enjoying salad since 1978.

Monday, July 20, 2009

Functional Refactoring #1: Replace conditional inside fold with filter.

Recently, while working on some code, I noticed that I have been making the same transform on lots of functional code in Scala. I think it's clearly a Refactoring and should have a name.

"Replace conditional inside fold with filter"

The idea is that if you're folding over items in a list and manipulating them based on a conditional, it might be clearer if you pull out the conditional and filter the list on that conditional first.

Here is a contrived example borrowed from a fake test suite. Let's say I had code with a conditional inside of a fold:

sampleUsers.values.foreach { u =>
  if (u.id != 0) {
    Authentication(u.id, u.hash).passwordToken mustEqual u.passwordToken

Let's apply this refactoring to make the test case clearer.

sampleUsers.values.filter(u => u.id != 0).foreach { u =>
  Authentication(u.id, u.hash).passwordToken mustEqual u.passwordToken

This works with multiple inner conditionals as well. You can fit them into one filter or chain multiple filters together.

Even though this is a simple refactoring, it seems like a worthwhile place to start cataloging.