From: Benoit Daloze <eregontp@...>
Date: 2009-10-30T03:14:24+09:00
Subject: Why (1..4).inject(&:+) works ?

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Hi rubyists,

I was just wondering why,

(1..4).inject(&:+) #=> 10

works ? (in Ruby 1.9.2)

I understand easily these:

(1..4).inject { |s, e| s + e } #=> 10
(1..4).inject(:+) #=> 10

I suppose it's calling :+.to_proc #=> #<Proc:0x2f52d8>

irb> def a(&b); b; end
irb> a(&:+)
=> #<Proc:0x2f52d8>
irb> :+.to_proc
=> #<Proc:0x2f52d8>

irb> p = a(&:+)
=> #<Proc:0x2f52d8>
irb> p.arity
=> -1
irb> p.call(1,2)
=> 3
irb> p.call(1)
ArgumentError: wrong number of arguments(0 for 1)
    ...
irb> p.call(1,2,3)
ArgumentError: wrong number of arguments(2 for 1)
   ...
irb> p.call
ArgumentError: no receiver given

Well, that's strange: arity = -1, so normally only optional arguments. And
it expects 1 argument but want 2 ?
"no receiver given" : That means it knows it has a argument to act on like
a.+(b). How come ?
Would &:+ knows it need some object to act with "+"(o) ?

Have a nice day

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