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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
case_statement ::= 
   case 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
The expression is expected to be of any discrete type. The expected type for each discrete_choice is the type of the expression.

Legality Rules

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The expressions and discrete_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.
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The possible values of the expression shall be covered as follows: 
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If the expression is a name (including a type_conversion or a function_call) having a static and constrained nominal subtype, or is a qualified_expression whose subtype_mark denotes a static and constrained scalar subtype, then each non-others discrete_choice shall cover only values in that subtype, and each value of that subtype shall be covered by some discrete_choice (either explicitly or by others). 
8
If the type of the 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
Otherwise, each value of the base range of the type of the 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

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For the execution of a case_statement the expression is first evaluated.
12
If the value of the expression is covered by the discrete_choice_list of some case_statement_alternative, then the sequence_of_statements of the _alternative is executed.
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Otherwise (the value is not covered by any discrete_choice_list, perhaps due to being outside the base range), Constraint_Error is raised.
NOTES
14
4  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 need be given explicitly. 

Examples

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Examples of case statements: 
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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;
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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;
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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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