Instances cannot be found when records are directly involved

[viluon]

state.ml

open import "prelude.ml"

type state 's 'a = State of 's -> 'a * 's

instance functor (state 's) begin
  let f <$> State a = State (fun s -> let (a, s) = a s in (f a, s))
end

instance applicative (state 's) begin
  let pure a = State (fun s -> (a, s))
  let State sf <*> State sa = State (fun st -> let (f, st) = sf st in let (a, st) = sa st in (f a, st))
end

instance monad (state 's) begin
  let State a >>= f = State (fun s -> let (a, s) = a s in let State sb = f a in sb s)
end

class monad_state 'm 's | 'm -> 's begin
  val get: 'm 's
  val put: 's -> 'm ()
end

instance monad_state (state 's) 's begin
  let get   = State (fun s -> (s,  s))
  let put s = State (fun _ -> ((), s))
end

open import "prelude.ml"
open import "./state.ml"

type int_state <- state {i: int}

let oof (): int_state () =
  let! n = get
  put (n.i + 1)

let rec run (): int_state () =
  let! _ = oof ()
  run ()

transcript

src/intstate.ml[11:9 ..14:1]: error (E2018)
  No instance for monad_state (state { i : int }) Public.int arising from use of the expression
   │ 
12 │   let! _ = oof ()
   │            ^^^^^^

  • Note: this constraint was not quantified over because
    recursive binding groups must have complete type signatures
    in all of their bindings.

   │ 
11 │ let rec run (): int_state () =
   │         ^^^^^^^^^^^^^^^^^^^^^^
   This binding should have had a complete type signature.

The following message has a detailed explanation: 2018.
Try 'amc explain 2018' to see it.