owaisqayum
Difficulty Understanding Program
I am going through Streams in Dave Thomas’s book and here I am having difficulties in understanding the following program.
For, example what is receive in sleep function and why to use it?
What is after in sleep function?
How this whole program is executed?
Why Dave Thomas suddenly jumped to such a complex program just after explaining the basics of Stream.
defmodule Countdown do
# sleep mode
def sleep(seconds) do
receive do
after
seconds * 1000 -> nil
end
end
def say(text) do
spawn(fn -> :os.cmd('say #{text}') end)
end
def timer do
Stream.resource(
# start of next minute or number of seconds in a minute
fn ->
{_h, _m, s} = :erlang.time()
60 - s - 1
end,
# wait for the next second
fn
0 ->
{:halt, 0}
count ->
sleep(1)
{[inspect(count)], count - 1}
end,
# nothing to deallocate
fn _ -> nil end
)
end
end
Most Liked Responses
hernytan
Streams, Enums and the functional idioms can be hard to memorize. My advice is to not worry too much about memorizing every function, but learn one everytime you solve a new problem. When I was learning F#, I started with map, reduce and filter only. Then, everytime i solve a problem, I would consult the list of functions, and see if any of them applied. Sometimes I’d miss out things I could do, but that’s ok.
Now when I am learning Elixir, I do the same thing. Also, you can always come here and ask for improvements 
sabiwara
I think you will get a better picture of how receive works when reading the chapter on Processes (chap 15), in the meantime you can just consider that this is a way of sleeping.
He could probably as well have used the built-in function :timer.sleep/1, e.g. seconds |> :timer.seconds() |> :timer.sleep().
The use of receive here is not very clear about the intent, but it is basically listening for a message that is never going to come (no call to send anywhere) with a timeout of seconds, after which it returns.
sabiwara
I suppose it depends what you are looking for, it is a bit hard for me to answer. Maybe this list might help?
If I had to recommend myself, I would suggest starting with the official Getting Started guide and check Elixir School as well.
A great way to explore different parts of the language through practice could be Elixir-koans.
And if you are planning to use Elixir for web development, the Phoenix official guide and Programming Phoenix are fantastic.
All those resources are free (except the book) and should help you get started, check what works best for you. Past some point nothing beats practicing on some real projects IMHO (and the official documentation will be a tremendous help once you do). And you can always come back at Programming Elixir later for a deeper dive into the concepts. 
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