failure.

Static functions

Either.either

:: (a -> c) -> (b -> c) -> Either a b -> c

Used to extract the value out of an Either by providing a function to handle the types of values contained in both Left and Right.

Either.of

:: b -> Either a b

Creates a pure instance of an Either. This is effectively a Right constructor.

Either.isLeft

:: Either a b -> Boolean

Returns true if the given Either instance is a Left, otherwise false.

Either.isRight

:: Either a b -> Boolean

Returns true if the given Either instance is a Right, otherwise false.

Instance methods

either.map

:: Either a b ~> (b -> c) -> Either a c

Modifies the value contained in a Right with the provided function. If the instance is a Left, the value is left unmodified.

either.ap

:: Either a (b -> c) ~> Either a b -> Either a c

Applies the function contained in the instance of a Right to the value contained in the provided Right, producing a Right containing the result. If the instance is Left, the result will be the Left instance. If the instance is Right and the provided either is Left, the result will be the provided Left.

either.chain

:: Either a b ~> (b -> Either a c) -> Either a c

Applies the provided function to the value contained in a Right instance, resulting in the return value of that function. If the instance is a Left, the function is ignored and the Left instance is returned unmodified.

either.extend

:: Either a b ~> (Either a b -> c) -> Either a c

Applies the provided function to the Right instance, returning a Right containing the result of the function application. If the instance is a Left, the function is ignored and the Left instance is returned unmodified.

either.bimap

:: Either a b ~> (a -> c) -> (b -> d) -> Either c d

Transforms an Either by applying the first function to the contained value if the instance is a Left, or the second function if the instance is a Right.

either.equals

:: Either a b ~> * -> Boolean

Determines whether the provided value is equal to this instance, ensuring both are of the same constructor and both contain equal values.

either.toString

:: Either a b ~> () -> String

Returns a string representation of the Either instance.

either.either

:: Either a b ~> (a -> c) -> (b -> c) -> c

Used to extract the value out of the Either by providing a function to handle the types of values contained in both Left and Right.