How can I test a multiline output in Elixir?

Question

How can I test the following code?

["one", "two", "three"]) |> Enum.each(&IO.puts(&1))
one
two
three
:ok

My test currently looks like this, but is failing because IO.puts returns :ok rather that the strings, and probably does not include newline characters in a complete string.

assert ["one", "two", "three"]) |> Enum.each(&IO.puts(&1)) == """
one
two
three
"""

Perhaps IO.puts is the wrong function for this use case. If so, what alternative might I use?

Thanks in advance.

wadie · Accepted Answer · 2018-09-27 14:33:43Z

5

Use capture_io.

fun = fn -> ["one", "two", "three"] |> Enum.each(&IO.puts/1) end
assert capture_io(fun) == "one\ntwo\nthree\n"

edited Sep 27, 2018 at 14:33

wadie

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answered Nov 1, 2016 at 21:51

Martin Svalin

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2 Comments

FelixFortis Over a year ago

Thanks for the great answer. Is there a way that doctest can handle leading whitespace? " my_string "

Martin Svalin Over a year ago

Sorry about the confusion, I looked it up and doctest does not (any longer?) support capturing io. I edited my answer, as the part about doctest was wrong.

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