Is it slower?

What penalty does it impose over two (or more), separate (differently named) subroutines?

Ie. Is the syntactic sugar worth the implementation costs?

A simple test I thought might be useful: 

my $t;

sub a {};
sub b {};

multi sub c (1) {};
multi sub c (2) {};

for ^7 {
  my $iterations = ^10 ** $_;
  say $iterations.max ~ " calls";

  $t = now;
  for ^$iterations { $_ %% 2 ?? a() !! b() };
  say "regular: {now - $t}";

  $t = now;
  for ^$iterations { c($_ %% 2 ?? 1 !! 2) };
  say "multi:   {now - $t}";
}

[download]

With a recent Rakudo on MoarVM: 

1 calls
regular: 0.00630924
multi:   0.0046663
10 calls
regular: 0.00344620
multi:   0.00392575
100 calls
regular: 0.0056468
multi:   0.0079017
1000 calls
regular: 0.0238749
multi:   0.02800533
10000 calls
regular: 0.0541161
multi:   0.287505
100000 calls
regular: 0.43776129
multi:   2.39229575
1000000 calls
regular: 4.45541020
multi:   24.2623499

[download]

Unfortunately the numbers are clearly pretty useless.

I hope to l follow up this upcoming week when I've got a better handle on how to usefully answer your question. I'm thinking I'll install perl6-bench which has robust logic for squeezing noise (startup, gc, etc.). out of results. Other than the noise in the numbers, perhaps you could confirm the basic benchmark I created is a reasonable test in principle to answer your question?

Update

I believe the "benchmark" I used above caused confusion in this thread due to its inclusion of implicit `where` constraints (where the argument had the value 1 or 2). These literal fixed value `where` constraints are supposed to one day be resolvable at compile-time but as I write this (April 2015) they are resolved at run-time.

So here's an alternative "benchmark": 

 
my $t;

class A {};
class B {};

sub a (A $a) {};
sub b (B $a) {};

multi sub c (A $a) {};
multi sub c (B $a) {};

for ^7 {
  my $iterations = ^10 ** $_;
  say $iterations.max ~ " calls";

  $t = now;
  for ^$iterations { $_ %% 2 ?? a(A) !! b(B) };
  say "regular: {now - $t}";

  $t = now;
  for ^$iterations { c($_ %% 2 ?? A !! B) };
  say "multi:   {now - $t}";
}

[download]

And the timing results: 

1 calls
regular: 0.00321471
multi:   0.00306454
10 calls
regular: 0.0029107
multi:   0.0029139
100 calls
regular: 0.0032379
multi:   0.00331093
1000 calls
regular: 0.0091466
multi:   0.00742758
10000 calls
regular: 0.04885046
multi:   0.053550
100000 calls
regular: 0.5330036
multi:   0.5973669
1000000 calls
regular: 5.6755910
multi:   5.946978

[download]

So, as I tried to explain in other comments in this thread, type checking and candidate ordering of multisub calls are generally completed at compile-time, just like regular subs.

»ö« . o O ( "the celebrity tell-all of the Perl-6 cult?" )

In reply to Re^5: rough approximation to pattern matching using local (Multi Subs) by raiph
in thread rough approximation to pattern matching using local by gregory-nisbet

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