Re: compilers, was Re: Press Release - Ada 2012 Language Standard Approved by ISO

[Shark8 <onewingedshark@gmail.com>]

On Thursday, December 27, 2012 2:12:59 PM UTC-6, tmo...@acm.org wrote:
> > I realize that the quality of proof needed to make this work might be
> > beyond the state-of-the-art today, but I doubt that will be the case by
> > the time a new version of Ada is defined.
>     I thought compilers were getting wider - handling more constructs -
> rather than deeper - more analysis and optimization.  Is that not the case?

I think that that is dependent on the language you're talking about, or in MS's case [sometimes] the platform (.NET)... and that's a C-ish language.

But then again, Ada's always been more for early-error detection -- static/compile-time checking is the earliest point the compiler can point out an error in our program-text.

In that vein I like Randy's suggestion (though I think an OR separated list would be better than a comma separated list); I absolutely agree that there needs to be a facility for naming a 'set' of exceptions.

The implementation shouldn't be too hard in the case of user-defined exceptions: the most difficult part would be recognizing that the following doesn't raise CONSTRAINT_ERROR (more correctly propagate):

  Type LString is Access String;

  -- Returns true when the strings are equal or both parameters are null.
  Function "=" (Left, Right: IN LString) Return Boolean is
     Subtype Constrained_LString is Not Null LString;

     Function "="( Left : LString; Right : String ) Return Boolean is
     begin
        Return Right = Left.All;
     exception
        When CONSTRAINT_ERROR => Return False;
     end "=";

  Begin
     Return "="(Left, Right.All);
  Exception
     When CONSTRAINT_ERROR =>
        begin
           Return "="(Right,Left.All);
        Exception
           When CONSTRAINT_ERROR => Return True;
        end;
  End "=";

-- Yes, it is horrid abuse of exceptions... but it'll work even in Ada 83.