Chapter 5. Names and Bindings

Goal

By the end of this chapter you will be able to give a value a name, and you will understand an important rule about those names.

Naming a value

A composer who writes a motif gives it a name, so that the rest of the score can refer back to it without writing the notes out again. A program does the same with a value. Giving a value a name is called binding it, and the name is called a binding.

A binding is made with the word let:

fn main() -> Word {
    let beats_per_bar = 4;
    let bars = 8;
    beats_per_bar * bars
}

Save that as phrase.kel and run it with keleusma run phrase.kel. The output is:

32

The program names two values, beats_per_bar and bars, and then uses both names in the final line. A piece of eight bars in four-four time has thirty-two beats.

Stating the type

The language works out the type of a binding on its own. 4 is a whole number, so beats_per_bar is a Word. The type may also be stated plainly, after a colon:

let beats_per_bar: Word = 4;

Stating the type is optional. It is useful when the value is complicated, or when writing the type down makes the program clearer to a reader.

Bindings do not change

Here is the important rule. Once a value has a name, that name keeps that value. A binding cannot be reassigned. Writing let total = 32; and then later trying to make total equal something else is not allowed.

This may sound limiting, and in one specific way it is. A binding cannot serve as a running total that a loop adds to, because adding to it would mean changing it. Chapter 8 returns to this point, and Part IV shows where changing state is actually done.

The benefit is large. When a name is read further down the program, it still holds exactly the value it was given. Nothing reassigned it in between. A reader, and the language, can trust the name. This is the same discipline as a written score, where a motif marked in the margin means the same thing every time the score points back to it.

What you now know

• let name = value; binds a value to a name.
• The type can be stated as let name: Type = value;, but the language can also work it out.
• A binding cannot be reassigned. A name keeps its value.

The next chapter groups statements into functions.