Ada Reference Manual (Ada 2022) — Legal Information
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5.4 Case Statements

1
A case_statement selects for execution one of a number of alternative sequences_of_statements; the chosen alternative is defined by the value of an expression. 

Syntax

2/3
case_statement ::= 
   case selecting_expression is
       case_statement_alternative
      {case_statement_alternative}
   end case;
3
case_statement_alternative ::= 
   when discrete_choice_list =>
      sequence_of_statements

Name Resolution Rules

4/3
The selecting_expression is expected to be of any discrete type. The expected type for each discrete_choice is the type of the selecting_expression.

Legality Rules

5/3
The choice_expressions, subtype_indications, and ranges given as discrete_choices of a case_statement shall be static. A discrete_choice others, if present, shall appear alone and in the last discrete_choice_list.
6/3
The possible values of the selecting_expression shall be covered (see 3.8.1) as follows:
7/4
If the selecting_expression is a name (including a type_conversion, qualified_expression, or function_call) having a static and constrained nominal subtype, then each non-others discrete_choice shall cover only values in that subtype that satisfy its predicates (see 3.2.4), and each value of that subtype that satisfies its predicates shall be covered by some discrete_choice (either explicitly or by others). 
8/3
If the type of the selecting_expression is root_integer, universal_integer, or a descendant of a formal scalar type, then the case_statement shall have an others discrete_choice.
9/3
Otherwise, each value of the base range of the type of the selecting_expression shall be covered (either explicitly or by others). 
10
Two distinct discrete_choices of a case_statement shall not cover the same value. 

Dynamic Semantics

11/3
For the execution of a case_statement, the selecting_expression is first evaluated.
12/3
If the value of the selecting_expression is covered by the discrete_choice_list of some case_statement_alternative, then the sequence_of_statements of the _alternative is executed.
13
Otherwise (the value is not covered by any discrete_choice_list, perhaps due to being outside the base range), Constraint_Error is raised.
14/5
NOTE   The execution of a case_statement chooses one and only one alternative. Qualification of the expression of a case_statement by a static subtype can often be used to limit the number of choices that can be given explicitly. 

Examples

15
Examples of case statements: 
16
case Sensor is
   when Elevation      => Record_Elevation(Sensor_Value);
   when Azimuth        => Record_Azimuth  (Sensor_Value);
   when Distance       => Record_Distance (Sensor_Value);
   when others         => null;
end case;
17
case Today is
   when Mon            => Compute_Initial_Balance;
   when Fri            => Compute_Closing_Balance;
   when Tue .. Thu     => Generate_Report(Today);
   when Sat .. Sun     => null;
end case;
18
case Bin_Number(Count) is
   when 1        => Update_Bin(1);
   when 2        => Update_Bin(2);
   when 3 | 4    =>
      Empty_Bin(1);
      Empty_Bin(2);
   when others   => raise Error;
end case;
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