Re: A bug in the interpreter
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?
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