From: 0x0dea+redmine@...
Date: 2015-06-19T02:25:37+00:00
Subject: [ruby-core:69673] [Ruby trunk - Bug #11283] Block assigned	implicitly

Issue #11283 has been updated by D.E. Akers.


> * Either provide a method with empty proc (`Proc.new`)

That is, in fact, exactly what Ruby is doing:

```ruby
def build
  Class.new { define_method :foo, Proc.new }
end

build{ :bar }.new.foo # => :bar
```

`Proc.new`, called without an explicit block, will instead attempt to use the one that was passed to the surrounding method:

```ruby
def foo
  Proc.new.call 1
end

foo { |x| x + 1 } # => 2
```

Note well that methods need not explicitly declare that they take a block. This double whammy of implicit behavior is certainly "astonishing" the first time you encounter it, but it all hangs together in the final analysis.

> * Or call `SyntaxError` as a strict way to ask for the programmer's intention

This is a semantic rather than syntactic concern, and Ruby does warn against it. That it doesn't raise an `ArgumentError` as it does for standalone `Proc.new` is indeed unexpected, and may well be a bug.

----------------------------------------
Bug #11283: Block assigned implicitly
https://bugs.ruby-lang.org/issues/11283#change-53033

* Author: Andrew Kozin
* Status: Open
* Priority: Normal
* Assignee: 
* ruby -v: 1.9.3, 2.0, 2.1, 2.2, ruby-head
* Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN, 2.2: UNKNOWN
----------------------------------------
That is how it works:

    module Test
      def self.build(&block)
        klass = Class.new(Object)
        klass.__send__(:define_method, :foo)
        klass.__send__(:define_method, :bar)
        klass
      end
    end

    Tested = Test.build { :foo }
    # warning: tried to create Proc object without a block
    # => Tested
    Tested.new.foo
    # => :foo
    Tested.new.bar
    # => :foo

The block is assigned to all calls to `:define_method` via `Object#__send__` implicitly, while it wasn't asked to.

The behaviour is tested under MRI 1.9.3, 2.0, 2.1, 2.2, ruby-head. It doesn't occur under rbx-2 and jruby (1.7, 9.0.0.0).

For the context look at this thread https://github.com/mbj/mutant/issues/356



-- 
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