should forms are reported like this:
F addition-test (ert-test-failed ((should (= (+ 1 2) 4)) :form (= 3 4) :value nil))
ERT shows what the
should expression looked like and what
values its subexpressions had: The source code of the assertion was
(should (= (+ 1 2) 4)), which applied the function
4, resulting in the value
nil. In this case, the test is wrong; it should expect 3
rather than 4.
If a predicate like
equal is used with
provides a so-called explanation:
F list-test (ert-test-failed ((should (equal (list 'a 'b 'c) '(a b d))) :form (equal (a b c) (a b d)) :value nil :explanation (list-elt 2 (different-atoms c d))))
In this case, the function
equal was applied to the arguments
(a b c) and
(a b d). ERT's explanation shows that
the item at index 2 differs between the two lists; in one list, it is
the atom c, in the other, it is the atom d.
In simple examples like the above, the explanation is unnecessary. But in cases where the difference is not immediately apparent, it can save time:
F test1 (ert-test-failed ((should (equal x y)) :form (equal a a) :value nil :explanation (different-symbols-with-the-same-name a a)))
ERT only provides explanations for predicates that have an explanation function registered. See Defining Explanation Functions.