IO in

About IO

Functions for handling input and output are provided by the IO module.

• IO.puts/2 writes a string to the standard output, followed by a newline. Returns :ok.
• IO.write/2 writes a string to the standard output, without adding a newline. Returns :ok.
• IO.gets/2 reads a line from the standard input.
• IO.inspect/2 writes anything to the standard output, accepts useful options like :label. Returns the value it was passed unchanged. Useful for debugging.

String.Chars protocol

If you try to pass to IO.puts/2 a value that it cannot print, you will see a Protocol.UndefinedError error.

IO.puts({:ok, 7})
# ** (Protocol.UndefinedError) protocol String.Chars not implemented for {:ok, 7} of type Tuple

When this happens, you might want to use IO.inspect/2 instead.

IO.inspect/2 for debugging

IO.inspect/2 is perfect for debugging because it returns its argument unchanged, it can print anything, and it offers a :label option.

"HELLO\n"
|> String.trim()
|> IO.inspect(label: "step 1")
|> String.downcase()
|> IO.inspect(label: "step 2")

# > step 1: "HELLO"
# > step 2: "hello"
# => "hello"

IO devices

The first argument to all those functions is an IO device, with the default value of :stdio (standard input/output). To write to the standard error device, use :stderr instead. An IO device could also be a process, for example one created by calling File.open/2, which would allow writing to a file.