Learning Elm - Part 1

Update 1 Look like it’s a bit unclear my end goal of this exercise so I list down the requirements here:-

  1. Have 2 values instead of single value like the original example.
  2. When button + clicked, it should increment value 1 by 1, and increment value 2 by 10.
  3. When button - clicked, it should decrement value 1 by 1, and increment value 2 by 10.
  4. When button Reset clicked, it should increment value 2 by 10.

As you can see above, the increment/decrement logic for value 1 is different but for value 2, the logic is similar in any conditions so it should be implement in a single place, instead of having to repeat it for each of the conditions.

#####*

I decided to try Elm yesterday but after spending a couple of hours, a bit frustrated after feeling I’m not going anywhere. Everywhere on the web people are praising Elm as easy to learn, even a 4th grader manage to build a simple games in just few hours, and I’m still glaring trying to understand the button examples from the docs.

The button example is quite simple, you have 2 buttons that when clicked, will increment or decrement a single value (model). The code look like this:-

import Html exposing (Html, button, div, text)
import Html.App as Html
import Html.Events exposing (onClick)



main =
  Html.beginnerProgram
    { model = model
    , view = view
    , update = update
    }



-- MODEL


type alias Model = Int


model : Model
model =
  0



-- UPDATE


type Msg
  = Increment
  | Decrement


update : Msg -> Model -> Model
update msg model =
  case msg of
    Increment ->
      model + 1

    Decrement ->
      model - 1



-- VIEW


view : Model -> Html Msg
view model =
  div []
    [ button [ onClick Decrement ] [ text "-" ]
    , div [] [ text (toString model) ]
    , button [ onClick Increment ] [ text "+" ]
    ]

This code work perfectly when I try with elm-reactor. It’s a good start. So I start to change it a bit by adding a reset button that will reset the model value back to 0. As claimed in the docs, it work quite intuitively, I managed to get around it even without fully understanding the core language yet.

diff --git a/button2.elm b/button2.elm
index a505170..5b20f65 100644
--- a/button2.elm
+++ b/button2.elm
@@ -31,6 +31,7 @@ model =
 type Msg
   = Increment
   | Decrement
+  | Reset


 update : Msg -> Model -> Model
@@ -42,6 +43,8 @@ update msg model =
     Decrement ->
       model - 1

+    Reset ->
+      0


 -- VIEW
@@ -53,4 +56,5 @@ view model =
     [ button [ onClick Decrement ] [ text "-" ]
     , div [] [ text (toString model) ]
     , button [ onClick Increment ] [ text "+" ]
+    , button [ onClick Reset ] [ text "Reset" ]
     ]

Actually it not quite straightforward as originally I try to this instead when reseting the model value back to 0:-

    Reset ->
      model = 0

And the compiler complained:-

Detected errors in 1 module.


-- SYNTAX PROBLEM -------------------------------------------------- button2.elm

I ran into something unexpected when parsing your code!

47|       model = 0
                ^
I am looking for one of the following things:

    end of input
    whitespace

I got pass through that after a couple of try and error. I think the error message will be more helpful if it say something like “assignment is not allowed here”, “Reassigning value not allowed”, or “Mutating value not allowed here”.

Now I got passed the first challenge, I started thinking what if I have 2 value to update when the button clicked ? So I defined model2:-

+model2 : Model
+model2 =
+  0

But I don’t know how to pass the model2 value into the update function. After googling around, I ended up defining a record model instead.

--- a/button2.elm
+++ b/button2.elm
 -- MODEL


-type alias Model = Int
+type alias Model = { one: Int, two: Int }


 model : Model
 model =
-  0
-
+  { one = 0, two = 0 }


 -- UPDATE
@@ -31,17 +30,20 @@ model =
 type Msg
   = Increment
   | Decrement
+  | Reset


 update : Msg -> Model -> Model
 update msg model =
   case msg of
     Increment ->
-      model + 1
+      { model | one = model.one + 1 }

     Decrement ->
-      model - 1
+      { model | one = model.one - 1 }

+    Reset ->
+      { model | one = 0, two = 0 }


 -- VIEW
@@ -51,6 +53,8 @@ view : Model -> Html Msg
 view model =
   div []
     [ button [ onClick Decrement ] [ text "-" ]
-    , div [] [ text (toString model) ]
+    , div [] [ text (toString model.one) ]
+    , div [] [ text (toString model.two) ]
     , button [ onClick Increment ] [ text "+" ]
+    , button [ onClick Reset ] [ text "Reset" ]
     ]

It look easy here but I don’t know how to explain how hard it is to get up until this state. Figuring the syntax how to update the record … sigh. Using record work now but it still no different that the original example. I can’t update model.two value when any button was clicked. My first attempt was to add code that updating model.two outside the case block.

--- a/button2.elm
+++ b/button2.elm
@@ -45,6 +45,8 @@ update msg model =
     Reset ->
       { model | one = 0, two = 0 }

+  { model | two = model.two + 10 }
+

 -- VIEW

Adding the line before the case block also didn’t work. It look like only single expression is allowed in the function body, I don’t know. Of cource I can update model.two in all of the conditions but that look a lot of repeatition and against DRY. At this point, I started to give up and call it a day.

This morning, I try to look at it again. Watch a couple of videos and try to think from different angle. Already join Elm Slack group since I think maybe just asking will lead me to the right direction instead of banging in the dark like this.

I think about functional programming, changing state and side effect. So probably what I’m trying to do yesterday - changing model.two value outside the case block was a side effect. FP is about calling function, pass a parameter and get back a result. So maybe I need to define another function that receive the updated model.one, and then update model.two inside that function. So I ended up with this code:-

import Html exposing (Html, button, div, text)
import Html.App as App
import Html.Events exposing (onClick)


main =
  App.beginnerProgram { model = model, view = view, update = update }


-- MODEL

type alias Model = { one: Int, two: Int }

model : Model
model =
  { one = 0, two = 10 }


-- UPDATE

type Msg = Increment | Decrement | Reset

update2 : Model -> Model
update2 model =
    { model | two = model.two + 10 }

update : Msg -> Model -> Model
update msg model =
  case msg of
    Increment ->
      update2 { model | one = model.one + 1 }

    Decrement ->
      update2 { model | one = model.one - 1 }

    Reset ->
      { model | one = 0, two = 0 }

-- VIEW

view : Model -> Html Msg
view model =
  div []
    [ button [ onClick Decrement ] [ text "-" ]
    , div [] [ text (toString model.one) ]
    , div [] [ text (toString model.two) ]
    , button [ onClick Increment ] [ text "+" ]
    , button [ onClick Reset ] [ text "Reset" ]
    ]

So now doesn’t matter which button clicked, model.two always incremented by 10. Not sure if this the correct solution but at least I got it moving.

Written on October 30, 2016