If block is a closure, why this code does not work? And how to make it work?
def R(arg)
Class.new do
def foo
puts arg
end
end
end
class A < R("Hello!")
end
A.new.foo #throws undefined local variable or method `arg' for #<A:0x2840538>
If block is a closure, why this code does not work? And how to make it work?
def R(arg)
Class.new do
def foo
puts arg
end
end
end
class A < R("Hello!")
end
A.new.foo #throws undefined local variable or method `arg' for #<A:0x2840538>
Blocks are closures and arg is indeed available inside the Class.new block. It's just not available inside the foo method because def
starts a new scope. If you replace def
with define_method, which takes a block, you'll see the result you want:
def R(arg)
Class.new do
define_method(:foo) do
puts arg
end
end
end
class A < R("Hello!")
end
A.new.foo # Prints: Hello!
If you define the class dynamically, you can alter it as you like:
def R(arg)
c = Class.new
# Send the block through as a closure, not as an inline method
# definition which interprets variables always as local to the block.
c.send(:define_method, :foo) do
arg
end
c
end
class A < R("Hello!")
end
puts A.new.foo.inspect
# => "Hello!"