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Re: [perl #123090] Not enough arguments for map
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From:
demerphq
Date:
October 31, 2014 07:26
Subject:
Re: [perl #123090] Not enough arguments for map
Message ID:
CANgJU+WvV_2i3HmseeN_jcyC7AOx_6ODmM=LeDdvHeiKK=bwkg@mail.gmail.com
On 30 October 2014 22:51, Father Chrysostomos via RT <
perlbug-followup@perl.org> wrote:
> On Thu Oct 30 13:55:41 2014, dean.herington@emc.com wrote:
> > This is a bug report for perl from dean.herington@emc.com,
> > generated with the help of perlbug 1.39 running under perl 5.16.3.
> >
> >
> > -----------------------------------------------------------------
> > I don't understand the following error. I think the result of the
> > following statement should be the same as the similar one shown later.
> >
> > DB<1> p map { a => 1 } 1,2,3
> > Not enough arguments for map at (eval 9)[C:/Perl/lib/perl5db.pl:646]
> > line 2, near "} 1"
> > syntax error at (eval 9)[C:/Perl/lib/perl5db.pl:646] line 2, near "}
> > 1"
> >
> > DB<2> $a = 'a'
> >
> > DB<3> p map { $a => 1 } 1,2,3
> > a1a1a1
>
> The map entry in perlfunc says:
>
> "{" starts both hash references and blocks, so "map { ..."
> could be
> either the start of map BLOCK LIST or map EXPR, LIST. Because
> Perl
> doesn't look ahead for the closing "}" it has to take a guess at
> which it's dealing with based on what it finds just after the
> "{".
> Usually it gets it right, but if it doesn't it won't realize
> something is wrong until it gets to the "}" and encounters the
> missing (or unexpected) comma. The syntax error will be
> reported
> close to the "}", but you'll need to change something near the
> "{"
> such as using a unary "+" to give Perl some help:
>
> %hash = map { "\L$_" => 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
> # comma at end
>
> to get a list of anonymous hashes each with only one entry
> apiece.
>
> So you could say this is not a bug, or a known limitation. However, I
> would like to try and fix the most obvious cases, like your examples, so
> that it just does the right thing. (That means doing look-ahead to see if
> there is a comma after the closing brace, which can only work for the most
> simple code.)
>
> Personally I usually do ‘map +(...), LIST’, since it always works, unless
> I need multiple statements in the block.
>
Yet most of us would prefer to see map EXPR, LIST removed in favour of map
BLOCK list.
Yves
--
perl -Mre=debug -e "/just|another|perl|hacker/"
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