why does io:get_line return "\n" in erlang shell?

Question

when using io:getline("prompt") in the erlang shell , the function returns immediately with a return value of "\n"

io:get_line("prompt"). 
prompt
"\n"

but as suggested in another thread doing the following reads from standard_io correctly.

spawn(fun() -> timer:sleep(100),io:get_line("prompt") end). 

waits for user input and reads from standard io (shell). It was mentioned that it was a race condition . can anyone tell me why is it so and how is it possible to read a value from the erlang shell ?

Answer 1

io:get_line/1 and io:get_line/2 returns data with \n every time.

get_line(Prompt) -> Data | server_no_data()

Where:

Data

The characters in the line terminated by a LF (or end of file). If the IO device supports Unicode, the data may represent codepoints larger than 255 (the latin1 range). If the I/O server is set to deliver binaries, they will be encoded in UTF-8 (regardless of if the IO device actually supports Unicode or not).

In first case you got \n, and try to get result of io:get_line in second case:

spawn(fun() -> 
      timer:sleep(100), 
      Result = io:get_line("prompt"), 
      io:format("Result: ~p~n", [Result]) 
end).

Answer 2

Let's break it down...

Why io:get_line/1 returns a \n?

io:get_line/1 returns a "line" and a \n ("end of a line" or "new line") constitutes a line together with the string you entered.

> io:get_line("Prompt:").
Prompt:TheStringIEntered
"TheStringIEntered\n"

How to read a value from the Erlang shell?

> Data = string:strip(io:get_line("Prompt:"), right, $\n).
Prompt:TheStringIEntered
"TheStringIEntered"
> Data.
"TheStringIEntered"

Note that the value (Data) here is always of string type. You can convert it into other types, but you always start with a string.

Why does spawn(fun() -> timer:sleep(100),io:get_line("prompt") end). behave differently?

Because spawn spawns a new process that temporarily takes over the shell. Once this process gets TheStringIEntered, it also reaches the end of its life. So it dies without having its return value (TheStringIEntered) printed to the shell.