Why does Perl function "map" give the error "Not enough arguments for map"

Question

Here is the thing I don't understand.

This script works correctly (notice the concatenation in the map functin):

#!/usr/bin/perl

use strict;
use warnings;
use Data::Dumper;

my %aa = map { 'a' . '' => 1 } (1..3);

print Dumper \%aa;

__END__
output:

$VAR1 = {
          'a' => 1
        };

But without concatenation the map does not work. Here is the script I expect to work, but it does not:

#!/usr/bin/perl

use strict;
use warnings;
use Data::Dumper;

my %aa = map { 'a' => 1 } (1..3);

print Dumper \%aa;
__END__
output:

Not enough arguments for map at e.pl line 7, near "} ("
syntax error at e.pl line 7, near "} ("
Global symbol "%aa" requires explicit package name at e.pl line 9.
Execution of e.pl aborted due to compilation errors.

Can you please explain such behaviour?

Answer 1

Perl uses heuristics to decide whether you're using:

map { STATEMENTS } LIST;   # or
map EXPR, LIST;

Because although "{" is often the start of a block, it might also be the start of a hashref.

These heuristics don't look ahead very far in the token stream (IIRC two tokens).

You can force "{" to be interpreted as a block using:

map {; STATEMENTS } LIST;    # the semicolon acts as a disambigator

You can force "{" to be interpreted as a hash using:

map +{ LIST }, LIST;    # the plus sign acts as a disambigator

grep suffers similarly. (Technically so does do, in that a hashref can be given as an argument, which will then be stringified and treated as if it were a filename. That's just weird though.)

Answer 2

Per the Documentation for map:

Because Perl doesn't look ahead for the closing } it has to take a guess at which it's dealing with based on what it finds just after the {. Usually it gets it right, but if it doesn't it won't realize something is wrong until it gets to the }

Giving the examples:

%hash = map {  "\L$_" => 1  } @array # perl guesses EXPR. wrong
%hash = map { +"\L$_" => 1  } @array # perl guesses BLOCK. right

So adding + will give you the same as the first example you've given

my %aa = map {  +'a'=> 1 } (1..3);