% Glossary

Not every Rustacean has a background in systems programming, nor in computer
science, so we've added explanations of terms that might be unfamiliar.

### Abstract Syntax Tree

When a compiler is compiling your program, it does a number of different things.
One of the things that it does is turn the text of your program into an
‘abstract syntax tree’, or ‘AST’. This tree is a representation of the structure
of your program. For example, `2 + 3` can be turned into a tree:

```text
  +
 / \
2   3
```

And `2 + (3 * 4)` would look like this:

```text
  +
 / \
2   *
   / \
  3   4
```

### Arity

Arity refers to the number of arguments a function or operation takes.

```rust
let x = (2, 3);
let y = (4, 6);
let z = (8, 2, 6);
```

In the example above `x` and `y` have arity 2. `z` has arity 3.

### Expression

In computer programming, an expression is a combination of values, constants,
variables, operators and functions that evaluate to a single value. For example,
`2 + (3 * 4)` is an expression that returns the value 14. It is worth noting
that expressions can have side-effects. For example, a function included in an
expression might perform actions other than simply returning a value.

### Expression-Oriented Language

In early programming languages, [expressions][expression] and
[statements][statement] were two separate syntactic categories: expressions had
a value and statements did things. However, later languages blurred this
distinction, allowing expressions to do things and statements to have a value.
In an expression-oriented language, (nearly) every statement is an expression
and therefore returns a value. Consequently, these expression statements can
themselves form part of larger expressions.

[expression]: glossary.html#expression
[statement]: glossary.html#statement

### Statement

In computer programming, a statement is the smallest standalone element of a
programming language that commands a computer to perform an action.