Stream: roctoberfest

Topic: 2023 day 8

Elias Mulhall (Oct 12 2023 at 22:11):

At long last https://github.com/mulias/roctoberfest/tree/main/advent_2022/day_08

• I got sidetracked on this one building out a module for fixed length 2D arrays. It's coming along, but is still a little rough. I feel like I need to do some survey work on multidimensional array APIs, because it turns out the small amount of python I did years ago did not magically give me all the insight I needed.
• Working with opaque types is nice. When I first read the docs I didn't like that opaque types have a special syntax, but in practice it makes it feel more like a deliberate language feature, vs in languages like Elm/OCaml where it feels more like a trick you need to be taught.
• I ran into a compiler issue trying to set a default value for an optional record element. I'll try to make a simple example, but the short version is I wanted to do this

walk = \array, startState, options, fn ->
    {
        direction ? Forward,
        orientation ? Rows,
        start ? defaultStartIndex array direction
    } = options

but I ran into a couple different compiler issues and couldn't get the value of start to rely conditionally on direction. I went with a different API for now.

• Not having any variable shadowing is super irritating. I wanted to have a public function called shape to get the dimensions of an Array2D. that meant I couldn't use shape as a variable in any other function. Doing the occasional gymnastics to not repeat variable names is not a deal breaker, but it's the most common papercut I encountered.
• The UNRECOGNIZED NAME error "The Foo module does not expose anything by the name bar" does not give a line number for the issue and can be a bit tedious to debug.

Luke Boswell (Oct 12 2023 at 22:48):

For the optional deconstruction, does it work if you do it all in the one line within the function? From memory I think there was a bug with the parser that meant it worked on one line but across multiple lines it didn't. So walk = \array, startState, {direction? Forward,orientation? Rows,start? defaultStartIndex array direction}, fn -> ?

I'm not sure I have seen optional field destructuring as a statement, i.e. within the body of a function and not the arguments, that looks like a really useful feature. I'm not sure but maybe we should raise a thread in #ideas for it? Or maybe that is already the intended use for that, in which case it would be good to add an explanation to the Tutorial.

Luke Boswell (Oct 12 2023 at 22:50):

Actually, I just realised.. I doubt you can use the array argument and call a function when providing a default value... :oops:

Elias Mulhall (Oct 12 2023 at 22:56):

I should really make an example with the different errors that came up, but yes having to be on one line was confusing. The formatter forces it which helped figure that one out.

This is currently working fine

WalkOptions : {
    direction ? [Forward, Backwards, ForwardFrom Index, BackwardsFrom Index],
    orientation ? [Rows, Cols],
}

walk : Array2D a, state, WalkOptions, (state, a, Index -> state) -> state
walk = \array, startState, options, fn ->
    { direction ? Forward, orientation ? Rows } = options

The main thing I ran into was that the default value for start couldn't use the value for default, which right now is specifically mentioned in the tutorial as something you can do.

Elias Mulhall (Oct 12 2023 at 22:57):

This section has an example where a default value is set with a function and a reference to a different value getting destructured https://www.roc-lang.org/tutorial#optional-record-fields

Elias Mulhall (Oct 12 2023 at 22:58):

{ height, width, title ? "", description ? Str.concat "A table called " title }

Anton (Oct 13 2023 at 09:19):

For reference, shadowing has been thoroughly debated.

Last updated: Jul 26 2025 at 12:14 UTC