Proposal for `toMaybeList` function

[Fabio Vitale]

Often times, when writing code for the UI, we get a list of elements to show. They have to be shown only if at least one element is available, so it would be useful to have a function which converts a List a in a Maybe (List a):

• if list has no elements, returns Nothing
• if list has at least one element, returns Just list

This allows, for instance

renderMetadata : List String -> Html msg
renderMetadata meta =
  meta
  |> List.toMaybeList
  |> Maybe.map renderMetadataBlock -- this function works on a proper list of strings, maybe builds some container which should not be rendered otherwise, etc
  |> Maybe.withDefault (text "No metadata set")

The code we currently use for this function is quite simple, and we believe it may be a nice addition to this lib. As far as we know, currently there is no way to achieve this using what is currently available.

toMaybeList : List a -> Maybe (List a)
toMaybeList list =
  if List.isEmpty list then
    Nothing
  else
    Just list

I will do the implementation and open a PR, if you feel this could be useful 😄

[Fabio Vitale]

Some of our collegues brought to our attention that this implementation would make the type system inconsistent, in some way. We could have a Just [] and we would be therefore unable to ensure that the inner list has at least one element. A better signature for this function would be toMaybeList : List a -> Maybe (a, List a), which is exactly what uncons does. We decided that correctness is more important, and we removed all instances of this function from our code 😄

[Chadtech]

Makes sense! Yes, it does seem like the correctness of uncons is ideal. Thanks for the time and consideration you put into this.