Non-descriptive error when encountering a typeclass mismatch

[JonathanFraser]

code:

module Main exposing (..)

import Html exposing (Html, text)
import Json.Decode as Json
import Set exposing (Set)

main : Html a
main =
    text "Hello, World!"

type Selection
    = SelInt Int
    | SelStr String

decodePath : Json.Decoder (List Selection)
decodePath =
    Json.list <|
        Json.oneOf
            [ Json.string |> Json.map SelStr
            , Json.int |> Json.map SelInt
            ]

decodeFlag : Json.Value -> Set (List Selection)
decodeFlag s =
    --the error from this line is nonsensical
    Json.decodeValue (Json.list decodePath |> Json.map Set.fromList) s
        |> Result.withDefault Set.empty

The problem here is that the user is trying to define a Set of items which are not comparable. This is of course not allowed. However, the error message given does not provide any indication of that.

The right side of (|>) is causing a type mismatch.
(|>) is expecting the right side to be a:

Json.Decoder (List (List Selection)) -> a

But the right side is:

Json.Decoder (List (List Selection)) -> Json.Decoder (Set (List Selection))

Hint: With operators like (|>) I always check the left side first. If it seems
fine, I assume it is correct and check the right side. So the problem may be in
how the left and right arguments interact.

According to the error message the code appears to be correct.

[JonathanFraser]

moved from elm-lang/elm-compiler#1619