Ada 2012 Reference Manual
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}
relation {and relation} | relation {and then relation}
| relation {or relation} | relation {or else relation}
| relation {xor relation}
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}
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}
relation ::=
simple_expression [relational_operator simple_expression]
| tested_simple_expression [not] in membership_choice_list
| raise_expression
simple_expression [relational_operator simple_expression]
| tested_simple_expression [not] in membership_choice_list
| raise_expression
primary ::=
numeric_literal | null | string_literal | aggregate
| name | allocator | (expression)
| (conditional_expression) | (quantified_expression)
numeric_literal | null | string_literal | aggregate
| name | allocator | (expression)
| (conditional_expression) | (quantified_expression)
Name Resolution Rules
Static Semantics
Each expression has a type; it specifies the computation or retrieval of a value of that type.
Dynamic Semantics
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
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
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)
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)
