Assert equal format

[orenbenkiki]

Current format is: ** (ExUnit.AssertionError) Expected v1 to be equal to (==) v2

This is sub-optimal for two reasons:

• It says "Expected v1" when v1 is the actual value and v2 is the expected value. Ok, it is technically correct in doing so, but it is confusing.
• It is hard to visually compare the values.

A better way would be:

** (ExUnit.AssertionError)
Expected: v1
But got : v2

This makes it clear what was expected and what we got, and makes it trivial to compare the values visually to see the difference.

[josevalim]

This is what we want to do with #265. We need someone to work on it, write a proposal considering the different formats available, etc. Pull requests are welcome.

[orenbenkiki]

The issue #265 is somewhat different, it talks about showing the source of the expression that triggered the assertion failure. I find that having the stack trace is enough, myself.

This current issue (#699) is about the format of showing the values (not the source expression) that triggered the assertion. The following trivial patch would solve the issue:

diff --git a/lib/ex_unit/lib/ex_unit/assertions.ex b/lib/ex_unit/lib/ex_unit/assertions.ex
index b61ef1b..599adc7 100644
--- a/lib/ex_unit/lib/ex_unit/assertions.ex
+++ b/lib/ex_unit/lib/ex_unit/assertions.ex
@@ -157,7 +157,7 @@ defmodule ExUnit.Assertions do
       left  = unquote(expected)
       right = unquote(actual)
       assert unquote(operator).(left, right),
-        "Expected #{inspect left} to #{unquote(text)} #{inspect right}"
+        "Expected #{unquote(text)} to hold between:\nProvided value: #{inspect right}\nComputed value: #{inspect left}"
     end
   end

@@ -219,7 +219,8 @@ defmodule ExUnit.Assertions do
   def assert_raise(exception, message, function) when is_binary(message) and is_function(function) do
     error = assert_raise(exception, function)
     assert error.message == message,
-      "Expected #{inspect exception} to be raised with message #{inspect message}, got: #{error.message}"
+      "Expected #{inspect exception} was raised with unexpected message:\n" \
+      <> "Provided message: #{message}\nComputed message: #{error.message}"
     error
   end