Tom White's Blog

More Literate Programming with jMock: Anaphora

According to the dictionary, an anaphor is a word used to avoid repetition. It refers back to something in the conversation. The word "it" in the previous sentence refers back to the word "anaphor" in the first sentence, so "it" is an anaphor for "anaphor". Natural language is often ambiguous, and one reason for this is that it may not be clear which word an anaphor such as "it" is referring to.

But ambiguity and programming languages don't go very well together - so why would anyone want to mix the two? Actually, there are circumstances in programming languages where there is no real ambiguity, and an anaphor can have a use in eliminating repetition. Think of it as applying the DRY (Don't Repeat Yourself) principle at the syntax level.

public void testMainMenu() throws Exception {

    goToTheIndexPage();
    assertThat(theCurrentPage, has(title("Home Menu")));
    assertThat(it, has(menuOptions().named("Departures and Arrivals",
                                           "Travel News")));
}

@Override
public void assertThat(Object target, Constraint constraint) {
    it = target;
    super.assertThat(target, constraint);
}

public void test() {
    Fruit fruit = lunchbox.getFruit();
    assertThat(fruit, isNotNull());
    assertThat(fruit, hasProperty("colour", eq("yellow")));
    assertThat(fruit, isIn(lunchbox.getItems()));
}

public void test() {
    assertThat(lunchbox.getFruit(),
        and(isNotNull(),
            hasProperty("colour", eq("yellow")),
            isIn(lunchbox.getItems())));
}

public void test() {
    assertThat(lunchbox.getFruit(), isNotNull());
    assertThat(it, hasProperty("colour", eq("yellow")));
    assertThat(it, isIn(lunchbox.getItems()));
}

Comments

• This is either really clever, or evil, or both :)

  If you want to go this way, how about:

  public void testFruitInLunchbox() {
  assertThat( lunchboxFruit, isNotNull() );
  andIt( isYellow());
  andIt( isIn( lunchboxItems ));

  Which would make a clearer distinction between the starting and subsequent clauses.

  Posted by: smgfreeman on May 14, 2006 at 10:31 AM

• 
  Nice suggestion - it also makes it even more readable. So the implementation would be (given the above extension of assertThat):

  public void andIt(Constraint constraint) {
  assertThat(it, constraint);
  }

  There's nothing to stop having both forms available.

  Will this be in the next jMock/Hamcrest, I wonder?

  
  Tom

  Posted by: tomwhite on May 14, 2006 at 11:03 AM

• 
  Our first implementation, as Tom showed, allows for fairly arbitrary manipulation of 'it' in the test, which could be viewed as evil.

  
  With Steve's suggestion, we could make 'it' private in the base testcase class, and only allow it to be set by assertThat() and read by andIt(). For grammatical correctness I think it should possibly by andThatIt(...), but I could be opening a whole can of worms :-)

  
  My example for when you want to use this sort of thing is when the target (subject?) of your assertion is a complex expression, e.g.:

  assertThat(theMug().on(theDesk()).by(theWindow()), isColoured(red));
  andThatIt(isFilledWith(tea));

  Robert

  Posted by: robertchatley on May 14, 2006 at 03:11 PM

• Jaggregate ("a collections library that is modeled after the ANSI Smalltalk collection protocols") has a similar class to Constraint - UnaryPredicate, with a "boolean matches(A target)" method.

  However, it's an abstract class instead of an interface, with methods such as "UnaryPredicate<A> and(UnaryPredicate<? super A> other)".

  So you can do "predicate1.and(predicate2)" instead of "and(predicate1, predicate2)".

  So you could have:

  
  public void test() {
  assertThat(lunchbox.getFruit(),
  isNotNull()
  .and(hasProperty("colour", eq("yellow")))
  .and(isIn(lunchbox.getItems())));
  }

  as an alternative.

  Posted by: calumm on May 17, 2006 at 01:58 PM

• What about that?

  public void test() {
  assertThat(lunchbox.getFruit()).
  .isNotNull()
  .and(hasProperty("colour", eq("yellow")))
  .and(isIn(lunchbox.getItems())));
  }

  Anyway very good idea.
  Keep it up!
  Check this http://piotrga.wordpress.com/2006/12/06/literate-testing-extending-jmock/

  Posted by: piotrga on December 06, 2006 at 09:05 AM

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