Re^2: strange syntax error with map (unary plus)

> perl -wMstrict -le 'print map { +2, $_ } 1 .. 2'

I'm not a big fan of the unary plus solution, since it's not easily understandable why the unary plus sometimes works or not.

Fun with Perl's parsing:

 DB<100> map {+2,$_} 1..3   # +2 is not a hash key
 => (2, 1, 2, 2, 2, 3)

 DB<101> map {2,$_} 1..3    # 2 is a hash key 
           ;;
Not enough arguments for map at (eval 21)[multi_perl5db.pl:644] line
2, near "} 1"
syntax error at (eval 21)[multi_perl5db.pl:644] line 2, near "} 1"

  DB<102> map {2,$_}, 1..3  # 2 is a hash key
 => ({ 2 => 1 }, { 2 => 2 }, { 2 => 3 })

  DB<103> map {+2,$_}, 1..3 # +2 is not a hash key
           ;;
syntax error at (eval 26)[multi_perl5db.pl:644] line 2, near "},"

  DB<104> map {+2 => $_}, 1..3 # +2 is not a hash key even with fat co
+mma
           ;;
syntax error at (eval 29)[multi_perl5db.pl:644] line 2, near "},"
[download]

so far so consistent, BUT

  DB<105> map +{+2 => $_}, 1..3 # +2 IS A HASH KEY
 => ({ 2 => 1 }, { 2 => 2 }, { 2 => 3 })

  DB<106> map +{+2 , $_}, 1..3  # +2 IS A HASH KEY even without =>
 => ({ 2 => 1 }, { 2 => 2 }, { 2 => 3 })
[download]

see my point? :)

Cheers Rolf
_{(addicted to the Perl Programming Language and ☆☆☆☆ :)

Je suis Charlie!}

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Re^3: strange syntax error with map (unary plus)
by haukex (Archbishop) on Apr 09, 2017 at 17:48 UTC

I'm not a big fan of the unary plus solution

I agree, I was just keeping the examples in line with what the map docs were saying. Personally in this case I'd prefer map { (...) } ..., since it makes it look obvious that returning a list is intentional. OTOH I also use map {$_=>1} ... very often, but that one doesn't cause any parse trouble.

The map +{...}, ... stuff is also mentioned in the map docs, although I haven't yet seen it used in the wild - personally I'd probably write map { {...} } .... As you can tell I usually prefer map BLOCK LIST over map EXPR, LIST.

... to force an anon hash constructor use +{:
my @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs
                                          # comma at end
[download]
to get a list of anonymous hashes each with only one entry apiece.

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