Compose the three parsing stages using the supplied fold, and accept only when no suffix remains:
eval :: String -> Maybe Int
eval input = foldMaybe first Nothing (parseInt input)
  where
    first (a,rest) = foldMaybe (second a) Nothing (parseOp rest)
    second a (op,rest) = foldMaybe (finish a op) Nothing (parseInt rest)
    finish a op (b,rest)
      | null rest = Just (apply op a b)
      | otherwise = Nothing
    apply '+' a b = a+b
    apply '-' a b = a-b
    apply '*' a b = a*b
    apply '/' a b = a `div` b
    apply _ _ _ = error "unreachable: parseOp rejected this operator"
Every failed intermediate parser is converted to Nothing by foldMaybe. Checking null rest rejects both a second operation and trailing letters. In particular, integer division makes eval "34/4" = Just 8. As permitted by the question, division by zero is outside the specified inputs; the bounded-Int proviso also applies to arithmetic results.

Ancestors (11)

  1. 3
  2. Functional programming, design and analysis of algorithms (resit)
  3. 2016
  4. Past exam of the year 1 of the computer science course of the University of Oxford
  5. Year 1 of the computer science course of the University of Oxford
  6. Computer science course of the University of Oxford
  7. Undergraduate course of the University of Oxford
  8. Course of the University of Oxford
  9. University of Oxford
  10. List of universities
  11. Home

View article source

Discussion (0) New discussion

There are no discussions about this article yet.

Articles by others on the same topic (0)

There are currently no matching articles.
See all articles in the same topic Create my own version