Conditional when constructs (GNAT Reference Manual)

#### 18.3.1 Conditional when constructs ¶

This feature extends the use of `when` as a way to condition a control-flow related statement, to all control-flow related statements.

To do a conditional return in a procedure the following syntax should be used:

    procedure P (Condition : Boolean) is
    begin
       return when Condition;
    end P;

This will return from the procedure if `Condition` is true.

When being used in a function the conditional part comes after the return value:

    function Is_Null (I : Integer) return Boolean is
    begin
       return True when I = 0;
       return False;
    end;

In a similar way to the `exit when` a `goto ... when` can be employed:

    procedure Low_Level_Optimized is
       Flags : Bitmapping;
    begin
       Do_1 (Flags);
       goto Cleanup when Flags (1);

       Do_2 (Flags);
       goto Cleanup when Flags (32);

       --  ...

    <<Cleanup>>
       --  ...
    end;

To use a conditional raise construct:

    procedure Foo is
    begin
       raise Error when Imported_C_Func /= 0;
    end;

An exception message can also be added:

    procedure Foo is
    begin
       raise Error with "Unix Error"
         when Imported_C_Func /= 0;
    end;