Re: Ada case-statement

["Robert I. Eachus" <rieachus@comcast.net>]

On 3/14/2018 1:49 PM, Dmitry A. Kazakov wrote:
> On 2018-03-14 18:35, Stephen Davies wrote:
>> I guess that this has probably been considered before,
>> but it would be nice if Ada allowed sub-cases, with
>> coverage checking of the appropriate alternatives.
>> This seems like a simpler change than some other recent
>> ones (e.g. conditional expressions), and would remove
>> one problem with not having non-contiguous subtypes.
>> For example:
>>
>>     case Calculate_Day is
>>        when Mon .. Fri =>
>>           ...
>>        when Weekend : Sat | Sun =>
>>           ...
>>           case Weekend is
>>              when Sat =>
>>                 ...
>>              when Sun =>
>>                 ...
>>           end case;
>>     end case;
>>
>> Ok, I know this isn't going to happen, I just wanted to
>> put it out there anyway.
> 
> Great idea, would be useful for variant records and exception handlers too.
> 
The variant record case might justify a language change.  For actual 
case statements I do:

     case Calculate_Day is
         when Mon .. Fri =>
            ...
         when Sat | Sun =>
            ...
            case Calculate_Day is
               when Sat =>
                  ...
               when Sun =>
                  ...
               when others => null; -- or raise Constrain_Error depending
                                    -- on style.
            end case;
      end case;

If Calculate_Day is a non-trivial function you might want to do:

     The_Day: Day := Calculate_Day
     ...
     case The_Day is
         when Mon .. Fri =>
            ...
         when Sat | Sun =>
            ...
            case The_Day is
               when Sat =>
                  ...
               when Sun =>
                  ...
               when others => null; -- or raise Constrain_Error depending
                                    -- on style.
            end case;
      end case;

It would be nice to allow compilers to allow the when others to be 
omitted based on what it knows about the value of The_Day but all that 
is syntactic sugar.  Or you could allow something like:
   case The_Day in Sat..Sunday is  -- very bad, better syntax needed.

What I usually end up doing in complex cases is to do:

       declare
         The_Day: constant Day := Calculate_Day;
       begin
          if The_Day in Mon..Fri then
             ...
          elsif The+Day = Saturday then
             ...
          else -- Day is Sunday
             ...
          end if;
       end;

Why? It seems to me that every time I end up with a complicated case, it 
is in the critical path of task scheduling.  (Or maybe I never need to 
look at others.)  The compiler probably will convert to an if statement, 
but by making the if statement explicit, I get to choose which case gets 
evaluated first.  (The compiler should also do any necessary branch tail 
merging.)  I might even choose to optimize for speed to get rid of the 
branches in the code.  On today's processors, branches can cost a lot 
more than other instructions.)