TDD with Elm

Elm-test……as a learning assistant

n_umiastowski

Remote meetup March 23rd 2017

Learning Elm

One goal : understand how Lists work in Elm (and in general)

One rule : don’t ask on Slack

Learning Elm

Let’s try the Test-first approach with a little exercise !

Learning Elm

A list…

1 2 3 4 5 6 7 8 9

List Int

…

Learning Elm

Getting to a List of Lists with a defined criteria

1

2

3

4

5

6

7

8

9

1st 2nd 3rd

…

…

…

…

Learning Elm

Defining ancestors and descendants :

1

2

3

4

5

6

7

8

9

1st 2nd 3rd

(8,9)

(9,0)

(1,2)

(2,3)

(3,0)

(4,5)

(5,6)

(6,7)

(7,0)

Learning Elm

[(1,2), (2,3),(3,0),(3,4),(4,5),(5,6),(6,0),(7,8),(8,9),(9,0),….,…]

[ [(1,2), (2,3)],[(3,4),(4,5),(5,6)],[(7,8),(8,9)] ,…,…]

->

From List (Int, Int) to List (List (Int, Int))

What would be the test?

How do I start?

[(1,2), (2,3),(3,0),(3,4),(4,5),(5,6),(6,0),(7,8),(8,9),(9,0)]

->

1/ Splitting criteria

2/ Let’s isolate it ->

The test does not work

Ouch !!!!!

So, what’s the process?

1/ Write a test 2/ The test fails->3/ Write type annotation

4/ Write the function

5/ Test passes !

Let’s design a solution

Now, can I split with that? Let’s think of a test…

->Split on Tuple.first < 3 ? OKBut then…Tuple.first < 6?Tuple.first < 9?Will be a nightmare!

Never forget

Keep

It

Stupid

Simple

KISS !

I’ve got an idea

Let’s think of another test….

->Split on Tuple.first between 0 and 3? OKTuple.first between 3 and 6? OKTuple.first between 6 and 9? OK

Let’s go for that!

Let’s get rid of unused values

Test ->

Code ->

Baby-steps

Test ->

Code->

Tail-recursion! How did I get there?

Recursion is easy

Clean code

Refactoring welcome

Got me there

Just one thing to add

Let’s add (0,3) at the beginning of the returned list

Done!

Test

->

Code->

We’re getting somewhere

And we know we can get this ->Now that we have this ->

Let’s write that function ->

The function

Let’s test the whole thing

[(0,3),(3,6),(6,9)]

[(1,2), (2,3),(3,0),(3,4),(4,5),(5,6),(6,0),(7,8),(8,9),(9,0)]

Simplify the test

Test->Code->

Keep only integration tests

Test the integration, and then go to fix units of code

Experiment 1

[(1,2), (2,3),(3,0) ,(3,4),(4,5),(5,6),(6,0) ,(7,8),(8,9),(9,0),(10,11),(11,0)]Experimentation:

< 3, < 6, < 9, < 11< 3 -> Test OK

< 6 -> Test KO

Drop this experiment, try another

Experiment 2[(1,2), (2,3),(3,0) ,(3,4),(4,5),(5,6),(6,0) ,(7,8),(8,9),(9,0),(10,11),(11,0) ]Experimentation:

< 3, then exclude this, and take < 6

Recursion nightmare

Drop this experiment, try another

2 failed attempts first!

As a conclusion

My exercise is documented with testsI can now test edge cases, wrong inputs, and write fuzz tests !If I come back in 2 months, the tests will remind me of the road to get to the result!

And most important… I succeded, and learnt, I stayed calm and focus, and it was FUN!

Questions ?