The Either type represents values with two possibilities:
a value of type Either a b is either Left a or Right b.
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").
The Either type represents values with two possibilities: a value of type Either a b is either Left a or Right b.
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").