# 4.4 Expressions

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An expression is a formula that defines the computation or retrieval of a value. In this International Standard, the term expression refers to a construct of the syntactic category expression or of any of the other five syntactic categories defined below.

## Syntax

```expression ::=
relation {and relation}  | relation {and then relation}
| relation {or relation}  | relation {or else relation}
| relation {xor relation}
```

```relation ::=
simple_expression [relational_operator simple_expression]
| simple_expression [not] in range
| simple_expression [not] in subtype_mark
```

```simple_expression ::= [unary_adding_operator] term {binary_adding_operator term}
```

```term ::= factor {multiplying_operator factor}
```

```factor ::= primary [** primary] | abs primary | not primary
```

```primary ::=
numeric_literal | null | string_literal | aggregate
| name | qualified_expression | allocator | (expression)
```

## Name Resolution Rules

A name used as a primary shall resolve to denote an object or a value.

## Static Semantics

Each expression has a type; it specifies the computation or retrieval of a value of that type.

## Dynamic Semantics

The value of a primary that is a name denoting an object is the value of the object.

## Implementation Permissions

For the evaluation of a primary that is a name denoting an object of an unconstrained numeric subtype, if the value of the object is outside the base range of its type, the implementation may either raise Constraint_Error or return the value of the object.

## Examples

Examples of primaries:

```4.0                --  real literal
Pi                 --  named number
(1 .. 10 => 0)     --  array aggregate
Sum                --  variable
Integer'Last       --  attribute
Sine(X)            --  function call
Color'(Blue)       --  qualified expression
Real(M*N)          --  conversion
(Line_Count + 10)  --  parenthesized expression
```

Examples of expressions:

```Volume                      -- primary
not Destroyed               -- factor
2*Line_Count                -- term
-4.0                        -- simple expression
-4.0 + A                    -- simple expression
B**2 - 4.0*A*C              -- simple expression
Password(1 .. 3) = "Bwv"    -- relation
Count in Small_Int          -- relation
Count not in Small_Int      -- relation
Index = 0 or Item_Hit       -- expression
(Cold and Sunny) or Warm    -- expression (parentheses are required)
A**(B**C)                   -- expression (parentheses are required)
```