Misleading TYPE MISMATCH error on the body of a function

[mordrax]

i'm finding the wording of the type mismatch to be confusing as it's lead me down the wrong path:

-- TYPE MISMATCH ------------------------------ ../frontend/src/Model/Person.elm

Something is off with the body of the `getCoverStartDate` definition:

18|>    Debug.log "Person cover start date:"
19|>        (person.coverInfoDetails
20|>            |> List.sortWith sorter
21|>            |> List.reverse
22|>            |> List.head
23|>            |> Maybe.map .startDate

The body is:

    Maybe DateTime

But the type annotation on `getCoverStartDate` says it should be:

    Maybe Date

In this example, I spent some time working out what the body is actually returning, and it's the correct Maybe Date. I'm 99% certain.

However I believe the compiler is inferring that the type is Maybe DateTime from the usage of the return of the function elsewhere. So it's misleading to say that the body is Maybe DateTime when the body is actually a Maybe Date being used as a Maybe DateTime.

The full function is:

getCoverStartDate : Person -> Maybe Date
getCoverStartDate person =
    let
        sorter : CoverInfoDetail -> CoverInfoDetail -> Order
        sorter a b =
            Alfred.DateTime.compare a.startDate b.startDate
    in
    Debug.log "Person cover start date:"
        (person.coverInfoDetails
            |> List.sortWith sorter
            |> List.reverse
            |> List.head
            |> Maybe.map .startDate
        )

CoverInfoDetail.startDate, Alfred.DateTime.compare both work with Date.

Once I get it compiling, I can confirm that it's the usage of the function and not the body that's Maybe DateTime