Elm and unrepresentable states

My Tailor-made Model

Nicolas Umiastowski@n_umiastowski

Remote meetup November 22nd 2016

First attempt : make it workIn a very « non-functional » kind of way

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Answered questions

There questions are really answered?

I want to be able to make sure that:

If one question is answered, then it has an answerId

Answered question that does not have an answerId : Possible

Invalid states?

Unrepresentable state:

Encoding the answered state of a question in the constructor means that I can’t write code that doesn’t check for whether a question is

answered or not

Making undesired state unrepresentable

Distinguish between answered and non answered questions.

But that is not a state, that is just two lists!

Unrepresentable states

I display only one list of questions

I check that there is no unanswered questions

What could happen… ?

I could have answered questions without an answerId

We can check if a questions has an answerId or not, but we cannot check that it has an “answered” or “unanswered” state.

Invalid states?

Define a real state for the questions

Modify an existing answer

Answer a question

If there are only answered questions….

… then we display the messsage

All questions have only two possible states : • Answered• Unanswered

Invalid states?

3rd attempt of unrepresentable statesWhy not have a state for the answer, instead of an Id ?

The answers also have different states.

State of the question

State of the answer

By the way… unrepresentable states does not prevent all impossible states!

I could have two different questions with the same Id, one question with an empy text, etc…

ConclusionAlways try to see if a good pattern can be appliedIt will help you learn and get betterTakes more time at the beginning, but then we go faster and bugs are avoided

Questions ?