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Edit summary for version 17 :

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Version 17

Idiom #59 Write to standard error stream

Print the message "x is negative" to standard error (stderr), with integer x value substitution (e.g. "-2 is negative").

Idiom #59 Write to standard error stream

Print the message "x is negative" to standard error (stderr), with integer x value substitution (e.g. "-2 is negative").

Code
IO.puts :stderr, "#{x} is negative"
Doc URL
http://elixir-lang.org/getting-started/io-and-the-file-system.html
Demo URL
http://elixirplayground.com?gist=7556b8f295e29e0e62fe