Why does a local variable lose its value when defining a method with define_method?
Trying to follow along with a metaprogramming screencast from pragpub and ran into some problems because of changes in Ruby since the release of screencast.
Hard to explain the problem w/o the code, so here's that:
class Discounter
def discount(*skus)
expensive_discount_calculation(*skus)
end
private
def expensive_discount_calculation(*skus)
puts "Expensive calculation for #{skus.inspect}"
skus.inject {|m, n| m + n }
end
end
def memoize(obj, method)
ghost = class << obj; self; end
ghost.class_eval do
define_method(method) do |*args|
memory ||= {}
memory.has_key?(args) ? memory[args] : memory[args] = super(*args)
end
end
end
d = Discounter.new
memoize(d, :d开发者_运维知识库iscount)
puts d.discount(1,2,3)
puts d.discount(1,2,3)
puts d.discount(2,3,4)
puts d.discount(2,3,4)
Problem: The local variable in the method memorize should only change (by taking the return value from Discounter#discount) if it is being passed different arguments than it previously was.
For example I expect the output from running the code above to look like:
Expensive calculation for [1, 2, 3]
6
6
Expensive calculation for [2, 3, 4]
9
9
But this is the actual output:
Expensive calculation for [1, 2, 3]
6
Expensive calculation for [1, 2, 3]
6
Expensive calculation for [2, 3, 4]
9
Expensive calculation for [2, 3, 4]
9
Why isn't the local variable persisting across the calls? What am I missing to make this code work?
Thanks
If you define a local variable inside a block, it will vanish when the end of the block is reached.
To achieve the lifetime you want, you need to define the memory
variable before the block:
def memoize(obj, method)
memory = {}
ghost = class << obj; self; end
ghost.class_eval do
define_method(method) do |*args|
memory.has_key?(args) ? memory[args] : memory[args] = super(*args)
end
end
end
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