Ada Reference Manual — Legal Information

4.4 Expressions

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 following categories: choice_expression, choice_relation, relation, simple_expression, term, factor, primary, conditional_expression, quantified_expression.

Syntax

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

2.1/3

choice_expression ::=
     choice_relation {and choice_relation}
   | choice_relation {or choice_relation}
   | choice_relation {xor choice_relation}
   | choice_relation {and then choice_relation}
   | choice_relation {or else choice_relation}

2.2/3

choice_relation ::=
simple_expression [relational_operator simple_expression]

3/4

relation ::=
     simple_expression [relational_operator simple_expression]
   | tested_simple_expression [not] in membership_choice_list
   | raise_expression

3.1/3

membership_choice_list ::= membership_choice {| membership_choice}

3.2/4

membership_choice ::= choice_simple_expression | range | subtype_mark

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

term ::= factor {multiplying_operator factor}

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

7/3

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:

15/2

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
R*Sin(θ)*Cos(φ)             -- 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)

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