why doesn't this work: x.map(&:+"A")

Question

from what I've read,

something {|i| i.foo } 
something(&:foo)

are equivalent. So if x = %w(a b c d), why aren't the following equivalent:

x.map {|s| s.+ "A"}
x.map {&:+ "A"}

?

The first one works as expected (I get ["aA","bA","cA","dA"]), but the second one gives an error no matter what I try.

You cannot pass argument to those Symbol#to_proc call. – oldergod Oct 02 '12 at 02:33 — oldergod, Oct 02 '12 at 02:33

score 6 · Accepted Answer · answered Oct 02 '12 at 02:55

6

Symbol::to_proc doesn't accept parameters.

You could add a to_proc method to Array.

class Array
  def to_proc
    lambda { |o| o.__send__(*self) }
  end
end

# then use it as below
x.map &[:+, "a"]

answered Oct 02 '12 at 02:55

xdazz

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That's a very interesting answer, thanks. Any reason why #send is not sufficient and __send__ is needed? – Boris Stitnicky Oct 02 '12 at 03:41

score 1 · Answer 2 · edited Oct 02 '12 at 10:52

1

If this worked, you'd have nothing to do as a rubyist. I wrote a whole postfix class built on #method_missing to remedy this. Simple dirty solution would be:

x = ?a, ?b, ?c

def x.rap( sym, arg )
  map {|e| e.send sym, arg }
end

x.rap :+, "A"

edited Oct 02 '12 at 10:52

Bill the Lizard

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answered Oct 02 '12 at 02:41

Boris Stitnicky

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tried this, then x.rap(&:+ "A"), got error. Ruby 1.9.3p125 – davej Oct 02 '12 at 02:48

why doesn't this work: x.map(&:+"A")

2 Answers2