Still a newbie in Elixir. My understanding so far was that we consume standard input with input = IO.read(:stdio,:line) and write a line to standard output with IO.puts(line). Now I found in elexir hexdocs the following example for producing on stdout an upper case version of stdin:
stream = IO.stream(:stdio, :line)
for line <- stream, into: stream do
String.upcase(line) <> "\n"
end
Here is my partial understanding of this code: The first line tells us that stream is a Stream representing all of stdin. This is an Enumerable, so the comprehension for line <- stream produces successively the individual lines.
What puzzles me is the into: stream clause. This means that the same stdin stream defined above is now used as Collector, and I don't see anything which would actually put something to stdout (no IO.puts).
Does stream somehow mysteriously represents both stdin and stdout in conjunction?
Yes, your analysis is correct (that is why is is named :stdio and not :stdin or :stdout).
To check that, you have to start from IO documentation ("Elixir provides :stdio and :stderr as shortcuts to Erlang's :standard_io and :standard_error.") to the Erlang io documentation ("By default all I/O sent to standard_io will en up in the user I/O device of the node that spawned the calling process. ")