Elephant variable reassignment

An example using the (.=) operator from microlens-mtl to write Haskell code that resembles reassignment of mutable variables.

This demonstration was inspired by a Stack Overflow question which asks:

How can I re-assign a variable in a function in Haskell?

For example,

elephant = 0

setElephant x =
  elephant = x

The short answer is that we don't have a language-level mechanism to model that sort of thing in Haskell, and it's generally not what you want anyway.

Here I give a long an…