Stream: beginners

Topic: the Json-able type

Warry (Aug 20 2026 at 08:42):

hi folks!

I'm toying with basic-webserver and have a question regarding Json.to_str. it works! but parts of the signature seem to be somehow internal, so I can't just copy paste it to my code, and every time I try to type anything around it I get errors.

can I express "a" as a "Json-able" value yet ?

thanks!

George (Aug 20 2026 at 14:54):

If you have a FooDto like this:

FooDto := {
  bar: Str,
  # ...
}

and you want to say "yo I want to turn this into JSON", then you can add this encoder_for : _ definition, and static dispatch should handle the rest. Your type would then look like this:

FooDto := {
  bar: Str,
  # ...
}. {
  encoder_for : _
}

If you have code like:

a = something_that_returns_foo_dto!()?

then you might need to add a type annotation like this:

a : FooDto
a = something_that_returns_foo_dto!()?

Does this answer your question? If not, a code example of yours might help.

Maxime Pigeon (Aug 24 2026 at 00:08):

I have a related issue where this:

respond : Status, a -> Response where [
    a.encoder_for : Encoding.JsonEncoding ->
        (a, Encoding.JsonEncodeState ->
            Try(Encoding.JsonEncodeState, [])),
]
respond = |code, body| {
    Response.from_status(code.to_u16())
        .with_headers([
            {
                name: "Content-Type",
                value: "application/json; charset=utf-8",
            },
        ])
        .with_body(Json.to_str(body).to_utf8())
}

doesn't compile because JsonEncoding and cie. are not in scope. Yet they seem to be part of the public documentation?

Maxime Pigeon (Aug 24 2026 at 00:09):

I know we can use a type hole, but it would be nice to make the "json-able" type explicit in the signature.

Warry (Aug 24 2026 at 09:54):

I was able to compile this code, the encoder_for had to be a one argument function though :

ok : a -> Try(Response, _)
    where [
        a.encoder_for : _ -> _,
    ]
ok = |response|
    Ok(
        Response.from_status(200)
            .with_headers([{ name: "Content-Type", value: "application/json; charset=utf-8" }])
            .with_body(Str.to_utf8(Json.to_str(response)))
    )

@Maxime Pigeon precisely ! JsonEncoding is there, the compiler can find it, but we can't reference it.

(PS: cool prénom, probablement le meilleur)

Last updated: Sep 03 2026 at 15:16 UTC