From: Jean-Hugues ROBERT <jean_hugues_robert@...>
Date: 2004-03-31T20:12:22+09:00
Subject: Re: Syntax: eval( "a = !a) != eval( "a = not a")

Hi,
OK, so precedence of: not, is lower than precedence of: =. Thanks.
Yet...
At 14:56 31/03/2004 +0900, Mehr, Assaph (Assaph) wrote:
> > Is there a reason why eval( "a = not a") raises a syntax error
> > whereas eval( "a = !a") works as I expected ?
>
>Probably because the statement "a = not a" is also a syntax error
>(regardless of the eval).
>The 'not' operator has a lower precedence than assignment, whereas the !
>operator has a higher precedence. Therefore the interpreter reads the
>second form as "a = not" which is a syntax error.
>If you really want to use the word form use: eval("a = (not a)"), which
>will take care of precedence.
>
>See:
>http://phrogz.net/ProgrammingRuby/tut_expressions.html#definedandorandno
>t
>and  http://phrogz.net/ProgrammingRuby/language.html the section on
>Boolean Expressions and table_18.4.
I now understand that thing are happening the way they are documented.

For the sake of curiosity, I would still like to know *why* things
were designed this way. A way that makes "not" rather useless to me
(besides the rare case where I would want to avoid a redefined !()).

i.e. why is "not" 's precedence lower than assignment's one ???

Thanks

Jean-Hugues


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