Stream: beginners

Topic: Importing types bug?

Johan Lövgren (Oct 22 2023 at 20:40):

I have run into an issue when trying to create a package... Let's say I define a generic type in a module, and then have another module import the type and define a more specific version of it, for example

package/Parser/Advanced/Generic.roc:

interface Parser.Advanced.Generic
    exposes [Parser ]
    imports []

Parser input value :=
    input -> Result value Str

package/Parser/Bytes.roc:

interface Parser.Bytes
    exposes [Parser]
    imports [Parser.Advanced.Generic]

Parser value : Parser.Advanced.Generic.Parser Str value

I also have a main file,
package/main.roc:

And then I have an example file which tries to use the less generic version,
examples/example.roc:

app "example"
    packages {
        cli: "https://github.com/roc-lang/basic-cli/releases/download/0.5.0/Cufzl36_SnJ4QbOoEmiJ5dIpUxBvdB3NEySvuH82Wio.tar.br",
        parser: "../package/main.roc",
    }
    imports [
        cli.Stdout,
        parser.Parser.Bytes.{ Parser }
    ]
    provides [main] to cli

main =
    Stdout.line "hello"

Then I get some errors when I try to build the example! But not when running `roc check package/main.roc´

Johan Lövgren (Oct 22 2023 at 20:41):

The errors I get are:

── MODULE NOT IMPORTED ─────────────────────────── ../package/Parser/Bytes.roc ─

The `Parser.Advanced.Generic` module is not imported:

8│  Parser value : Parser.Advanced.Generic.Parser Str value
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Did you mean to import it?


── MISSING DEFINITION ──────────────────────────── ../package/Parser/Bytes.roc ─

Parser is listed as exposed, but it isn't defined in this module.

You can fix this by adding a definition for Parser, or by removing it
from exposes.

────────────────────────────────────────────────────────────────────────────────

Johan Lövgren (Oct 22 2023 at 20:43):

I can fix the error by also importing Parser.Advanced.Generic.Parserin the example. But should I not be able to use the more specific type and not have to even know/care about the generic one? (this example is inspired by the elm parser, which is built in this way with one advanced parser and then a simpler parser which is a more specific type with fewer type variables)

Brendan Hansknecht (Oct 22 2023 at 21:00):

Not sure this is a fix, but if you import Parser.Advanced.Generic, I would assume you would have to references things as Generic.Parser

Luke Boswell (Oct 22 2023 at 22:11):

Is that the same issue as this? https://github.com/roc-lang/roc/issues/5477

Johan Lövgren (Oct 23 2023 at 06:00):

Oh! Yes that seems to be the same. Ok then I know it’s a known issue at least

Johan Lövgren (Oct 23 2023 at 06:08):

Brendan Hansknecht said:

Not sure this is a fix, but if you import Parser.Advanced.Generic, I would assume you would have to references things as Generic.Parser

Hmm if I try to reference like this Parser value : Generic.Parser Str value I get a panic:

thread '<unnamed>' panicked at 'called `Option::unwrap()` on a `None` value', crates/compiler/load_internal/src/docs.rs:382:71
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Last updated: Jul 26 2025 at 12:14 UTC