Info Catalog ert: How to Debug Tests ert: Interactive Debugging

ert: Understanding Explanations

 
 4.1 Understanding Explanations
 ==============================
 
 Failed ‘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 ‘=’ to the arguments ‘3’ and
 ‘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 ‘should’, ERT 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.  Defining Explanation
 Functions.
 

Info Catalog ert: How to Debug Tests ert: Interactive Debugging