Per your Medium post and the search for Elm "patterns", let me suggest
casing on tuples. This is useful in update functions for state machines but
it is also useful in this case. I also broke things apart into slightly
smaller functions to keep them simpler.
type Destination
= NotChosen
| ToCountry Country
| ToCity Country City
type alias Model =
{ destination : Destination }
type Msg
= SetCountry Country
| SetCity City
updateDestination : Msg -> Destination -> Destination
updateDestination msg oldDestination =
case ( msg, oldDestination ) of
( SetCountry newCountry, NotChosen ) ->
ToCountry newCountry
( SetCountry newCountry, ToCountry oldCountry ) ->
ToCountry newCountry
( SetCountry newCountry, ToCity oldCountry oldCity ) ->
if oldCountry == newCountry then
oldDestination
else
ToCountry newCountry
( SetCity newCity, ToCountry oldCountry ) ->
ToCity oldCountry newCity
( SetCity newCity, ToCity oldCountry oldCity ) ->
ToCity oldCountry newCity
( _, _ ) ->
Debug.log "Illegal message for destination (ignoring)" (msg,
oldDestination)
|> (always oldDestination)
update : Msg -> Model -> Model
update msg model =
{ model
| destination = model.destination |> updateDestination msg
}
The chief reason to keep the logic out of the view is that it's probably
easier to test the model by itself than via the view.
I would probably be tempted to put destination into its own module and make
the representation private. This module could then be subjected to various
external tests with property testing confirming things like: we can't have
a city without a country; we can always set the country; and, if we have a
country, we can always set the city. Only the first of these follows from
the data structure itself. The rest are assertions about the update logic.
Mark
On Tue, Dec 13, 2016 at 8:14 AM, Wouter In t Velt <[email protected]>
wrote:
> Op zondag 11 december 2016 20:01:09 UTC+1 schreef Brian Marick:
>>
>> Since FP has been around for a long time, too, I’m hoping that those
>> idioms and patterns have been written down, with a good effort toward
>> explaining them (with examples!) Where?
>>
>
> As it happens, I just wrote a post on this topic over on Medium:
> https://medium.com/@wintvelt/how-to-make-impossible-states-
> impossible-c12a07e907b5
>
> It is more of an experiment with different directions. It would be
> interesting if there were more extensive patterns around from people with
> more of an FP background..
>
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