How to change the variable value outside of the block?

[lijie]

In ruby/irb, I can do this:

$ irb
irb(main):001:0> a = 1
=> 1
irb(main):002:0> f = proc do
irb(main):003:1* a = 2
irb(main):004:1> end
=> #<Proc:0x007fa7fd02f740@(irb):2>
irb(main):005:0> f.call
=> 2
irb(main):006:0> a
=> 2

But in Elixir, variable not changed:

$ iex
Erlang R16B02 (erts-5.10.3) [source] [64-bit] [smp:8:8] [async-threads:10] [hipe] [kernel-poll:false] [dtrace]
Interactive Elixir (0.10.4-dev) - press Ctrl+C to exit (type h() ENTER for help) 
iex(1)> a = 1
1
iex(2)> f = fn () -> a = 2 end
#Function<20.80484245 in :erl_eval.expr/5>
iex(3)> f.()
2
iex(4)> a
1

How to change it? I want to change the variable in callback function.

Best regards.

[José Valim]

TLDR: You can't. 

In Elixir, a closure/function creates a new binding. This means a function cannot change a variable defined "above", nor "below" can change the variable of a function. This is very important for concurrency because it avoids race conditions. Imagine this example in Ruby:

x = 1

execute_in_thread { x = 2 }

# Do something more

puts x

Where execute_in_thread will execute the given block concurrently. So, what is the value of x? It is hard to tell. 

It may be 1, if the block was not executed yet. Or 2, if the block was. That's a race condition and behaviour like that makes it really hard to write concurrency software. Even more in a language like Elixir which was designed to run thousands of processes concurrently.

That's why in Elixir we end up using functions like Enum.map and Enum.reduce more frequently because whatever computation we do, the results need to be explicitly returned.

José Valim

www.plataformatec.com.br

Skype: jv.ptec

Founder and Lead Developer

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[lijie]

That is good, I should use ets.

Thanks.