Re: Ternary operator patch

[David Kastrup <dak@...>]

Ryota Hirose <hirose.ryota@gmail.com> writes:

> Hello, Lua Hackers,
>
> 2010/9/15 David Kastrup <dak@gnu.org>
>
>     (x and y or else z)
>
>
> I think this notation is very ambiguous for the programmer, and not
> be friendly for parser.  If we introduced this notation, the keyword
> 'and' has two meanings, logical operator, and a half of ternary
> operator.  How do you feel about the following lines?
>
> x = A and B and C or else D

((A and B) and C or else D)

> y = A or B and C or else D

A or (B and C or else D)

> z = A and B or C and D or else E

(A and B) or (C and D or else E)

The real ugliness is something else:

A and B and C or else D or else E

would appear to have to be read as

(A and (B and C or else D) or else E)

and that introduces some not strict left-to-right ordering into the
construct that is not appealing.

> In the view point of the parser, the logical operator 'and' has
> higher priority than 'or', but, for programmers, a ternary operator
> must have lower priority than logical operator.

Why?

>  So, when parser meets 'and', it cannot decide the priority of  the
> operator.  It must reserve the process, and search 'or else'
> forward.  If we assign same priority of logical 'and' and the ternary
> operator, it must confuse many programmers.

Lua has quite fewer operator priorities than C (and Pascal has even
fewer), and I should like to see evidence that more people get confused
by the operator priorities of Pascal than those of C.

-- 
David Kastrup