Fold

Creates a new TState value by applying the given folder functions to state and Either<TLeft, TRight> value according to its state.

Usage

This function applies the folder function to the Either<TLeft, TRight> value and to the state according its state.

When Either IsRight

int state = 20
Either<string, int> eitherValue = 22;
int result = eitherValue.Fold(
    state,
    (_state, right) => right + _state,
    (_state, left)  => _state + 10);

//result = 42

When Either IsLeft

int state = 20
Either<string, int> eitherValue = "ERROR";
int result = eitherValue.Fold(
    state,
    (_state, right) => right + _state,
    (_state, left)  => _state + 10);

//result = 30

One sided approach

You can also use the FoldLeft and FoldRight to produce the same results, but with these methods the folder is applied just to one of the possible values.

When the target type is different from Either current value the result always will be equals to state.

FoldRight when Either IsRight

int state = 20
Either<string, int> eitherValue = 22;
int result = eitherValue.FoldRight(
    state,
    (_state, right) => right + _state);

//result = 42

FoldRight when Either IsLeft

int state = 20
Either<string, int> eitherValue = "ERROR";
int result = eitherValue.FoldRight(
    state,
    (_state, right) => right + _state);

//result = 20

FoldLeft Either IsRight

int state = 20
Either<string, int> eitherValue = 22;
int result = eitherValue.FoldLeft(
    state,
    (_state, left) => _state + 10);

//result = 20