Alex Burr wrote:
[EMAIL PROTECTED] (Luke Palmer) writes:
I would hope the former. However, what about this compile-time
integral power macro[1]?
macro power ($x, $p) {
if $p > 0 {
{ $x * power($x, $p-1) }
}
else {
{ 1 }
}
}
That would hopefully turn:
my $var = 10;
print power $var, 4;
Into
print($var * $var * $var * $var * 1);
The complete answer to this would be partial evaluation. Partial
evaluators exist for langauges such as lisp, ML, and even C. See
http://www.dina.dk/~sestoft/pebook/pebook.html
A partial evaluator simplifies a function with respect to some of its
inputs, if possible. This can be quite powerful: If P is a partial
evaluator, I an interpreter, and C some code, the P(I,C) compiles C
(in a rudimentary sort of way) and P(P,I) produces a compiler. But
code has to be written with an eye to making it possible to simplify it,
otherwise you just get the original code back.
In theory you could write one as a perl6 macro, although it would be
more convenient if there was someway of obtaining the syntax tree of a
previously defined function other than quoting it (unless I've missed
that?).
But I confidently predict that no-one with write a useful partial
evaluator for perl6. The language is simply too big.
As it is currently defined the default "is parsed" trait includes
evaluating the arguments before the marco is called, so the above
macro doesn't do what you want with out adding some magic.
One way to do what you want is to use a string returning macro with
an "is parsed" trait that return the unevaluated args as strings and
then concat up the string for the expression you want.
Another possibility is a built-in trait orthoginal to "is parsed"
that turns off argument evaluation and allow the args to be passed
in as syntax trees, allowing the macro to built the syntax tree
to be returned.
I imagine that by the time all is done there will be a whole set
if trait definitions to make various types of macros easy to do.
Which brings us back to the need for a way to bundle up a set of
traits and give it a name.
--
[EMAIL PROTECTED]
[EMAIL PROTECTED]
