Editing Monad (functional programming) (section)

=== Writer monad (JavaScript) <span id="Writer monad"></span> ===
Another common situation is keeping a [[log file]] or otherwise reporting a program's progress.
Sometimes, a programmer may want to log even more specific, technical data for later [[profiling (computer programming)|profiling]] or [[debugging]].
The '''Writer monad''' can handle these tasks by generating auxiliary output that accumulates step-by-step.

To show how the monad pattern is not restricted to primarily functional languages, this example implements a <code>Writer</code> monad in [[JavaScript]].
First, an array (with nested tails) allows constructing the <code>Writer</code> type as a [[linked list]].
The underlying output value will live in position 0 of the array, and position 1 will implicitly hold a chain of auxiliary notes:
<syntaxhighlight lang="Javascript">const writer = value => [value, []];</syntaxhighlight>

Defining {{mvar|unit}} is also very simple:
<syntaxhighlight lang="Javascript">const unit = value => [value, []];</syntaxhighlight>

Only {{mvar|unit}} is needed to define simple functions that output <code>Writer</code> objects with debugging notes:
<syntaxhighlight lang="Javascript">
const squared = x => [x * x, [`${x} was squared.`]];
const halved = x => [x / 2, [`${x} was halved.`]];
</syntaxhighlight>

A true monad still requires {{mvar|bind}}, but for <code>Writer</code>, this amounts simply to concatenating a function's output to the monad's linked list:
<syntaxhighlight lang="Javascript">
const bind = (writer, transform) => {
    const [value, log] = writer;
    const [result, updates] = transform(value);
    return [result, log.concat(updates)];
};
</syntaxhighlight>

The sample functions can now be chained together using {{mvar|bind}}, but defining a version of monadic composition (called <code>pipelog</code> here) allows applying these functions even more succinctly:
<syntaxhighlight lang="Javascript">
const pipelog = (writer, ...transforms) =>
    transforms.reduce(bind, writer);
</syntaxhighlight>

The final result is a clean separation of concerns between stepping through computations and logging them to audit later: 
<syntaxhighlight lang="Javascript">
pipelog(unit(4), squared, halved);
// Resulting writer object = [8, ['4 was squared.', '16 was halved.']]
</syntaxhighlight>