How to pass an instance variable as an argument to a block?
This doesn't work in Ruby 1.9 (formal argument cannot be an instance variable).
awesome do |foo, @bar|
puts 'yay'
end
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2 Answers
Ruby 1.9 makes block parameters local to the block. This also means that block parameters can no longer be global or instance variables. You can use closures for your purposes:
@foo = 10
1.times do |i|
puts @foo
end
# => "10"
UPDATE Instead of using instance variables, local variable can help you:
foo = 10
1.times do |i|
puts foo
end
# => "10"
In such cases, you won't get problems related to code execution inside different context.
2 Comments
m33lky
I can't do that. That's why I was confused! Take a look at my new question.
WarHog
Ok, got it. I've updated my post and answered on your question.
There are 3 cases here which could be relevant:
- Instance variable of the definer of the block that references the instance variable (in the example the usage of
awesome2. - Instance variable of the definer of the block that gives the instance variable as argument to the receiver (no chance to give it as argument to the block). This is usage of
awesome. - Instance variable of the user of the block that gives the instance variable as argument to the block. This is usage of
awesome3.
So lets try to implement both examples to see them. The example is lengthy, the relevant lines are:
awesomegets the instance variable as argument, which is then used by the receiver in the call of the blockawesome2gets no instance variable, it is bound bySender#use_in_block. No chance for the receiver to change that binding.awesome3gets the instance variable of the sender, but uses its own instance variable for calling the block
So depending on what you want to reach, one of the three is the better solution.
class Sender
attr_accessor :send_var
def call_a_block(var)
rec = Receiver.new(5)
@my_var = var
res = rec.awesome(@my_var) do |arg1|
arg1 + 3
end
res2 = rec.awesome3(@my_var) do |arg1|
arg1 + 3
end
p "Result of calling awesome with: 3 and #{@my_var} is #{res}"
p "Result of calling awesome3 with: 3 and #{@my_var} is #{res2}"
end
def use_in_block(var)
rec = Receiver.new(6)
@my_var = var
res = rec.awesome2 do
4 + @my_var
end
p "Result of calling awesome2 with: 4 and #{@my_var} is #{res}"
end
end
class Receiver
attr_accessor :rec_var
def initialize(var)
@rec_var = var
end
def awesome(arg1)
res = yield(arg1)
res * 2
end
def awesome3(arg1)
res = yield(@rec_var)
res * 2
end
def awesome2
res = yield
res * 2
end
end
s = Sender.new
s.call_a_block(7)
s.call_a_block(20)
s.use_in_block(7)
s.use_in_block(20)
Results are:
c:\Temp>ruby call.rb
"Result of calling awesome with: 3 and 7 is 20"
"Result of calling awesome3 with: 3 and 7 is 16"
"Result of calling awesome with: 3 and 20 is 46"
"Result of calling awesome3 with: 3 and 20 is 16"
"Result of calling awesome2 with: 4 and 7 is 22"
"Result of calling awesome2 with: 4 and 20 is 48"
