At first I wondered if a type is really what you need for this application,
as opposed to say a list of strings. Anyway one idea is to write a function
to explicitly convert the type to a string. I think this is better than
simply taking the type name and making it a string because the string
representation may need spaces or other punctuation. Using an explicit case
means the compiler won't let you forget if you add a new type.
module Main exposing (..)
import Html exposing (Html, text)
type Province = Alberta | BritishColumbia | Saskatchewan
toString : Province -> String
toString p =
case p of
Alberta -> "Alberta"
BritishColumbia -> "British Columbia"
Saskatchewan -> "Saskatchewan"
main : Html a
main =
text (toString Alberta)
mch
On Tuesday, May 2, 2017 at 9:44:34 PM UTC-6, Matthew Buscemi wrote:
>
> I have run into a problematic use case a couple of times, and based on
> Slack discussions earlier today, it seems I'm not alone.
>
> The situation occurs when, for some reason, I need to maintain a list of
> all constructors of a union type. The example given in Slack by mltsy:
>
> type State = Alabama | Alaska | Arizona | Arkansas | ...
>
> allStates = [ Alabama, Alaska, Arizona, Arkansas, ...]
>
> stateSelectBox : List State -> Html Msg
> stateSelectBox states =
> let stateValues = List.map toString states
> in ...
>
> In the example above, simply turning allStates into a list of strings
> would be fine for this particular use case, but in the context of a larger
> system, it could be inadequate–other functions besides stateSelectBox may
> want to utilize the power of performing a case over the union type.
> However, the above solution is also structurally undesirable–if a new type
> is added to State, then the programmer must also remember to update
> allStates separately. The duplication and tight coupling are disconcerting,
> and yet I can't see a viable alternative that preserves the power of types
> without incurring the duplication.
>
> I have wondered if a language-level construct that takes a union type and
> returns a list of all constructors for that type would be appropriate for
> Elm. Just a thought.
>
> - Matt
>
>
>
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