Assertions with Enum.any?

Questions / Help
1

I am trying to unit test a function that returns a very long list (by necessity), every element of which should satisfy a certain condition. I am presently testing it like this…

assert Enum.all?(some_func(...), fn x ->
         ...
       end)

…which works, but I have to look through the list manually to see which element failed the test. How could I get better reporting that gives me this information?

2

How about using Enum.filter/2 where you keep all the elements that does not satisfy the condition?
The list should be smaller and it’d be easier to inspect.

1 Like
3

You can build your own version of Enum.all? based on Enum.reduce_while. all? is meant to only return a boolean value.

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4

Maybe in this way:

some_func(...)
|> Enum.with_index()
|> Enum.each(fn {element, index} ->
  assert check(element), "#{index} fails: #{inspect(element)}"
end)

or

assert some_func(...)
       |> Enum.with_index()
       |> Enum.reject(fn {element, _index} ->
         check(element)
       end) == []
3 Likes
5

@Marcus gave you an excellent suggestion IMO. This will give you a context – the index of the element and the element itself – after an assertion fails.

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6

Excellent, thanks! (I went with the second option because in this case I only need to see the first thing that failed.)