OpenEuphoria

Re: About GOTO (was RE: fixed windows)

• Posted by David Cuny <dcuny at LANSET.COM> Aug 07, 2003
• 401 views

I've implemented REDO, BREAK and CONTINUE in my own BASIC interpreter. 
Additionally, there is a ELSE clause on all the control structures, so if 
they complete without hitting a BREAK clause, the THEN executes:

   for i = 1 to length( s )
      if s[i] = 999 then
         break
      end if
   else
      printf( "999 was not found" )
   end for

It saves having to set up a flag marking that something was found, and is 
cheap to implement.

What none of these provides is the ability to jump out an arbitrary number of 
levels - for example:

   for i = 1 to 10
      while a < length( s )
         for j = 1 to 10
            for k = 1 to 10
                -- LEAVE HERE...
            end for
         end for
      end while
      -- ... AND JUMP TO HERE
   end for

For this, TRY/CATCH is probably the best thing:

   integer MyError = NEW_ERROR( "My custom error" )
   for i = 1 to 10
      TRY
         while a < length( s )
            for j = 1 to 10
               for k = 1 to 10
                   THROW MyError
               end for
            end for
         end while
      CATCH { MyError }
      END TRY
   end for

Another option would be to implement a more limited version of TRY/CATCH, like 
this:

   for i = 1 to 10
      while a < length( s )
         for j = 1 to 10
            for k = 1 to 10
                THROW MyError
            end for
         end for
      CATCH { MyError }
      end while
   end for

Note the CATCH clause is attached to the WHILE loop. THROW would cause a 
continuous BREAK until a CATCH clause is found.

The problem with this is that it gives you less granularity than if you had a 
TRY control structure.

Also, TRY has a cool FINALLY clause, which guarantees to always execute, 
whether or not an error was thrown, even if a RETURN is encountered in the 
clause. This helps ensure that resources that were taken are always disposed 
of.

-- David Cuny