# 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:

      +
     / \
    2   3

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

      +
     / \
    2   *
       / \
      3   4

### Arity

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

    # #![allow(unused_variables)]
    #fn main() {
    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.

### Bounds

Bounds are constraints on a type or trait. For example, if a bound is placed on the argument a function takes, types passed to that function must abide by that constraint.

### Combinators

Combinators are higher-order functions that apply only functions and earlier defined combinators to provide a result from its arguments. They can be used to manage control flow in a modular fashion.

### DST (Dynamically Sized Type)

A type without a statically known size or alignment. (more info)

### 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 and statements 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.

### Statement

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