[tpm] Dealing with nested map { grep {} }

[Shlomi Fish]

Hi Tom,

On Tue, 26 Sep 2017 13:53:25 -0400
Tom Legrady <legrady at gmail.com> wrote:

> As an exercise, I'm implementing a RosettaCode challenge for a game 
> between human and computer.
> 
>      const my $PLAYERS => qw( human computer );
> 
> I want to automatically access the appropriate opponent for each player, 
> which allows core game logic of
> 
>      while ( $no_one_won ) {
>          for my $player ( @PLAYERS ) {
>              $self->player( $player )->take_turn( $OPPONENT{$player} );
>          }
>      }
> 
> But to initialize %OPPONENT, I would need to nest a grep{} inside a 
> map{}, which leads to two instances of $_. Now perhaps the specification 
> of the opponent should be part of the game object or of a player object, 
> but I'm still curious how to resolve this problem in a single stage. Now 
> obviously you can't compare $_ with $_ ...
> 
>      const my %OPPONENT => map { $_ => grep { $_ ne $_ } @PLAYERS } @PLAYERS
> 
> If it weren't a const object, we could do this in stages, though it's 
> verbose and ugly:
> 
>      my %OPPONENT;
>      for my $player ( @PLAYERS ) {
>          $OPPONENT{$player} = grep { $_ ne $player } @PLAYERS
>      }
> 
> But in the case of 'const my %OPPONENT', it's an error to attempt to 
> vivify a key after the object has become restricted.
> 
> It worked fine if I replaced the inner grep{} with a subroutine call, 
> not_me(), which would perform the grep away from the map{}. But then it 
> occurred to me to perform a variable assignment within the map{}. Once 
> we assign $a, we can safely redefined $_.
> 
>      const my %OPPONENT => map { my $a = $_; $a => grep { $_ ne $a} 
> @PLAYERS } @PLAYERS
> 

this seems like a good solution but you should not use $a as the name of a
lexical var. See:

http://perl-begin.org/tutorials/bad-elements/#overuse_dollar_underscore

http://perl-begin.org/tutorials/bad-elements/#vars-a-and-b



> Tom
> _______________________________________________
> toronto-pm mailing list
> toronto-pm at pm.org
> http://mail.pm.org/mailman/listinfo/toronto-pm



-- 
-----------------------------------------------------------------
Shlomi Fish       http://www.shlomifish.org/
My Aphorisms - http://www.shlomifish.org/humour.html

 * rindolf demands equal rights for years, minutes, hours and days to also get
   YAAKOV’S GREAT HUGE LOVE.
    —  http://www.shlomifish.org/humour/fortunes/sharp-perl.html

Please reply to list if it's a mailing list post - http://shlom.in/reply .