Bug #10684: Block arity changes through Enumerable methods

• Status: Rejected
• Assignee: -
• Target version: -
• ruby -v: 2.2.0
• Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN, 2.2: UNKNOWN
• Tags: core, maybenotbug

[jakesower (Jake Sower)]

Blocks traveling through methods in Enumerable have their arity changed before reaching #each. Example:

class MyEnumerator
include Enumerable

def initialize(ary)
@ary = ary
end

def each(&block)
puts block.arity
@ary.each(&block)
end
end

my_enum = MyEnumerator.new([1,2,3])
my_enum.each{|x| x} # outputs 1
my_enum.detect{|x| x} # outputs -1

This is surprising behavior. I would expect the output to be 1 in both cases since the blocks appear identical to me as a programmer.

[jakesower (Jake Sower)]

Blocks traveling through methods in Enumerable have their arity changed before reaching #each. Example:

class MyEnumerable
  include Enumerable

  def initialize(ary)
    @ary = ary
  end
  def each(&block)
    puts block.arity
    @ary.each(&block)
  end
end

my_enum = MyEnumerable.new([1,2,3])
my_enum.each{|x| x}    # outputs 1
my_enum.detect{|x| x}  # outputs -1

This is surprising behavior. I would expect the output to be 1 in both cases since the blocks appear identical to me as a programmer.

[austin (Austin Ziegler)]

It’s not that surprising to me.

While Enumerable#detect is written in C, in Ruby I might implement it as:

module Enumerable
  def detect2
    return enum_for(:detect2) unless block_given?
    v = each { |x| break x if yield x }
    v == self ? nil : v
  end
end

In this case, the C code is instructive:

static VALUE
enum_find(int argc, VALUE *argv, VALUE obj)
{
    NODE *memo;
    VALUE if_none;

    rb_scan_args(argc, argv, "01", &if_none);
    RETURN_ENUMERATOR(obj, argc, argv);
    memo = NEW_MEMO(Qundef, 0, 0);
    rb_block_call(obj, id_each, 0, 0, find_i, (VALUE)memo);
    if (memo->u3.cnt) {
	return memo->u1.value;
    }
    if (!NIL_P(if_none)) {
	return rb_funcall(if_none, id_call, 0, 0);
    }
    return Qnil;
}

In practice, it's doing it more like:

module Enumerable
  def detect3
    return enum_for(:detect3) unless block_given?
    inject(nil) { |m, x|
      v = yield x
      m ||= v if v
      m
    }
  end
end

[jeremyevans0 (Jeremy Evans)]

• Status changed from Open to Rejected