map EXPR - what's an EXPR?

missingthepoint has asked for the wisdom of the Perl Monks concerning the following question:

Greetings, monks.

Let's say I have:

my %foo = map( $_ => 2*$_, 1..10 );
use Data::Dumper;
print Dumper(\%foo);
[download]

... and I (naively) expect output like:

$VAR1 = {
          '6' => 12,
          '3' => 6,
          '7' => 14,
          '9' => 18,
          '2' => 4,
          '8' => 16,
          '1' => 2,
          '4' => 8,
          '10' => 20,
          '5' => 10
        };
[download]

But that's not the result. It looks like it's being parsed as:

my %foo = map( $_, 2 * $_, 1..10 )

... where all but the first argument are flattened into a single list.

Why doesn't Perl DWIM here? In general terms, what's an EXPR and what's not (and why can't $_ => 2*$_ be one)?

Cheers,

Life is denied by lack of attention,
whether it be to cleaning windows
or trying to write a masterpiece...
-- Nadia Boulanger

Comment on map EXPR - what's an EXPR? Select or Download Code

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Re: map EXPR - what's an EXPR? by moritz (Cardinal) on Nov 28, 2008 at 12:08 UTC
The problem is that `=>` is just a comma, and if the lhs is a bare word it's automatically quoted. So what you write is parse as `$_, 2 * $_, 1..10`, so `2 * $_` is the first item of the list. `$ perl -MO=Deparse -e 'my %foo = map( $_ => 2$_, 1..10 );' my(%foo) = map($_, 2 $_, 1..10);` [download] In general terms an EXPR is everything that returns a value, but `EXPR, LIST` means that there has to be a decision where EXPR ends and LIST starts, and that's the first comma.	[reply] [d/l] [select]
Re: map EXPR - what's an EXPR? by johngg (Canon) on Nov 28, 2008 at 15:05 UTC
map says that both `EXPR` and `BLOCK` are evaluated in list context. Using a parenthetical list is ambiguous as they are taken as enclosing the arguments to `map` I think. `$ perl -le ' > %h = map ( $_, 2 * $_ ), 1 .. 3; > print qq{$_ - $h{$_}} for sort {$a <=> $b} keys %h;' 0 - $` [download] Using an anonymous array emits a single scalar containing the reference so you then need another `map` to dereference it. `$ perl -le ' %h = map @$_, map [ $_, 2 * $_ ], 1 .. 3; print qq{$_ - $h{$_}} for sort {$a <=> $b} keys %h;' 1 - 2 2 - 4 3 - 6 $` [download] You could construct a subroutine that returns a list in the fly. `$ perl -le ' %h = map sub { return ( $_, 2 * $_ ) }->( $_ ), 1 .. 3; print qq{$_ - $h{$_}} for sort {$a <=> $b} keys %h;' 1 - 2 2 - 4 3 - 6 $` [download] Simplest is to use `map BLOCK list` as already suggested. `$ perl -le ' %h = map { $_, 2 * $_ } 1 .. 3; print qq{$_ - $h{$_}} for sort {$a <=> $b} keys %h;' 1 - 2 2 - 4 3 - 6 $` [download] I hope this is of interest. Cheers, JohnGG Update: Couldn't see the wood for the parentheses, just add an extra pair so there's no ambiguity. `$ perl -le ' %h = map ( ( $_, 2 * $_ ), 1 .. 3 ); print qq{$_ - $h{$_}} for sort {$a <=> $b} keys %h;' 1 - 2 2 - 4 3 - 6 $` [download] Update 2: Another way is to use a do block. `$ perl -le ' > %h = map do { ( $_, 2 * $_ ) }, 1 .. 3; > print qq{$_ - $h{$_}} for sort {$a <=> $b} keys %h;' 1 - 2 2 - 4 3 - 6 $` [download]	[reply] [d/l] [select]
Re: map EXPR - what's an EXPR? by ikegami (Patriarch) on Nov 28, 2008 at 17:32 UTC
First some solutions, then I'll explain. `map(($_ => 2$_), @list) map +($_ => 2$_), @list map { $_ => 2$_ } @list` [download] In general terms, what's an EXPR and what's not (and why can't $_ => 2$_ be one)? You gotta keep in mind the entire list of arguments is also an expression (a list operator), so the expression is bounded by a comma. "`=>`" is a fancy form of the comma operator. So why you have is basically a precedence issue. And when you have a precedence issue, you use parens. `map(($_ => 2$_), @list)` [download] `map` is often used without parens, but that's a problem here since `map ($_ => 2$_), @list # XXX` [download] means `map(($_ => 2$_), ()), @list # XXX` [download] The usual trick is to use the unary `+` operator. `map +($_ => 2$_), @list` [download] Omitting parens around function in Perl is frought with peril.	[reply] [d/l] [select]
Re: map EXPR - what's an EXPR? by parv (Parson) on Nov 28, 2008 at 13:24 UTC
Try other syntax: `map { EXPR } LIST`.	[reply] [d/l]