src/doc/book/glossary.md

   1 % Glossary
   2
   3 Not every Rustacean has a background in systems programming, nor in computer
   4 science, so we've added explanations of terms that might be unfamiliar.
   5
   6 ### Abstract Syntax Tree
   7
   8 When a compiler is compiling your program, it does a number of different things.
   9 One of the things that it does is turn the text of your program into an
  10 ‘abstract syntax tree’, or ‘AST’. This tree is a representation of the structure
  11 of your program. For example, `2 + 3` can be turned into a tree:
  12
  13 ```text
  14   +
  15  / \
  16 2   3
  17 ```
  18
  19 And `2 + (3 * 4)` would look like this:
  20
  21 ```text
  22   +
  23  / \
  24 2   *
  25    / \
  26   3   4
  27 ```
  28
  29 ### Arity
  30
  31 Arity refers to the number of arguments a function or operation takes.
  32
  33 ```rust
  34 let x = (2, 3);
  35 let y = (4, 6);
  36 let z = (8, 2, 6);
  37 ```
  38
  39 In the example above `x` and `y` have arity 2. `z` has arity 3.
  40
  41 ### Bounds
  42
  43 Bounds are constraints on a type or [trait][traits]. For example, if a bound
  44 is placed on the argument a function takes, types passed to that function
  45 must abide by that constraint.
  46
  47 [traits]: traits.html
  48
  49 ### DST (Dynamically Sized Type)
  50
  51 A type without a statically known size or alignment. ([more info][link])
  52
  53 [link]: ../nomicon/exotic-sizes.html#dynamically-sized-types-dsts
  54
  55 ### Expression
  56
  57 In computer programming, an expression is a combination of values, constants,
  58 variables, operators and functions that evaluate to a single value. For example,
  59 `2 + (3 * 4)` is an expression that returns the value 14. It is worth noting
  60 that expressions can have side-effects. For example, a function included in an
  61 expression might perform actions other than simply returning a value.
  62
  63 ### Expression-Oriented Language
  64
  65 In early programming languages, [expressions][expression] and
  66 [statements][statement] were two separate syntactic categories: expressions had
  67 a value and statements did things. However, later languages blurred this
  68 distinction, allowing expressions to do things and statements to have a value.
  69 In an expression-oriented language, (nearly) every statement is an expression
  70 and therefore returns a value. Consequently, these expression statements can
  71 themselves form part of larger expressions.
  72
  73 [expression]: glossary.html#expression
  74 [statement]: glossary.html#statement
  75
  76 ### Statement
  77
  78 In computer programming, a statement is the smallest standalone element of a
  79 programming language that commands a computer to perform an action.