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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 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)

About DocAda Light: Preface / Preliminary / Help / TOC / Copyright
DocAda Online at the Ada Home: Complete RM95 / Updates / News
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