Mandatory assignment

In a first-in, first-out (FIFO) queue, elements are consumed from the queue in the same order as they arrive to the queue. Test-driven development (TDD) is a software development process that relies on the repetition of a very short development cycle where the developer:

  1. writes one or more failing automated test cases that defines a desired improvement or new functionality.
  2. writes code to implement the desired improvement or new functionality.
  3. runs the automated tests to verify the implementation.
  4. Finally refactors the new code to acceptable standards.

You will now use TDD to implement a FIFO queue in Erlang.


Erlang comes with a unit test framework named EUnit.

File to use

In module-8/src/fifo.erl you find the skeleton code for a FIFO module.

Test driven development (TDD)

In the true spirit of TDD, the tests should be written before the implementation. Therefore you are provided with a number of EUnit test cases. The test cases captures design decisions.

First compile

From the Linux shell:

$ rm ebin/fifo.beam
$ make

You will see a number of warnings:

src/fifo.erl:26: Warning: variable 'X' is unused
src/fifo.erl:35: Warning: variable 'In' is unused
src/fifo.erl:37: Warning: variable 'H' is unused
src/fifo.erl:37: Warning: variable 'In' is unused
src/fifo.erl:37: Warning: variable 'T' is unused

Warnings - but success!


From the Linux shell, run EUnit in verbose mode:

$ make testv_fifo

EUnit will now run all the test cases and report the results.

======================== EUnit ========================
module 'fifo'
 fifo:60: new_test_...ok
 fifo:61: new_test_...ok
 fifo:62: new_test_...ok
 fifo: push_test...ok
 fifo: push_pop_test...*failed*
in function fifo:pop/1 (src/fifo.erl, line 41)
  called as pop(tbi)
  in call from fifo:'-push_pop_test/0-fun-0-'/0 (src/fifo.erl, line 84)

    *** test generator fifo:size_test_/0 failed ***
    **in function fifo:push/2 (src/fifo.erl, line 32)
      called as push(tbi,bar)
      in call from fifo:f1/0 (src/fifo.erl, line 88)
      in call from fifo:size_test_/0 (src/fifo.erl, line 91)

        Failed: 1.  Skipped: 0.  Passed: 4.
        One or more tests were cancelled.

From the EUnit report we see that four tests passed with status ok. The first test that failed was the push_pop_test() which terminated with reason: ::function_clause. This means that no matching function clause is found when evaluating a function call. Read here for more information about the various exit reasons.

Dig deeper

To investigate this further we will test the fifo module manually from the Erlang shell:

erlang> F1 = fifo:new().

A new fifo was created. The new fifo is represented by a 3-tuple where the first element is the atom fifo and the second and third elements are empty lists. Now, lets try to push a value to the new fifo:

erlang> F2 = fifo:push(F1, a).

Aha! When pushing the atom a to the fifo F1, the atom tbi is returned. What happens if we use the value of F2 in a pop:

erlang> F3 = fifo:pop(F2).
** exception error: no function clause matching fifo:pop(tbi) (src/fifo.erl,
line 33)

When we try to pop, we call fifo:pop(tbi) and there is no function clause matching. In other words, the function fifo:pop/1 is not defined when called with the atom tbi as argument. The problem is that fifo:push/2 returns the atom tbi.


Change the implementation of fifo:push/2 according to the comments in the source.

Make all tests pass

Your task is to make all test pass by making necessary changes to the implementation of fifo:pop/1 and fifo:push/2.

Small steps

It is often recommended to make a few small changes, then compile and run the tests again.

Additional tests from the Erlang shell

In addition to the EUnit test cases, it is often very helpful to test the functions in a module manually from the Erlang shell.


EDoc lets you write the documentation of an Erlang program as comments in the source code itself, using tags on the form @Name .... A source file does not have to contain tags for EDoc to generate its documentation, but without tags the result will only contain the basic available information that can be extracted from the module.


Provide proper descriptions for all @doc tags.

Generate new html doc files:

$ make doc

Now you can reload the web page and see the new information added to the html documentation page for fifo.erl. To open the documentation from scratch:

$ ./help

u may also use the Makefile to print the url to the documentation index page to the terminal:

$ make doc_url

Type specifications

Erlang is a dynamically typed language. Still, it comes with a notation for declaring sets of Erlang terms to form a particular type, effectively forming a specific sub-type of the set of all Erlang terms. EDoc will detect any type declarations and include this information in the generated documentation.

Here you can read more how to declare types and function specifications using EDoc.


Use -spec type declarations to add type declarations to fifo:push/2 and fifo:pop/1.


Look at the type declarations provided for fifo:new/0, fifo:empty/1 and fifo:size/1. You may also look at the type declarations for the functions in tutorial.erl.