OpenEuphoria

Re: How to tell the difference bewteen comparing and assigning

• Posted by "Carl W." <euphoria at cyreksoft.yorks.com> May 13, 2002
• 415 views

jbrown wrote:

> Aside from not covering all situations, what do i do about this:
>
> if a > b then a = b else b = a end if
>
> i could try looking only between the "if" and "then" tokens (and ditto
> for while..do) but it would be hard to do that for all possible
> situations:
>
> ?(a=b) --check for (
> ?a[b=b] --check for [
> ? a=b --check for ?
> a = a = b --check for previous =
> etc etc etc
>
> as there are too many exceptions and to make a parser which takes care
> of this cleanly would be quite difficult. i was thinking about making my
> preprocessor use == for comparision and := for assignment and ignore the
> = sign completely.
> (Perhaps even make the = sign an illegal token, although that would be
> going to the extreme.) I'd prefer not to do that however, so if you or
> anyone else has an idea on how this could be done, it would be greatly
> appreciated.

I think what you need to do is parse the source by context. i.e. Judge what
you need to do by what has gone before.

There's only one statement form using '=' that is an assignment; All others
are boolean statements. Assignments *always* begin with a variable or a
sequence index, so you can safely ignore 'what comes next' even if it does
contain a second equals sign. In fact, the *only* valid statements
_beginning_ with a variable are assignments.

Here's a little euphoria-like pseudocode:

while Parsing_Source do
    get
        next token
    end get
    look for a typename, if there is one, it's a variable declaration
        store variable declarations in a symbol table for future ref.
    end look
    parse
        other syntax
        -- Any '=' found in here is part of a boolean expression.
    end parse
    look for so-far-unmatched token in the symbol table. if found:
        -- must be an assignment
        get
            next non-whitespace char
        end get
        check for a '['; if it is:
            -- skip sequence subscript
            skip past the next ']'
            get
                next non-whitespace char
            end get
        end check
        check to see if it's an '='; fail with error if not.
        get
            an expression
            -- Any '=' found here is part of a boolean expression.
        end get
    end look
end while

HTH,
Carl
--
[ Carl R White -=- aka -=- Cyrek the Illogical ]
[ () E-mail...:     cyrek{}cyreksoft.yorks.com ]
[ /\ URL......: http://www.cyreksoft.yorks.com ]