On Wed, Nov 22, 2006 at 11:20:58PM +0100, Leopold Toetsch wrote:
> Am Mittwoch, 22. November 2006 21:03 schrieb Leopold Toetsch:
> > Am Mittwoch, 22. November 2006 18:03 schrieb Patrick R.Michaud:
> > > Is this a bug (I think it is), or does the underscore in
> > >
> > > :multi mean something other than "any argument"?
> >
> > The meaning is 'any PMC' [1], and it of course can't be a bug as there are
> > no specs ;)
>
> Implementing '_' as 'any type' wouldn't be that hard.
I'll take the position that implementing '_' as 'any type' is
the semantic we really want here. We already have a way to
say 'any PMC':
.sub foo :multi(pmc)
Here's a more detailed use case of why the current semantics
aren't useful. For subroutine calls, PAST-pm tries to pass
constants directly to subroutines (when it can) rather than
creating temporary PMCs and passing those. For example, a HLL
expression such as
3 .op. 4
gets turned into PIR like
$P0 = "infix:.op."(3, 4)
instead of
$P0 = new .Integer
assign $P0, 3
$P1 = new .Integer
assign $P0, 4
$P2 = "infix:.op."($P0, $P1)
The present case I've run into is the Perl 6 "smart match"
operator (infix:~~), where the operation to be performed depends
on the types of the arguments. For example, a pattern match
is defined as:
.sub "infix:~~" :multi(_, Sub)
For an expression such as $x ~~ /abc/, this works okay:
find_lex $P0, '$x' # get $x
find_name $P1, 'abc_sub' # get sub for /abc/
$P2 = "infix:~~"($P0, $P1) # call infix:~~
But for something like 'abc' ~~ /abc/, we end up with
find_name $P1, 'abc_sub' # get sub for /abc/
$P2 = "infix:~~"('abc', $P1) # call infix:~~
and it won't find the :multi(_, Sub) given above. So far I've
only found two workarounds -- one is to convert all arguments
to PMCs before calling any sub (this is what PAST-pm is
doing now), the other is to create additional
:multi(string|int|num, ...) subs that redispatch to the '_'
version:
.sub "infix:~~" :multi(int, Sub)
.param pmc x
.param pmc y
.return "infix:~~"(x, y)
.end
.sub "infix:~~" :multi(string, Sub)
.param pmc x
.param pmc y
.return "infix:~~"(x, y)
.end
# etc.
The first approach basically means we build a PMC for
every constant value, the second means that we end up
with three additional subs for every underscore in :multi.
And from a PIR-programmer perspective I don't see a lot
of downside to having '_' mean 'any value', but I can't
speak to the difficulty of implementing this in imcc.
Pm
