Why does this map statement in Perl not compile?
Question
This fails:
my @a = ("a", "b", "c", "d", "e");
my %h = map { "prefix-$_" => 1 } @a;
with this error:
Not enough arguments for map at foo.pl line 4, near "} @a"
but this works:
my @a = ("a", "b", "c", "d", "e");
my %h = map { "prefix-" . $_ => 1 } @a;
why?
Solution
Because Perl is guessing an EXPR (a hash reference, for example) instead of a BLOCK. This should work (note the '+' symbol):
my @a = ("a", "b", "c", "d", "e");
my %h = map { +"prefix-$_" => 1 } @a;
OTHER TIPS
I prefer to write that as
my %h = map { ("prefix-$_" => 1) } @a;
to show the intent, that I am returning a 2-element list.
From perldoc -f map
:
"{" starts both hash references and blocks, so "map { ..."
could be either the start of map BLOCK LIST or map EXPR, LIST.
Because perl doesn’t look ahead for the closing "}" it has to
take a guess at which its dealing with based what it finds just
after the "{". Usually it gets it right, but if it doesn’t it
won’t realize something is wrong until it gets to the "}" and
encounters the missing (or unexpected) comma. The syntax error
will be reported close to the "}" but you’ll need to change
something near the "{" such as using a unary "+" to give perl
some help:
%hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
%hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
%hash = map { ("\L$_", 1) } @array # this also works
%hash = map { lc($_), 1 } @array # as does this.
%hash = map +( lc($_), 1 ), @array # this is EXPR and works!
%hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
or to force an anon hash constructor use "+{"
@hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
and you get list of anonymous hashes each with only 1 entry.
Also, the other way to do what you're doing, initializing the hash, you can do like this:
my @a = qw( a b c d e );
my %h;
@h{@a} = ();
That will create undef entries for each of the five keys. If you want to give them all true values, then do this.
@h{@a} = (1) x @a;
You can also do it explicitly with a loop;
@h{$_} = 1 for @a;
I think that
map { ; "prefix-$_" => 1 } @a;
is more idiomatic, as far as specifying that it is a block of statements and not a hash ref. You're just kicking it off with a null statement.
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