Either

This is a DLang implementation for the Either monad.

The Either type represents values with two possibilities: a Left value or a Right value, and never both.

The Either type is sometimes used to represent a value which is either correct or an error; by convention, the Left constructor is used to hold an error value and the Right constructor is used to hold a correct value (mnemonic: "right" also means "correct").

How to use

Instantiation

You can initialize a struct directly or by using the bind function. All of these instantiations are equivalent:

  auto myEither = bind!(Left, Right)(someValue); /// Instantiate Either, and assign the side based on the type of `someValue`.
  auto myEither = bind(someValue);     /// Instantiate Either with the Right value. Left Will have the "Any" type.
  auto myEither = bindLeft(someValue); /// Instantiate Either with the Left value. Right Will have the "Any" type.

Type matching

Most of the time, the Right type matcher is used for the algorithm and the Left type matcher for error handling.

  auto either = Either!(int, bool)(1);

  either
    .when((int value) {
      // ... handle the value when it has the Left type
    })
    .when((bool value) {
      // ... handle the value when it has the Right type
    });

Value matching

  auto either = Either!(int, bool)(1);

  auto result = either
    .when!1 ({
      // ... do something when the value has the Left type and is 1
    })
    .when!false {
      // ... do something when the value has the Right type and is false
    })
    .when!true {
      return 2; // when the value is true it is replaced with 2
    })
    .when!true (2); // this is the shortened version of the previous matcher

Function matching

  bool function isOdd(int value) {
    return value % 2 == 1;
  }

  bool function isEven(int value) {
    return value % 2 == 0;
  }

  auto either = Either!(int, bool)(1);

  auto result = either
    .when!isOdd ({
      return true; // set the value to true, when the Left value is an odd number
    })
    .when!isEven ((int a) {
      return a / 2; // divide by two if the value is an even number
    });

Example

This is a function that can safe divide two numbers. It returns an Either struct, where the Left type(the error) is a string and the Right value is a numeric type.

  auto divideBy(NumericType)(NumericType numerator, NumericType denominator) {
    enum NumericType zero = 0;

    return denominator.bind
      .when!(zero) ("Division by zero!".bindLeft)
      .when!(isNaN!NumericType) ("Denominator is NaN.")
      .when((NumericType a) =>
        numerator.bind
          .when!(isNaN!NumericType) ("Numerator is NaN.".bindLeft)
          .when((NumericType b) => b / a)
      );
  }

The above function can be used like this:

  auto result = 30.divideBy(4);

or you can add a to string method that can help you to print the value:

  string toString(T)(Either!(string, T) result) {
    import std.conv;

    string message;

    result
      .when ((string error) { message = "Error: " ~ error; })
      .when ((T value) { message = value.to!string; });

    return "\t" ~ message ~ "\n";
  }

and print the result to the console:

  writeln("30 / 4 =");
  30.divideBy(4).toString.writeln;

... for a complete example you can check the source code here.

License

MIT