I'm not an expert, but I'd say you should use a Dict whenever you have
key-value data and you need insert, remove and/or query operations.
You can always achieve the same with a List, but with a naive
implementation you wouldn't get the same performance.
I bet the TodoMVC uses a List for simplicity, especially towards beginners.
A List is definitely simpler to understand than Dict at a first glance.
On Saturday, 3 September 2016 09:28:04 UTC+1, Brian John Farrar wrote:
>
> I am looking for design guidelines on when to model a collection of
> records as a Dict vs. a List.
>
> The TodoMVC uses a List, though at first glance a Dict seems like a more
> natural match:
> - There are several "id based" Msg's (ex. EditingEntry, UpdateEntry,
> Delete, Check)
> - Using Html.Keyed should be (slightly) easier to use
>
> But I think I understand why a List is more pragmatic.
> In (my) practice, things get muddy attempting to implement TodoMVC with a
> Dict.
> Dict.update wants a (Maybe v -> Maybe v) resulting in nested case
> statements which are okay but preferable to avoid.
> (...it could be that my FP-fu is too weak to factor a concise expression.
> Maybe.map doesn't appear useful as record updates aren't compose-able?)
>
> So, the "mapWhen" idiom (via an "updateEntry" helper) does seem to be the
> simplest approach
>
> let
> updateEntry t =
> if ti.id = id then { t | field = newValue } else t
> in
> { model | entries = List.map updateEntry model.entries }
>
> So if Dict isn't the best choice for a use case like TodoMVC...what kinds
> of use cases is it good for?
>
>
>
>
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