kokolegorille
Different closure between Erlang and Elixir
Hello everyone,
Here is some erlang code I wanted to translate to Elixir… here is a simple example in the console.
$ erl
Erlang/OTP 22 [erts-10.4.4] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [hipe]
Eshell V10.4.4 (abort with ^G)
1> Pid=self().
<0.82.0>
2> Ref = monitor(process, Pid).
#Ref<0.2147538226.2721841158.105393>
3> receive
3> {'DOWN', Ref, process, Pid, Why} -> Why
3> end.
and here is the Elixir version I came up with…
$ iex
Erlang/OTP 22 [erts-10.4.4] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [hipe]
Interactive Elixir (1.9.1) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> pid = self()
#PID<0.108.0>
iex(2)> ref = Process.monitor(pid)
#Reference<0.417878796.1379926020.235519>
iex(3)> receive do
...(3)> {:DOWN, ref, :process, pid, why} -> why
...(3)> end
warning: variable "pid" is unused (if the variable is not meant to be used, prefix it with an underscore)
iex:4
warning: variable "ref" is unused (if the variable is not meant to be used, prefix it with an underscore)
iex:4
Code is quite similar, but in the Elixir version, ref and pid are not seen by the receive block, while the Erlang version will match on a ‘DOWN’ message where Ref and Pid are set.
Why is that different?
Thanks for taking time
Marked As Solved
hauleth
Elixir allows to rebind the variables, so this is perfectly possible to do:
a = 1
a = 2
Which will then be translated to Erlang as a something like:
A@1 = 1
A@2 = 2
To have the same behaviour as in Erlang, you need to use pin operator (^):
a = 1
^a = 2 # will fail with no match
So in your case you need to write it as:
receive do
{:DOWN, ^ref, :process, ^pid, why} -> why
end
To “pin down” the variables to their current binding instead of creating new ones.
Also Liked
al2o3cr
Erlang doesn’t distinguish between binding a value and pattern-matching on an already-bound value (part of its Prolog heritage IIRC). Elixir disambiguates between these two cases with the ^ operator, referred to as the “pin operator”:
So this (from your example) binds new values to ref and pid:
receive do
{:DOWN, ref, :process, pid, why} -> why
end
while this pins them to their already-bound values:
receive do
{:DOWN, ^ref, :process, ^pid, why} -> why
end
kokolegorille
Thanks, I forgot the pin 
kokolegorille
Thanks, I need a 
This is the full function
def on_exit(pid, fun) do
spawn(fn ->
ref = Process.monitor(pid)
receive do
{:DOWN, ^ref, :process, ^pid, reason} -> fun.(reason)
end
end)
end
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