13.6 Additional PyMonad features
One of the other features of PyMonad is the confusingly named monoid. This comes directly from mathematics and it refers to a group of data elements that have an operator and an identity element, and the group is closed with respect to that operator. Here’s an example of what this means: when we think of natural numbers, the add operator, and an identity element 0, this is a proper monoid. For positive integers, with an operator *, and an identity value of 1, we also have a monoid; strings using + as an operator and an empty string as an identity element also qualify.
PyMonad includes a number of predefined monoid classes. We can extend this to add our own monoid class. The intent is to limit a compiler to certain kinds of optimization. We can also use the monoid class to create data structures which accumulate a complex value, perhaps including a history of previous operations.
The pymonad.list is an example of a monoid. The identity element is...
