"The pattern can never match the type." error from dialyzer

Are you sure the check_rating_to_play/2 takes a user_id? Maybe the function’s implementation expects a user like the other check function, chokes on the user_id
and will always return the error tuple?

What is the full error?

From the sparse bits of the error, some pattern matching seems to be involved, though the only pattern matching here is hidden in the conds expansion.

The full error is

lib/teiserver/coordinator/consul_server.ex:826:pattern_match
The pattern can never match the type.

Pattern:
true

Type:
false

Line 826

1921×418 23.5 KB

Are you piping the result of the cond do statement into another statement?

No. The full code for that function is here.

Strange, don’t see anything that might explain your dializer warning.

spec check_rating_to_play seems to either return :ok or indeed an {:error, msg}-tuple.

To dialyzer doesn‘t matter what it can theoretically return. The only thing that matters is what dialyzer inferred for it to return. And if inference says it only ever returns ˋ:okˋ then the error message makes sense. Now the question becomes why dialyzer would infer that ˋ:okˋ is the only possible return value for those functions.

I know, that’s why I followed the trail of function calls (backwards). As far as I could tell it could return either one. The only thing that stood out was a check on whether state.minimum_rating_to_play and state.maximum_rating_to_play was nil or not, while it looks like it is set to 0 when a new state is created and could not find any code that sets it to nil (rating is a float). Which would make it always return :ok.

It looks the state is never nil, so :ok can never be reached…

    cond do
      state.minimum_rating_to_play != nil and player_rating < state.minimum_rating_to_play ->
        msg = get_failed_rating_check_text(player_rating, state, rating_type)
        {:error, msg}

      state.maximum_rating_to_play != nil and player_rating > state.maximum_rating_to_play ->
        msg = get_failed_rating_check_text(player_rating, state, rating_type)
        {:error, msg}

      true ->
        # All good
        :ok 

In this file…

https://github.com/beyond-all-reason/teiserver/blob/08351a1af2be5021041f447c881e330f45216d77/lib/teiserver/lobby/libs/lobby_restrictions.ex

Dialyzer has inferred that check_rating_to_play and check_rank_to_play can ONLY return :ok, so it deduces that those branches of the cond are dead.

The issue is sneaky, but ultimately comes from get_failed_rating_check_text and get_failed_rank_check_text returning the wrong type.

A walkthrough of what’s happening. Start in this code, from check_rating_to_play:

    cond do
      state.minimum_rating_to_play != nil and player_rating < state.minimum_rating_to_play ->
        msg = get_failed_rating_check_text(player_rating, state, rating_type)
        {:error, msg}
# maximum clause omitted
      true ->
        # All good
        :ok
    end

What’s the type of msg? It’s the return type of get_failed_rating_check_text.

What’s that type? It’s [String.t()]!

  def get_failed_rating_check_text(player_rating, consul_state, rating_type) do
    bounds = get_rating_bounds_for_title(consul_state)
    player_rating_text = player_rating |> Decimal.from_float() |> Decimal.round(2)

    [
      @splitter,
      "You don't meet the rating requirements for this lobby (#{bounds}). Your #{rating_type} match rating is #{player_rating_text}. Learn more about rating here:",
      "https://www.beyondallreason.info/guide/rating-and-lobby-balance#openskill"
    ]
  end

Dialyzer sees this and infers that the minimum_rating_to_play branch of the cond would make the function return {:error, [String.t()]}.

The function is not specced to return that type, so Dialyzer infers that that branch does not execute, so check_rating_to_play “always” returns :ok.

When I run Dialyzer locally, it also complains about get_coordinator_userid not being called - further evidence it has deduced that those branches are dead.

Replacing the lists in get_failed_rating_check_text and get_failed_rank_check_text with a literal string makes all three messages go away, verifying the diagnosis.

Possible fixes:

• join the strings together in get_failed_rating_check_text etc

• widen the type of check_rating_to_play etc to :ok, {:error, iodata()}