I was glad when the author of the article advised against learning Monads by analogy, because it's one of those subjects where analogies serve only to confuse people. Personally, my brain maps badly to Haskell syntax, so let's do this in JavaScript (I know, I know, not so powerful and everything):

In fact if you used jQuery, you already know what a Monad is, probably without realizing it. Consider the following code:

  $('.myelements').filter(filterFunction).css('color', '#ff0');

You can follow along what this code does by following the dots. First, all elements matching the ".myelements" class are returned, these are then filtered, and the result of that gets a css modification.

Every time you wrote a method that returns an object, you already did monads in principle. They can be used to chain things together. While Haskell provides semantics that explicitly deal with the mechanics of monads, pretty much every dynamic language has them.

Here's what it looks like in PHP, for example:

  $obj
    ->bla()
    ->beep()
    ->borp();

You can use monads to make your code more concise, to simply chain function calls, or to iterate over a list, but they're also often used when sifting through databases since filtering and applying attributes to items. For example, an SQL-like database query might be expressed like this:

  table1.select(all).where({ ref : 1 }).orderby('id');

Of course, that's an API which is a bit dishonest since it acts as if the code really goes through all those items (which would be a performance disaster with huge databases), so when you see something like this it's often just a shortcut to assemble a query. And this goes for monads in general, they're really more of an API style. A monad itself is an abstract concept, a paradigm.

> Also, it's suspicious that pg has never once mentioned monads

For a theory why pg specifically hasn't written anything about monads yet, here's an article that I believe was on HN in the past about the difficulties of comfortably using monads in Lisp: http://marijnhaverbeke.nl/monad.html - but I guess the short explanation would probably be that Lispers tend to deal with monad-worthy problems in other ways.