2022-10-15-gren-arithmetic.elm

module Main exposing (main)

main : String -> Int
main args =
  let
    tokens =
      String.words args

    result =
      List.foldl evaluator initialEvaluatorState tokens
  in
    result.value


type alias EvaluatorState =
  { operator : Operator
  , value : Int
  }

type Operator
  = Unknown
  | Plus
  | Minus

initialEvaluatorState : EvaluatorState
initialEvaluatorState =
  { operator = Unknown
  , value = 0
  }

evaluator : String -> EvaluatorState -> EvaluatorState
evaluator token state =
  case String.toInt token of
    Just num ->
      case state.operator of
        Unknown ->
          { state | value = num }
        Plus ->
          { state | value = state.value + num }
        Minus ->
          { state | value = state.value - num }

    Nothing ->
      case token of
        "+" ->
          { state | operator = Plus }
        "-" ->
          { state | operator = Minus }
        otherwise ->
          { state | operator = Unknown }