boolean-blindness.md

Boolean Blindness

It is tempting to use bool to represent a choice between different things. Unfortunately

• true or false have no semantic meaning whatsoever, in any particular context, unless the context explicitly assigns meaning to them.

• bool has only two values, and if our choices increase we can't add more values to it.

• To add a third option, I need a second boolean value, and the amount of possible states increases in powers of 2.

Fortunately we can make use of both Variants and Polymorphic Variants to give meaning to our programs.

So whenever I see this:

let can_request = true;
let expect_deleted_files = false;

if (can_request && expect_deleted_files) {
  /** do something */
};

I feel I should rewrite it to this:

let request_status = `Can_request;
let expectation = `Files_were_deleted;

switch (request_status, expectation) {
| (`Can_request, `Files_were_deleted) => /** do something */
};

The advantages being that

• I no longer rely on the name of a variable to understand what the value means.

• If the request_status needs more than 2 options, it's trivial to add another one and the type-system will help me refactor along to make sure I miss nothing.

• Because I have exactly as many options as I need, the surface area for illegal states is minimized, and the possible states scale linearly.

In particular I prefer polymorphic variants because they do not need to be declared beforehand and the type-system will help me unify them as I use them.

You can read more about them here: Polymorphic Variants, Real World OCaml.