Stream: beginners

Topic: Remove record field from inference?

view this post on Zulip Trevor Settles (Jun 26 2024 at 03:36):

I've got a use case that involves type inference, and I'd like to ensure that the data earlier in the program doesn't infer to have a field, in this case style: Str. Heres a code example of what I'd like to do, but it doesn't compile:

style = "some css"
addStyle : {}a -> { style : Str }a
addStyle = \r -> { r & style }

My though is that the a would 'eat' the style: Str, and give the intended behavior

If addStyle infers that it should return {style: Str, foo: Bool}*, then it would infer that it should take {foo: Bool}*. Do I have the wrong mental model, or is this not possible right now?

view this post on Zulip Sam Mohr (Jun 26 2024 at 04:30):

@Trevor Settles the & record update syntax cannot change the structure of the record, nor can anything really in Roc. The only way to do that is to manually copy over the fields you want to a new record.

view this post on Zulip Sam Mohr (Jun 26 2024 at 04:31):

And there's not any way (as far as I know) to do what you're trying here, besides with the current style of record builders

view this post on Zulip Sam Mohr (Jun 26 2024 at 04:31):

But that's much fancier than what you're trying to do

view this post on Zulip Sam Mohr (Jun 26 2024 at 04:34):

So the way is to define something like

Element : { tag: Str, content: Str, style: Str }

newElem : Str -> Element
newElem = \tag -> { tag, content: "", style: "" }

addStyle : { style : Str }a -> { style : Str }a
addStyle = \r -> { r & style: "some css" }

coloredDiv =
    newElem
    |> addStyle

view this post on Zulip Trevor Settles (Jun 26 2024 at 04:38):

Thanks for the suggestion! I'm doing json parsing, and HTML gen with RTL, so I'm really trying to lean into the type inference. I ended up getting around this by adding a style key in the json I was taking in

view this post on Zulip Sam Mohr (Jun 26 2024 at 04:40):

Oh, if you're trying to decode JSON with an optional field, the OptionOrNull type can handle that https://github.com/lukewilliamboswell/roc-json/blob/main/package/OptionOrNull.roc

view this post on Zulip Sam Mohr (Jun 26 2024 at 04:41):

Your Roc code will still need the field, but you can avoid needing to add optional fields in the JSON.

view this post on Zulip Brendan Hansknecht (Jun 26 2024 at 04:42):

addStyle = \r -> { r & "some css" }

should be:
addStyle = \r -> { r & style: "some css" }

view this post on Zulip Sam Mohr (Jun 26 2024 at 04:43):

But you would need to extract it from the OptionOrNull type using OptionOrNull.getResult |> Result.withDefault

view this post on Zulip Sam Mohr (Jun 26 2024 at 04:43):

Yeah, whoops

Last updated: Jul 26 2025 at 12:14 UTC