Re-defined method with a yield throws no block given - ruby

Question

Can someone explain why the last yielder throws a no block given?
class Foo
def yielder
yield "hello"
end
end
class Mod
def initialize
##foo = Foo.new
end
def self.foo
##foo
end
end
worker = Mod.new
Mod.foo.yielder do |hello|
puts hello
end
Mod.foo.class.send(:define_method,:yielder) do
yield "new hello"
end
Mod.foo.yielder do |hello|
puts hello
end
Gives:
hello
test.rb:27:in `block in ': no block given (yield) (LocalJumpError)
from test.rb:30:in `'

Answer 1

A short introduction:
You don't need the Mod-instance, if you define ##foo outside initialize.
You don't need the Mod class to get the problem:
class Foo
def yielder
p 2
yield "hello"
end
end
foo = Foo.new
foo.yielder do |hello|
puts hello
end
foo.class.send(:define_method,:yielder) do
p 1
yield "new hello"
end
foo.yielder do |hello|
puts hello
end
You may shorten your example again:
class Foo
end
foo = Foo.new
foo.class.send(:define_method,:yielder) do
yield "new hello"
end
foo.yielder do |hello|
puts hello
end
This is the same as:
class Foo
define_method(:yielder) do
yield "new hello"
end
end
foo = Foo.new
foo.yielder do |hello|
puts hello
end
End of Introduction.
And now, I'm not sure if I understood correct what you want (and if I understand ruby correct ;) )
define_method accepts a block and use it as method body.
If the new method should receive a block on its own, you must define it in the interface of the definition and call it:
class Foo
define_method(:yielder) do | &prc |
prc.call("new hello")
end
end
foo = Foo.new
foo.yielder do |hello|
puts hello
end
Or the same logic in your example:
class Foo
def yielder
yield "hello"
end
end
class Mod
def initialize
##foo = Foo.new
end
def self.foo
##foo
end
end
worker = Mod.new
Mod.foo.yielder do |hello|
puts hello
end
Mod.foo.class.send(:define_method,:yielder) do | &prc |
prc.call "new hello"
end
Mod.foo.yielder do |hello|
puts hello
end
To make the code more robust, I would recommend some checks with block_given?.