create-report.elm

type alias Book =
    { isbn : String
    , title : String
    , authors : List String
    , copyright : Int
    , edition : Maybe String
    }

type alias LibraryReport =
    { numBooks : Int
    , oldestCopyright : Int
    , newestCopyright : Int
    , uniqueAuthors : Int
    }

createReport : List Book -> LibraryReport
createReport books =
    let
        copyrightYears =
            List.map .copyright books
    in
        { numBooks = List.length books
        , oldestCopyright =
            List.minimum copyrightYears
                |> Maybe.withDefault 0
        , newestCopyright =
            List.maximum copyrightYears
                |> Maybe.withDefault 0
        , uniqueAuthors =
            books
                |> List.map .authors
                |> List.concat
                |> Set.fromList
                |> Set.size
        }