How to handle Http POST

sign-in.elm

module SignIn exposing (..)

import Html exposing (..)
import Html.App
import Http
import Json.Decode
import Task


type alias Model =
    { signInMessage : String
    }


model : Model
model =
    { signInMessage = "Request not sent"
    }


type Message
    = SignInStart
    | SignInSucceed String
    | SignInFail Http.Error


view : Model -> Html Message
view model =
    label [] [ text model.signInMessage ]


update : Message -> Model -> ( Model, Cmd Message )
update message model =
    case message of
        SignInStart ->
            ( { model | signInMessage = "Signing in..." }, Cmd.none )

        SignInSucceed _ ->
            ( { model | signInMessage = "Sign in succeeded" }, Cmd.none )

        SignInFail error ->
            ( { model | signInMessage = "Sign in failed" ++ toString error }, Cmd.none )

init =
    ( model, Task.perform SignInFail SignInSucceed (Http.post Json.Decode.succeed "sign-in" (Http.string """{"email":"[email protected]","password":"bar"}""")) )


main : Program Never
main =
    Html.App.program
        { init = init
        , view = view
        , update = update
        , subscriptions = (\model -> Sub.none)
        }

Compiler error message is:

The 3rd argument to function `perform` is causing a mismatch.

45|              Task.perform SignInFail SignInSucceed (Http.post Json.Decode.value "sign-in" (Http.string """{"email":"[email protected]","password":"bar"}""")
                                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Function `perform` is expecting the 3rd argument to be:

    Task.Task Http.Error String

But it is:

    Task.Task Http.Error Json.Encode.Value

Hint: I always figure out the type of arguments from left to right. If an
argument is acceptable when I check it, I assume it is "correct" in subsequent
checks. So the problem may actually be in how previous arguments interact with
the 3rd.

Detected errors in 1 module.