Using monads for trivial tasks like list manipulation?

Question

Whenever I read about Monad example, they always present IO as a case study.

Are there any examples of monads doing list manipulation which somebody could present? I apr

Answer 1

A nice example for list manipulation is querying an HTML/XML document, a la jQuery. jQuery is a monad. Let's examine an example:

$(doc).find('> body')
      .find('> *')
      .is('table')
      .is('.bar')
      .find('> caption')
      .text()

This gives you the captions of all the top-level tables having a CSS class bar. This is an unusual way to use jQuery (usually you'd simply do body > table.bar > caption), but that's because I wanted to show the steps.

In xml-conduit you do it similarly:

child cursor >>= element "body" >>= child
        >>= element "table" >=> attributeIs "class" "bar"
        >>= child >>= element "caption"
        >>= descendants >>= content

What the list monad does is combine the steps. The monad in itself knows nothing about HTML/XML.

In HXT you do it in almost the same way. Newer HXT versions use what are called arrows instead of monads, but the underlying principle is the same.