You deleted the case that returns the final result. You may want to check out @wolfadex 's solution again.

Btw, I do not think replacing the `Result String String`

with just a `String`

is the right call here. Elm is really good at modelling the exact data types you are dealing with. Moving everything to String is the opposite approach - it gets you closer to something you would write in Javascript for example. This is commonly called a “stringly typed” system. Doing so makes everything harder to understand and maintain, since it is extremly easy to just mix up 2 completely unrelated values.

I would try to get towards these types:

```
type Operator
= Add
| Subtract
| Multiply
| Divide
type Input
= Operator Operator
| Number Int
type Problem
= InvalidOperator String
| InvalidNumber String
| NoNumbersForOperator
| SingleNumberForOperator
| MoreThanOneInputLeft
parsePolishNotation : String -> Result Problem (List Input)
getPolishSumHelper : List Input -> List Int -> Result Problem Int
calcNext : Operator -> List Input -> Int -> Int -> List Int -> Int
```