Representing a selected Dict Item

Learn
1

Hi,

Inside my application I have a list of addresses, a user can select a single address, edit a field and click save.

I’m struggling with the best way to model the ‘selected’ address element of this process, currently I have:

type Address = Address (Dict Int AddressFields)

What’s the correct way to model that an address is being edited? So far I’ve come up with the following, but it’s pretty ugly.

type Address = Address (Dict Int AddressFields) (Maybe (Int, AddressFields)

Or another idea:

type Address
    = Address
        { previous : Dict Int AddressData
        , current : Maybe AddressData
        }

But again, how do I store which field I want to update once a user clicks save? Or put another way, once a user clicks save I don’t want any address to be selected, I want the address that was updated to go back into the list and nothing be selected.

I also thought about adding an selected attribute to the record but this could introduce impossible states (eg two elements selected)

Looking forward to hearing your ideas :+1:

2

Perhaps something like this list selection library? https://package.elm-lang.org/packages/NoRedInk/list-selection/latest/

3

I would model it something like

type Model =
    { addresses : Dict Int AdressField
    , addressForm : Maybe (Int,AddressFieldForm)
    , ...
    }

And then adding and editing should be handled by the same function, only initiation would be handled differently: Adding would create an empty AddressFieldForm and editing would insert the necessary data.

I’m not quite sure though why you are using an opaque type. Maybe I’m missing something. :thinking:

1 Like
4

You could also do it as:

type Model =
    { addresses : Dict Int AdressField
    , addressForm : Maybe Int
    , ... 
    }

That way you don’t duplicate the AddressField.

Then call Dict.get id model.addresses in the view, and case on it:

view model = 
    case `Dict.get id model.addresses` of
        Nothing -> ... -- render view when not editing anything.

        Just addressField -> ... -- render view editing the address.

Which isn’t any more complex, since the addressField is inside a Maybe anyway, when it is duplicated.

5

Yeah, but once you edit the Field you need to start storing the Field form somewhere. You can’t store it in the dict, as the user might decide to scrap it.

1 Like
6

My personal preference here would be to use a custom type instead of a Maybe, because I think it would be more descriptive.

type EditingStatus
    = NotEditing
    | Editing Int AddressFieldForm

type Model =
    { addresses : Dict Int AdressField
    , editingStatus : EditingStatus
    , ...
    }
7

This topic was automatically closed 10 days after the last reply. New replies are no longer allowed.