Seven More Languages in Seven Weeks: Elm

Between the move and the election and the holidays, took me a long time to finish this chapter.

But I’m glad I did, because Elm is — dare I say — fun?

The error messages are fantastic. The syntax feels like Haskell without being as obtuse.  Even the package management system just feels nice.

A sign of how much I liked working in Elm: the examples for Day Two and Three of the book were written for Elm 0.14, using a concept called signals. Unfortunately, signals were completely removed in Elm 0.17 (!). So to get the book examples working in Elm 0.18, I had to basically rebuild them. Which meant spending a lot of time with the (admittedly great) Elm tutorial and trial-and-erroring things until they worked again.

None of which I minded because, well, Elm is a great language to work in.

Here’s the results of my efforts:

And here’s what I learned:

Day One

  • haskell-inspired
  • elm-installer: damn, that was easy
  • it’s got a repl!
  • emacs mode also
  • types come back with all the values (expression results)
  • holy sh*t: “Maybe you forgot some parentheses? Or a comma?”
  • omg: “Hint: All elements should be the same type of value so that we can iterate through the list without running into unexpected values.”
  • type inferred: don’t have to explicitly declare the type of every variable
  • polymorphism via type classes
  • single-assignment, but the repl is a little looser
  • pipe syntax for if statement in book is gone in elm 0.17
  • case statement allows pattern matching
  • case statement needs the newlines, even in the repl (use `\`)
  • can build own complex data types (but not type classes)
  • case also needs indentation to work (especially if using result for assignment in the repl)
  • records: abstract types for people without beards
  • changing records: use `=` instead of `<-`: { blackQueen | color = White }
  • records look like they’re immutable now, when they weren’t before? code altering them in day one doesn’t work
  • parens around function calls are optional
  • infers types of function parameters
  • both left and right (!) function composition <| and |>
  • got map and filter based off the List type (?)
  • no special syntax for defining a function versus a regular variable, just set a name equal to the function body (with the function args before the equal sign)
  • head::tail pattern matching in function definition no longer works; elm is now stricter about requiring you to define all the possible inputs, including the empty list
  • elm is a curried language (!)
  • no reduce: foldr or foldl
  • have to paren the infix functions to use in foldr: List.foldr (*) 1 list
  • hard exercise seems to depend on elm being looser than it is; afaict, it won’t let you pass in a list of records with differing fields (type volation), nor will it let you try to access a field that isn’t there (another type violation)

Day Two

  • section is built around signals, which were removed in Elm 0.17 (!)
  • elm has actually deliberately moved away from FRP as a paradigm
  • looks like will need to completely rewrite the sample code for each one as we go…thankfully, there’s good examples in the elm docs (whew!)
  • [check gists for rewritten code]
  • module elm-lang/keyboard isn’t imported in the elm online editor by default anymore

Day Three

  • can fix the errors from loading Collage and Element into main by using toHtml method of the Collage object
  • elm-reactor will give you a hot-updated project listening on port 8000 (so, refresh web page of localhost:8000 and get updated view of what your project looks like)
  • error messages are very descriptive, can work through upgrading a project just by following along (and refreshing alot)
  • critical to getting game working: https://ohanhi.github.io/base-for-game-elm-017.html (multiple subscriptions)