"Austin Hastings" <[EMAIL PROTECTED]> writes:
> [Eirik wrote:]
> > Let us see ... somewhat speculative and probably short-of-the-mark
> > generalization coming up:
> >
> >
> > macro infix:[ ($lhs, $op, $rhs)
> > is parsed(/(<Perl6.expr>) \] (<Perl6.expr>)/) {
> > return {
> > $op($lhs, $rhs)
> > };
> > }
> >
> > (Precedence? Err ... the left hand side has already been parsed,
> > so infix:[ must be of fixed precedence to the left hand side, right?
> > Damn, I thought I had it ...)
>
> This is text replacement, not expression evaluation. You do have it from
> where I sit.
Text replacement? This is returning a closure, not text. But even
if this is a case of text replacement, I get the first parameter,
$lhs, from text that has already been parsed.
> # Note: Need a way to parse nested []'s
> macro [ ($whosit) is parsed(/(<?:\s[) (<expr>) \]/) {
> eval $whosit;
> }
I may not be up-to-date on macros and is parsed, but ITYM:
macro prefix:[ ($whosit) is parsed(/(<Perl6.expr>) \]/) {
return eval $whosit;
}
This should return text, I believe. But the parser will not be
expecting a term (or prefix:-operator) here ... it is looking for
an infix:-operator or a postfix:-operator, right?
(And it will eval() at compile time, so my phase-of-the-moon example
will not be evaluated at run time, even if this works.)
> The macro immediately evaluates the expression, so it has to be a deferrable
> reference.
> Then:
>
> macro infix:eqn($n) is equiv (&infix:eq) {
> "[&String::strncmp.assuming(n => 4)]"
> }
(s/4/\$n/, I believe.)
But the infix:-operators by default get their LHS and RHS parse
trees as parameters, last I heard, so your parameter list is too
short. Still, you may be on to something ... what about this?
macro infix:eqn ($lhs, $n, $rsh) is equiv(&infix:eq)
is parsed(/ \( (<Perl6.expr>) \)
(<Perl6.expr but speculatively_equiv(&infix:eq)>)/) {
return "String::strncmp($lhs, ($rhs), ($n))";
}
if "Dough" eqn(4) "Douglas" {...}
becomes
if String::strncmp("Dough", ("Douglas"), (4)) {...}
Okay, that may be evil. But it DWYW, right?
No such luck with my phase-of-the-moon example, though. And come to
think of it, finding the right precedence might be tricky, even if
this (or more complicated re-parsing tricks) should happen to work:
macro infix:[ ($lhs, Sub $op, $rhs)
is equiv($op)
is parsed(/(<Perl6.expr>)
\]
(<Perl6.expr but speculatively_equiv($op)>)/) {
return "($op).(($lhs), ($rhs))";
}
# This might be determinable at compile time:
if $test [$moon.is_waxing ? &infix:< : &infix:>=] $target {...}
# This would need explicit "but equiv" or something like that:
if $test [($lookup{$key} || &infix:eq) but equiv(&infix:eq)] $target {...}
So, probably just a bad idea.
Eirik
--
All bridge hands are equally likely, but some are more equally likely
than others.
-- Alan Truscott
