13.4 Monad bind() function

The name of the PyMonad library comes from the functional programming concept of a monad, a function that has a strict order. The underlying assumption behind much functional programming is that functional evaluation is liberal: it can be optimized or rearranged as necessary. A monad provides an exception that imposes a strict left-to-right order.

Python, as we have seen, is already strict. It doesn’t require monads. We can, however, still apply the concept in places where it can help clarify a complex algorithm. We’ll look at an example, below, of using a monad-based approach to designing a simulation based on Markov chains.

The technology for imposing strict evaluation is a binding between a monad and a function that will return a monad. A flat expression will become nested bindings that can’t be reordered by an optimizing compiler. The then() method of a monad imposes this strict ordering.

In other languages, such as Haskell, a monad...