Convert Time.now to Date - Elm - functional-programming

Question

Hello I am brand new to Elm and I am having some difficulty with getting the current time and converting it into a date in Elm.
I have a message type - Message and a function to add a new message to the model. I am trying to store the time the message was posted along with the text and user id.
However I keep getting this error -
The argument to function `fromTime` is causing a mismatch.
59| Date.fromTime (currentTime Time.now)
^^^^^^^^^^^^^^^^^^^^
Function `fromTime` is expecting the argument to be:
Time
But it is:
x -> Time -> Time
Hint: It looks like a function needs 2 more arguments.
Here is the code
type alias Message =
{ text : String,
date : Date,
userId : Int
}
currentTime : task -> x -> Time -> Time
currentTime _ _ time =
time
newMessage : String -> Int -> Message
newMessage message id =
{ text = message
, date = Date.fromTime (currentTime Time.now)
, userId = id
}
I really can't figure out what is going on. Any help would be much appreciated. Thanks.

Answer 1

Elm is a pure language, where function calls are deterministic. Requesting the current time is a little more complex, because there is no function we can call that will return us a different time depending on the time of day. Function calls with the same input will always return the same thing.
Getting the current time lies in the land of side effects. We have to ask the architecture to give us the time in a way that is pure. The way Elm deals with that is through the Task and Program functionality. You send a Task to the Elm Architecture through a Cmd in your update function. The Elm Architecture then does its own thing behind the scenes to fetch the current time, then it responds in pure code with another call to your update function.
Here is a simple example you can paste in at http://elm-lang.org/try, where you click a button to see the current time converted to a Date.
import Html exposing (..)
import Html.Events exposing (onClick)
import Time exposing (..)
import Date
import Task
main =
Html.program
{ init = { message = "Click the button to see the time" } ! []
, view = view
, update = update
, subscriptions = \_ -> Sub.none
}
type alias Model = { message: String }
view model =
div []
[ button [ onClick FetchTime ] [ text "Fetch the current time" ]
, div [] [ text model.message ]
]
type Msg
= FetchTime
| Now Time
update msg model =
case msg of
FetchTime ->
model ! [ Task.perform Now Time.now ]
Now t ->
{ model | message = "The date is now " ++ (toString (Date.fromTime t)) } ! []
If you are familiar with javascript, the purpose of the Now message could be loosely thought of as a callback function, where the argument it provides is the time sent by the Elm Architecture.