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% 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.
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 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.