state monad using record as a state

[vzaliva]

I am observing a strange error when using ExtLib. I am unsure if it is a bug or some sort of limitation, related to universes. In short, I've modified examples/StateGame.v ExtLib example, changing state type from Definition GameState: Type := (prod bool Z) with Record GameState : Type := mkGameState {on:bool; score:Z}.. This lead to the following error message:

Error:
Unable to satisfy the following constraints:

?State_m : "MonadState GameState (state GameState)"

At first glance MonadState_state instance should satisfy this constraint, but there is universe mismatch.
The full example is enclosed below:

Require Import Coq.ZArith.ZArith_base Coq.Strings.String.
Require Import ExtLib.Data.Monads.StateMonad ExtLib.Structures.Monads.

Section StateGame.
  
  Import MonadNotation.
  Local Open Scope Z_scope.
  Local Open Scope char_scope.
  Local Open Scope monad_scope.

  Definition GameValue : Type := Z.
  Record GameState : Type := mkGameState {on:bool; score:Z}.

  Variable m : Type -> Type.
  Context {Monad_m: Monad m}.
  Context {State_m: MonadState GameState m}.

  Fixpoint playGame (s: string) {struct s}: m GameValue :=
    match s with
    |  EmptyString =>
       v <- get ;; ret (score v)
    |  String x xs =>
       v <- get ;;
         match x, (on v) with
         | "a", true =>  put {| on := on v ; score := (score v) + 1 |}
         | "b", true => put {| on := on v ; score := (score v) - 1 |}
         | "c", _ =>   put {| on := negb (on v) ; score := score v |}
         |  _, _  =>   put v
         end ;; playGame xs
    end.

  Definition startState: GameState := {| on:=false; score:=0 |}.

End StateGame.

Definition main : GameValue :=
  (@evalState GameState GameValue (playGame (state GameState) "abcaaacbbcabbab") startState).

(* The following should return '2%Z' *)
Compute main.

[gmalecha]

Interesting. Coq shouldn't care about the record vs the pair, so a universe problem is possible, though I would highly doubt it, you aren't doing anything interesting with universes. Can you print the relevant universes?

[vzaliva]

Changing definition of main as follows:

Definition main : GameValue :=
  (@evalState GameState GameValue
              (@playGame
                 (state GameState)
                 _
                 (MonadState_state GameState)
                 "abcaaacbbcabbab"
              ) startState).

Gives the following error:

Error:
The term "MonadState_state GameState" has type
 "MonadState@{ExtLib.Data.Monads.StateMonad.1 ExtLib.Data.Monads.StateMonad.2
    ExtLib.Data.Monads.StateMonad.2} GameState (state GameState)"
while it is expected to have type
 "MonadState@{Set Top.20455 Top.20456} GameState (state GameState)".

[gmalecha]

This seems to be fixed by making state universe polymorphic.

[gmalecha]

Is this still an issue?