-- State Monad.
-- (C) Dmitry Boulytchev, JetBrains Research, St. Petersburg State University, 2020
--
-- This unit provides a state

import List;
import Fun;

public infixl => before $ (x, f) {
  fun (state) {
    case x (state) of
      [state, x] -> [state, f (x)]
    esac
  }
}

public infix =>> at => (x, f) {
  fun (state) {
    case x (state) of
      [state, x] -> f (x) (state)
    esac
  }
}

public fun returnST (x) {
  fun (state) {[state, x]}
}

public fun chainST (xs) {
  fun (state) {
    case
      foldl (fun (f, x) {
               fun (state) {
                 case f (state) of
                   [state, xs] -> case x (state) of
                                  [state, x] -> [state, x : xs]
                                esac
                 esac
               }
             }, returnST $ {}, xs) (state) of    
      [state, xs] -> [state, reverse (xs)]
    esac
  }
}