Nested Comment Tree Decoder

Main.elm

module Main exposing (main)

import Browser
import Html exposing (Html, div, text)
import Json.Decode as D exposing (Decoder)

{-
  This module demonstrates how to decode a nested tree of nodes that are NOT infinitely recursive.
-}
type alias CommentTree =
    CommentWithReplies
        (CommentWithReplies
            LeafComment 
        )
   



type alias CommentWithReplies a =
    { id : String
    , anonymousAuthorName : String
    , replies : List a
    , body : String
    }
        
        
type alias LeafComment =
    { id : String
    , anonymousAuthorName : String
    , body : String
    }


apiResponseDecoder : Decoder a -> Decoder a
apiResponseDecoder dataDecoder =
    D.field "data" dataDecoder
    


-- intentionally ommiting type here
commonFields =
    { id = D.field "id" D.string
    , anonymousAuthorName = D.field "anon_author_name" D.string
    }



leafCommentDecoder : Decoder LeafComment
leafCommentDecoder =
    D.map2 LeafComment
        commonFields.id
        commonFields.anonymousAuthorName
        
    

makeCommentTreeDecoder : Decoder a -> Decoder (CommentWithReplies a)
makeCommentTreeDecoder nextDecoder =
    D.map3 CommentWithReplies
        commonFields.id
        commonFields.anonymousAuthorName
        (D.field "replies" <| D.list nextDecoder)


commentListDecoder : Decoder (List CommentTree)
commentListDecoder =
    let
        commentTreeDecoder =
            makeCommentTreeDecoder
                (makeCommentTreeDecoder leafCommentDecoder
                )
    in
    D.field "comments"
        (D.list commentTreeDecoder)
        
 
json : String
json = """
{
    "data": {
        "comments": [
            {
                "id": "ckhxpcv2l0056b7t28lc2qnii",
                "anon_author_name": "unbeatable-caped-terror",
                "body": "A top-level comment",
                "votes": 0,
                "post_id": "ckhxpc7sh0032b7t24cmlli2j",
                "created_at": 1606326555981,
                "updated_at": 1606326555981,
                "parent_comment_id": null,
                "author_id": null,
                "author": null,
                "replies": []
            },
            {
                "id": "ckhxpde9v0069b7t2tuddh7lm",
                "anon_author_name": "infamous-sir-master",
                "body": "Another top-level comment",
                "votes": 0,
                "post_id": "ckhxpc7sh0032b7t24cmlli2j",
                "created_at": 1606326580868,
                "updated_at": 1606326580868,
                "parent_comment_id": null,
                "author_id": null,
                "author": null,
                "replies": [
                    {
                        "id": "ckhxpdorx0077b7t2y28g02dj",
                        "anon_author_name": "criminal-tomorrow-storm",
                        "body": "Yo this is a reply",
                        "votes": 0,
                        "post_id": "ckhxpc7sh0032b7t24cmlli2j",
                        "created_at": 1606326594477,
                        "updated_at": 1606326594478,
                        "parent_comment_id": "ckhxpde9v0069b7t2tuddh7lm",
                        "author_id": null,
                        "author": null,
                        "replies": []
                    }
                ]
            }
        ],
        "siteVerified": false,
        "postId": "ckhxpc7sh0032b7t24cmlli2j",
        "leafIds": [
            "ckhxpcv2l0056b7t28lc2qnii",
            "ckhxpdorx0077b7t2y28g02dj"
        ]
    }
}
"""

errorToHtml : D.Error -> Html.Html msg
errorToHtml error =
  Html.pre [] [ Html.text (D.errorToString error) ]


decoded : Html msg 
decoded =
    let
        decoder = apiResponseDecoder commentListDecoder
    in
    case D.decodeString decoder json of
        Ok data ->
            text "Yay! Successfully parsed a comment tree"
        
        Err e ->
            errorToHtml e
    



main : Program () () Never
main =
    Browser.sandbox
        { init = ()
        , view = \_ -> div [] [ decoded ]
        , update = \_ _ -> ()
        }