Re: Pascal design, was Testing strategy for compiler

On Tue, 22 Jun 2010 11:17:52 +0000 (UTC), Marco van de Voort
<marcov@turtle.stack.nl> wrote:


>On 2010-06-21, George Neuner <gneuner2@comcast.net> wrote:
>> Pascal's FOR loop is required to terminate in all cases - deliberately
>> causing an infinite loop is illegal. The compiler is required to
>> check that step <> zero on entry to the loop and it is illegal to
>> modify any of the loop control values (index, finish, step) from
>> within the body of the loop.
>
>(isn't the loopvar also required to be a ordinal local var? Or is that
>Borland dialects only? Anyway it is a rule that also excludes a lot of
>dodgy use cases)


Yes, the named loop variable must be a local variable. However, the
named variable may not be in control of the loop:


The standard (ISO-10206) says:


for v := e1 to e2 do BODY


shall be equivalent to


      begin
          temp1 := e1;
          temp2 := e2;
          if temp1 <= temp2 then
              begin
                  v := temp1;
                  BODY;
                  while v <> temp2 do
                      begin
                          v := succ(v);
                          BODY
                      end
              end
      end


There's an analogous equivalence using pred() for


for v := e1 downto e2 do BODY




It doesn't take much thought to simplify the code above to


          temp1 := e1;
          temp2 := e2;
          if temp1 <= temp2 then
              repeat
v := temp1;
                  BODY
                  temp1 := succ(temp1);
              until ( temp1 > temp2 );


which both separates the named variable from loop control and ensures
the variable has the proper value at the start of each iteration.


Separation of the loop variable from control is not strictly necessary
because it is illegal to assign to the loop variable within the loop
body, but it is what a lot of compilers do because it is not illegal
to pass the loop variable to a function by reference and it can be
difficult to statically find every indirect modification.


George

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