Ok thanks for the explanation. I'm still too new to completely grok how to
use Focus but I understand well enough why what I was trying to do wouldn't
work.
The extensible records link sounds very useful for a couple of reasons -
thanks for the tip.
On Sunday, 19 June 2016 23:47:40 UTC+7, Aaron VonderHaar wrote:
>
> To do this without focus, you’d write something like:
>
> updateCanvas : (Canvas -> Canvas) -> Model -> Model
> updateCanvas fn model =
> { model | canvas = fn model.canvas }
>
> updateScale : (Int -> Int) -> Canvas -> Canvas -- probably put this in Canvas
> module
> updateScale fn canvas =
> { canvas | scale = fn canvas.scale }
>
> inc x = x + 1
>
> -- then in update...
> newModel = updateCanvas (updateScale inc) model
>
> Another technique to avoid using focus is to have a flatter data structure
> using extensible records (as described here
> https://groups.google.com/d/msg/elm-discuss/g_hxu-tjzUQ/ah30ic4zHTQJ )
>
> Finally, to answer your question, when using Focus, you have to define
> canvas and scale yourself—they are not generated automatically. You would
> write something like this (note the similarity to the updateCanvas and
> updateScale functions in my first suggestion above):
>
> canvas : Focus Model Canvas
> canvas =
> Focus.create
> .canvas
> (\fn model -> { model | canvas = fn model.canvas })
>
> scale : Focus Canvas Int
> scale =
> Focus.create
> .scale
> (\fn canvas -> { canvas | scale = fn canvas.scale })
>
>
>
> On Sun, Jun 19, 2016 at 3:58 AM, <[email protected] <javascript:>>
> wrote:
>
>> I want to use the focus library to update a nested property per this
>> example from the docs
>> <http://package.elm-lang.org/packages/evancz/focus/2.0.1/Focus>:
>>
>> set (physics => velocity => x) 0 object
>>
>>
>>
>> My model is like this:
>>
>> 0 { canvas =
>> 1 { width = 800
>> 2 , height = 600
>> 3 , scale = 1
>> 4 , ratioToWorld = 10
>> 5 }
>> 6 , player = player
>> 7 , enemy = enemy
>> 8 }
>>
>>
>> My update code is this:
>>
>> 0 update : Msg -> Model -> (Model, Cmd Msg)
>> 1 update action model =
>> 2 case action of
>> 3 Step time ->
>> 4 let
>> 5 newScale = model.canvas.scale + 1
>> 6 newModel = Focus.set (canvas => scale) newScale model
>> 7 in
>> 8 (newModel, Cmd.none)
>> 9 NoOp ->
>> 10 (model, Cmd.none)
>>
>> Initially I had a great deal of trouble with the compiler complaining
>> about the "=>" operator but I got lucky and found a code example in github
>> somewhere that showed me how to import it from the module with this:
>>
>> 0 import Focus
>> 1 import Focus exposing ((=>))
>>
>>
>> But I'm still getting syntax errors where it complains that it doesn't
>> know what canvas, scale etc are:
>> Detected errors in 1 module.
>>
>>
>>
>> -- NAMING ERROR ------------------------------------------------- ././
>> Update.elm
>>
>> Cannot find variable `canvas`
>>
>> 17| newModel = Focus.set (canvas => scale) newScale model
>> ^^^^^^
>>
>> -- NAMING ERROR ------------------------------------------------- ././
>> Update.elm
>>
>> Cannot find variable `scale`
>>
>> 17| newModel = Focus.set (canvas => scale) newScale model
>> ^^^^^
>> Maybe you want one of the following?
>>
>> List.scanl
>>
>>
>> Any help with what I'm doing wrong would be appreciated. I could work
>> how to do it without using Focus but I plan/hope to use Focus a lot.
>>
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>
>
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