How do I get distinct items from a list? - elm

Question

It's not clear to me how to retrieve distinct items from a list.
I have the following code:
topicsFromLinks : List Link -> List Topic
topicsFromLinks links =
links
|> List.map (\l -> l.topics)
|> List.concat
|> Set.fromList
|> Set.toList
Error:
The definition of topicsFromLinks does not match its type
annotation. - The type annotation for topicsFromLinks says it always
returns:
List Topic
But the returned value (shown above) is a:
List comparable
I expect the following lines to just work in regards to structural equality:
|> Set.fromList
|> Set.toList
Why am I receiving a List of Comparables?
How do I resolve this compilation error?
Appendix:
type alias Topic =
{ name : String, isFeatured : Bool }
type alias Link =
{
...
, topics : List Topic
...
}

Answer 1

According to the documentation for Set:
The values can be any comparable type. This includes Int, Float, Time, Char, String, and tuples or lists of comparable types.
You are attempting to put a Topic value in where only a comparable type will work.
Thankfully there is the elm-community/list-extra package which exposes a uniqueBy function that lets you specify your own function to turn something into a comparable.
If you want to get the distinct list of topics, matching both on the name and isFeatured fields, then you can use toString to serialize the value into something comparable:
import List.Extra exposing (uniqueBy)
topicsFromLinks : List Link -> List Topic
topicsFromLinks links =
links
|> List.map .topics
|> List.concat
|> uniqueBy toString

Answer 2

In 2021, rather than using an external library and writing a clumsy toString method (since Debug.toString isn't available for production use anymore), consider using a fold:
unique : List a -> List a
unique list =
List.foldl
(\a uniques ->
if List.member a uniques then
uniques
else
uniques ++ [ a ]
)
[]
list