fsharp-formatting.fsx

namespace Foo

module Bar =
    let RelationsReducer (state:RelationState) action =
        match action with
        | ActivitiesRequestSucceeded (id, items) 
            -> { state with FacilityActivity = state.FacilityActivity |> Array.filter (fun (f, _) -> f <> id)
                                                                      |> Array.append (items |> Array.map (fun item -> (id, item.Id))) }
        | _ -> state

Extract variable:

match action with
  | ActivitiesRequestSucceeded (id, items) ->
    let facilityActivity =
      state.FacilityActivity
        |> Array.filter (fun (f, _) -> f <> id)
        |> Array.append (items |> Array.map (fun item -> (id, item.Id)))

      { state with FacilityActivity = facilityActivity }
  | _ -> state