Question

While I was reading this tutorial on monads, found the following expression.

data M a = Raise Exception | Return a

type Exception = String

It says that a is used as a type variable and a range of values in Raise Exception and Return a, but I don’t understand the use (or the meaning) of M here. If M is a data type Why is it used like M a ?

Answer 1

M is the name of the type constructor being defined, a is a type parameter to this constructor. To use the data type later you will have to give the constructor M and supply a type argument for a, e.g. M Int.