Blag

I briefly covered a mapping function in "The Functional Shape of Loops" which can be used for one-to-one mapping, meaning that one item in the input collection produces one value in the output collection:

[1, 2, 3] |> map((n) => n * 2); // [2, 4, 6]

Sometimes it's useful to have a one-to-many mapping, where one input produces multiple output values.

I was writing some code in a template that looked a lot like:

if (collection.HasItems)
{
  foreach (var item in collection)
  {
    <some markup>
  }
}
else
{
  <some message about no items>
}

and I realized that it would be awfully nice for for, foreach, and while loops to have an else clause.

Learning to program, it's quite common to come across many resources and code samples that include imperative loops of various sorts (particularly for and while).

Frequently when I see examples that include loops, I cringe. They often include many steps that make them harder to understand because there is so much happening every iteration.

Instead, I much prefer to use a functional approach to iteration. Each step can be converted to an individual function and those functions can be applied to the entire collection to produce a result.

There are several advantages to the functional approach over imperative:

• Named functions are essentially declarative; they describe what will happen rather than how it will happen.
• Splitting functionality into individual functions allows for code reuse.
• Pure functions can easily be tested individually.

This is a follow-up to my "Multisort via Composition" post, and won't make sense if you haven't read that post.

When sorting on a single dimension, changing the sort direction can usually be handled by changing the sorter function directly.

const data = [33, 1, 95, 77, 54, 91, 38, 89, 48, 24];

// ascending
data.sort((a, b) => a - b);

// descending
data.sort((a, b) => b - a);

making this act dynamically based on some external input can be reasonably handled by updating the function as well:

const ascending = getAscending(); // true or false
data.sort((a, b) => (ascending ? a - b : b - a));

This works fine for a single sorter, but quickly becomes untenable once multiple sorting dimensions are used:

Sorting a collection of data in a single dimension is pretty straightforward in most programming languages. It's quite common to use some sort of comparison interface that exposes a function with two parameters of the type being sorted, and expected return values of -1, 0, or 1.

For example, JavaScript has Array.prototype.sort, C# has IComparer, and PHP has usort.

This works quite well when you want to sort on a single property: