MonadReader in elm

Classic.elm

type alias Config =
    { httpEndpoint : String
    , portNumber : Int
    }


type alias Person =
    { name : String
    , address : String
    }


getPersonName : Config -> String -> String
getPersonName config pnr =
    personInfo config pnr |> .name


personInfo : Config -> String -> Person
personInfo config pnr =
    -- use config to make wscall
    Person config.httpEndpoint "Södra kungsgatan"

use : String
use =
    let
        c = Config "http://localhost" 8080
    in        
        getPersonName c "123"

MonadReader.elm

type Reader ctx a
    = Reader (ctx -> a)


unit : a -> Reader any a
unit x =
    Reader (\_ -> x)


runReader : Reader ctx a -> ctx -> a
runReader r ctx =
    case r of
        Reader f ->
            f ctx


withContext : context -> Reader context a -> a
withContext =
    flip runReader


andThen : Reader ctx a -> (a -> Reader ctx b) -> Reader ctx b
andThen reader bindFn =
    Reader
        (\ctx ->
            let
                a =
                    runReader reader ctx

                newReader =
                    bindFn a
            in
                runReader newReader ctx
        )


(>>=) : Reader ctx a -> (a -> Reader ctx b) -> Reader ctx b
(>>=) =
    andThen


ask : Reader ctx ctx
ask =
    Reader identity

UsingReader.elm

getPersonNameM : String -> Reader Config String
getPersonNameM pnr =
    -- does not know anything about the config
    personInfoM pnr >>= (.name >> unit)


personInfoM : String -> Reader Config Person
personInfoM pnr =
    ask >>= \c -> unit (Person ("Got endpoint " ++ c.httpEndpoint) "Södra kungs")

useM : String
useM =
    let
        r = getPersonNameM "123"
        c = Config "endpoint" 8080
    in        
        runReader r c