On 06/01/11 18:01, Peter Michaux wrote:
On Wed, Jun 1, 2011 at 5:52 PM, Waldemar Horwat<[email protected]> wrote:
On 06/01/11 16:07, Peter Michaux wrote:
Could some of JavaScript's statements also be allowed as expressions?
[snip]
Theoretically it's possible to allow statements as expressions, but you run
into a number of practical problems:
- Precedence inversion:
a = if (b) c
So far so good. However, the substatement of an if can also be a comma
expression, which causes trouble:
a = if (b) c, d
Also, what should be stored in a if b is false? You can argue between
false, null, and undefined.
Indeed.
- Conflict between blocks and statements:
a = {x: while (x) { ... break x;}}
is either an object initializer or a block containing a labeled while
statement.
Ouch.
- Semicolon insertion:
return
if (longcondition) {
...
} else {
...
}
will not do what you want. The entire if is dead code.
This one is already an issue for JavaScript programmers.
--
Ok so perhaps only some statements could be converted to expressions
easily without ambiguity. Would a throw expression be possible without
much grief, for example? I think it could be useful (since I wanted to
do it exactly two days ago hence my post here.)
You'd have to deal with the same precedence inversion as in the if statement
example above.
If you make <throw expr> bind the loosest, then there is no inversion, but you
can't write things like:
a = b ? c : throw d;
because you made ?: bind tighter than throw.
If you make <throw expr> bind tighter than, say, ?:, then you run into an
ambiguity with = or , operators:
a = b ? c : throw d, e;
This can parse as either:
a = b ? c : throw (d, e);
or:
(a = b ? c : throw d), e;
Waldemar
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