4.4 Expressions

1. 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
   

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

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

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

5. term ::= factor {multiplying_operator factor}
   

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

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

   Name Resolution Rules
   
8. A name used as a primary shall resolve to denote an object or a value.
   Static Semantics
   
9. Each expression has a type; it specifies the computation or retrieval of a value of that type.
   Dynamic Semantics
   
10. The value of a primary that is a name denoting an object is the value of the object.
    Implementation Permissions
    
11. 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
    
12. Examples of primaries:

13. 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
    

14. Examples of expressions:

15. 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)
    

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