Struct Embedding in Elm

Elm supports composition of types through a concept similar to struct embedding. This is achieved through the use of extensible records and type aliases.

```elm
module Main exposing (main)

import Html exposing (Html, div, text)


type alias Base =
    { num : Int }


describe : Base -> String
describe base =
    "base with num=" ++ String.fromInt base.num


type alias Container =
    { base : Base
    , str : String
    }


main : Html msg
main =
    let
        co =
            { base = { num = 1 }
            , str = "some name"
            }
    in
    div []
        [ text ("co={num: " ++ String.fromInt co.base.num ++ ", str: " ++ co.str ++ "}")
        , Html.br [] []
        , text ("also num: " ++ String.fromInt co.base.num)
        , Html.br [] []
        , text ("describe: " ++ describe co.base)
        , Html.br [] []
        , text ("describer: " ++ describe co.base)
        ]

In Elm, we don’t have struct embedding as in some other languages, but we can achieve similar composition using record types and type aliases.

We define a Base type alias with a num field, and a describe function that works with Base values.

The Container type alias includes a base field of type Base, along with an additional str field. This is similar to embedding in other languages.

In the main function, we create a Container value. We can access the num field through co.base.num.

Elm doesn’t have method-like syntax, so we call the describe function by passing co.base as an argument.

Elm uses a more functional approach to polymorphism, so we don’t need to explicitly define interfaces. The describe function can work with any record that has a num field.

To run this Elm program, you would typically compile it to JavaScript and run it in a web browser. The output would be displayed on the web page.