7GUIs in Elm

I'm happy to finally share my implementation of 7GUIs (source) in Elm.

The last two tasks, Circle Drawer and Cells were the most challenging and interesting of the seven tasks. However, I was delighted 😄 to see how effective Elm was at modeling both tasks.

Change propagation for Cells

One of the main highlights for me was figuring out a neat and performant implementation of change propagation for Cells. I modeled it as a directed graph and to figure out the order in which to propagate the changes through the cells I used a topological sort. I also achieved a clean separation of the domain model for the spreadsheet and the UI which allowed me to easily write tests for the spreadsheet. Finally, testing allowed me to come up with a well-crafted test case that breaks naive implementations of change propagation.

Comments

[Joan Llenas Masó]

I did the 7GUIs like 7 years ago when I was learning Elm, but I didn't even try to implement the Cells exercise because I didn't know where to start. I may give it a try now. There's so much interesting stuff in it :)

May I ask what is the grammar.ebnf file for? Did you auto generate the formula parser? or is it just part of your thought process?

[Dwayne Crooks]

I may give it a try now.

Go for it. There's much to be learned by attempting the Cells task.

May I ask what is the grammar.ebnf file for?

The grammar.ebnf is an artifact of my process.