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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

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expression ::= 
     relation {and relation}  | relation {and then relation}
   | relation {or relation}  | relation {or else relation}
   | relation {xor relation}
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relation ::= 
     simple_expression [relational_operator simple_expression]
   | simple_expression [notin range
   | simple_expression [notin subtype_mark
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simple_expression ::= [unary_adding_operatorterm {binary_adding_operator term}
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term ::= factor {multiplying_operator factor}
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factor ::= primary [** primary] | abs primary | not primary
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primary ::= 
   numeric_literal | null | string_literal | aggregate
 | name | qualified_expression | allocator | (expression)

Name Resolution Rules

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

Static Semantics

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

Dynamic Semantics

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

Implementation Permissions

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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

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Examples of primaries:
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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 
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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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