As another way to think about this, and I am unsure how Elm'ish this is (but it is very functional), you could invert your lists into being functions, then pass in commands to operate on the internal lists. This is how some functional list implementations are developed especially when missing HKT's like Elm is (when is Elm getting HKT's anyway?).
On Monday, August 1, 2016 at 1:46:53 AM UTC-6, Ryan W wrote: > > I want to be able to store several different types of lists in a single > data structure, and and each type of list should be treated differently by > the view function. > > What I've tried: > type ThingList > = FooList (List Foo) > | BarList (List Bar) > | BazList (List Baz) > > > This works okay, but if I want to do something like get the length of a > ThingList without caring about whether it is a FooList, BarList or BazList > it seems I have to have to write something like this: > > length : ThingList > length thingList = > case thingList of > FooList list -> > List.length list > > BarList list -> > List.length list > > BazList list -> > List.length list > > Since in my actual program I plan to have several more sub-types in the > actual counterpart to ThingList this becomes somewhat cumbersome and it > definitely isn't DRY. I guess this is a form of the expression problem? > <https://en.wikipedia.org/wiki/Expression_problem> I tried to generalize > it a bit so I wouldn't have to update as many of these functions when I add > a new sub-type like this: > reduce : (List a -> b) -> EntityList -> b > reduce f entityList = > case entityList of > FooList list -> > f list > > BarList list -> > f list > > BazList list -> > f list > > but that gave me an error like this: > > Function `f` is expecting the argument to be: List Foo But it is: List Bar > > In my particular case, at least for now, I only wanted the length of a > ThingList literal so I can work around it, but I'm curious if there is > way to structure the types that avoids this problem. > > -- You received this message because you are subscribed to the Google Groups "Elm Discuss" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/d/optout.
