OpenEuphoria

Re: A bug in the interpreter

• Posted by Daryl Border <Darylb5 at aol.com> Feb 28, 2006
• 457 views

Ed Davis wrote:
> 
> Darly Border wrote:
> 
> >I was experimenting with some code recently, when I discovered a
> >bug in the parser. The following code will parse without
> >complaint, but does not work properly.
> >
> >for n = 1 to 10 do
> >   if 4 < n < 8 then
> >      ? n
> >   end if
> >end for
> >
> >This program should display the numbers 5,6,and 7. It actually
> >displays the number 1 through 10.
> >
> >The problem is with function rexpr() in parser.e which parses
> >the expression 4 < n < 8. If the value of n is 1, it calcualtes
> >4 < 1 which is FALSE. It then calculates FALSE < 8 which is
> >meaningless because FALSE is not a number. Because TRUE and
> >FALSE are represented by the numbers 1 and 0, the computer can
> >perform the calcualtion. And because both 0 and 1 are less than
> >8, the expression will evaluate to TRUE no matter what value n
> >contains.
> 
> While I think "4 < n < 8" is a nice shortcut, what about
> "(4 < n) < 8"?  It is also accepted by Euphoria.  How should it
> be evaluated?
> 
> Perhaps a question to ask is are Euphoria expressions meant to be
> more C-like or Pascal-like.  Or something in between?  Something
> entirely different?
> 
> C will accept both of the above expressions, but will evaluate
> both as the latter expression, where "4 < n" returns a 0 or 1
> depending on the truth value of the expression.  Basically the
> way Euphoria is evaluating it now.
> 
> Pascal will accept neither expression, because relational
> operators require an integer expression, and "4 < n" is a boolean
> expression, parenthesized or not.
> 
> It would be kind of cool if Euphoria could properly handle "4 < n
> < 8", but you'd have to define 'properly', and you'd also have to
> have special code for "(4 < n) < 8", so that the relational
> operators behaved consistently.
> 
> I think it would bother me if Euphoria handled "4 < n < 8"
> 'properly' (e.g., not as it does now), but continued to support
> "(4 < n) < 8".  That seems awfully inconsistent.
> 
> I think the viable solutions are:
> 
> 1) Leave it the way it is.  It might be nice to document the
> behavior.  Essentially C-like behavior.
> 
> 2) Make expressions more Pascal-like, and thus prohibit "4 < n <
> 8" and "(4 < n) < 8" altogether.  The former expression is easy
> to prohibit via a grammar change.  The latter would require
> expressions to be typed as number vs. boolean. This might be a
> lot of work, and it might introduce unwanted side effects.
> 
> 3)  Make "4 < n < 8" work the way a non-C programmer thinks it
> should work, e.g., "4 < n" and "n < 8".  And then prohibit "(4 <
> n) < 8", as it would make no sense anymore.  You've already
> solved the first problem.  However, as in number 2 above,
> prohibiting the latter expression might be a lot of work, and it
> might introduce unwanted side effects.
> 
> This reminds me of a question I've had:  is there an official
> Euphoria grammar?

Ed, 
   It is not the responsibility of the interpreter to prevent the programmer
from using poorly formed expressions such as (4 < n) < 8. It is the
responsibility
of the interpreter to evaluate the expressions according to the rules of
mathmatics. My code evaluates both expressions correctly. Euphoria currently
does not.
Daryl Border