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.


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 [notin range
  | simple_expression [notin 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 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)

Copyright © 1992,1993,1994,1995 Intermetrics, Inc.
Copyright © 2000 The MITRE Corporation, Inc. Ada Reference Manual