Stream: beginners

Topic: Convert one field of a record

Oskar Hahn (Jan 08 2025 at 19:16):

I have a record with many fields. One of the fields has the type Utc.Utc from basic-webserver.

To convert the record to json, I have to convert the field to something else that implements encodable. What I want to do is something likeencodable_object = {object & time: object.time |> Utc.to_nanos_since_epoch}. But this does not work, since the type of { object & ...} has to have the same type as object. So you can change the value, but not the type.

Is there an easy way to create a new record with a similar type? Or do I have to type out all fields?

Richard Feldman (Jan 08 2025 at 19:17):

we should give Utc the Encoding ability

Sam Mohr (Jan 08 2025 at 19:17):

For now, you have to type out all of the fields, yes.

Sam Mohr (Jan 08 2025 at 19:18):

We didn't want to enable using record updates to change the underlying type because that would make it really easy for beginners to require moving a lot of data around

Sam Mohr (Jan 08 2025 at 19:18):

In general, we should push for all custom types in the future to have as many of the generated methods as possible

Sam Mohr (Jan 08 2025 at 19:18):

In our stdlib/platforms

Oskar Hahn (Jan 08 2025 at 19:20):

Sam Mohr said:

We didn't want to enable using record updates to change the underlying type because that would make it really easy for beginners to require moving a lot of data around

Only if the ref count is higher then 1 (which is true for everything in Roc) and the new type has a different size or alignment then the old type.

It does not seem so different then List.map. It also copies a lot of memory if the new size is different.

Oskar Hahn (Jan 08 2025 at 19:21):

But I get your point.

Sam Mohr (Jan 08 2025 at 19:21):

I think the door is still open to implementing it

Sam Mohr (Jan 08 2025 at 19:24):

#contributing > record `..` update syntax @ 💬

Sam Mohr (Jan 08 2025 at 19:24):

Here's the most recent discussion

Sam Mohr (Jan 08 2025 at 19:24):

Feel free to suggest against the conclusion we made in there

Last updated: Jul 26 2025 at 12:14 UTC