in reply to D'Orcish Maneuver

Dorkish indeed. I don't see how that makes any sense.

First, it gives tons of warnings. Second, it doesn't make sense to treat an error signal as a valid value. 

$ perl -wE'
   sub expensive_call { $_[0] == -1 ? undef : $_[0] }
   say $_ // "[undef]"
      for
         sort {
            ( $c{$a} //= expensive_call($a) )
               <=>
            ( $c{$b} //= expensive_call($b) )
         }
         0,1,-1,-1,0,2;
'
Use of uninitialized value $c{"-1"} in numeric comparison (<=>) at -e 
+line 3.
Use of uninitialized value $c{"-1"} in numeric comparison (<=>) at -e 
+line 3.
Use of uninitialized value $c{"-1"} in numeric comparison (<=>) at -e 
+line 3.
Use of uninitialized value $c{"-1"} in numeric comparison (<=>) at -e 
+line 3.
Use of uninitialized value $c{"-1"} in numeric comparison (<=>) at -e 
+line 3.
Use of uninitialized value $c{"-1"} in numeric comparison (<=>) at -e 
+line 3.
Use of uninitialized value $c{"-1"} in numeric comparison (<=>) at -e 
+line 3.
0
-1
-1
0
1
2

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It would make more sense to handle the error, perhaps by dying or by using a sensible default. 

$ perl -wE'
   sub expensive_call { $_[0] == -1 ? undef : $_[0] }
   say $_ // "[undef]"
      for
         sort {
            ( $c{$a} ||= expensive_call($a) // -1 )
               <=>
            ( $c{$b} ||= expensive_call($b) // -1  )
         }
         0,1,-1,-1,0,2;
'
-1
-1
0
0
1
2

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Re^2: D'Orcish Maneuver
by JavaFan (Canon) on Apr 28, 2011 at 09:47 UTC
    Well, duh. Of course it gives a ton of warnings. But that's not the fault of the orkish maneuver - it's the fault of the set of elements you want to sort. You'd get the same warnings if you'd used a plain sort, or an ST - the warnings come from returning undefined values in the function that maps elements to sort to <=> operands. 
    #!/usr/bin/perl

use 5.010;

use strict;
use warnings;

sub expensive_call { $_[0] == -1 ? undef : $_[0] }
say $_ // "[undef]" for sort {expensive_call($a) <=> expensive_call($b
+)}
                             0,1,-1,-1,0,2;
__END__
Use of uninitialized value in numeric comparison (<=>) at /tmp/s line 
+9.
Use of uninitialized value in numeric comparison (<=>) at /tmp/s line 
+9.
Use of uninitialized value in numeric comparison (<=>) at /tmp/s line 
+9.
Use of uninitialized value in numeric comparison (<=>) at /tmp/s line 
+9.
Use of uninitialized value in numeric comparison (<=>) at /tmp/s line 
+9.
Use of uninitialized value in numeric comparison (<=>) at /tmp/s line 
+9.
Use of uninitialized value in numeric comparison (<=>) at /tmp/s line 
+9.
0
-1
-1
0
1
2

    [download]
    I complete fail to get the point you're trying to make.
[reply]
[d/l]
[select]

      You'd get the same warnings if you'd used a plain sort, or an ST

      So just use those, then. The whole point of the OP's code is that it's suppose to be able to handle undef. Since it doesn't, it's a bug.

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