molleweide

molleweide

Effective Haskell: Calculator throws errors in parse'

@RebeccaSkinner,

Allright, so I have put all of the example code for Calculator in chapter 4 on page ~140 in a file and when loading it into GHCI it throws this:

    λ: :reload
    [1 of 1] Compiling Calculator       ( Calculator.hs, interpreted )

    Calculator.hs:40:9: error:
        • Couldn't match expected type ‘String -> Either String a1’
                      with actual type ‘Either String b0’
        • In the pattern: Left err
          In a case alternative: Left err -> Left err
          In the expression:
            case readEither of
              Left err -> Left err
              Right lit' -> Right (Lit lit', rest)
       |
    40 |         Left err -> Left err
       |         ^^^^^^^^

    Calculator.hs:41:9: error:
        • Couldn't match expected type ‘String -> Either String a1’
                      with actual type ‘Either a0 Int’
        • In the pattern: Right lit'
          In a case alternative: Right lit' -> Right (Lit lit', rest)
          In the expression:
            case readEither of
              Left err -> Left err
              Right lit' -> Right (Lit lit', rest)
       |
    41 |         Right lit' -> Right (Lit lit', rest)
       |         ^^^^^^^^^^
    Failed, no modules loaded.

I am not sure if maybe there is something that you forgot to add or that should be added.

Below I post my full calculator module:

module Calculator where

import Text.Read (readEither)

data Expr = Lit Int
  | Add Expr Expr
  | Sub Expr Expr
  | Mul Expr Expr
  | Div Expr Expr

eval :: Expr -> Int
eval expr =
  case expr of
    Lit num -> num
    Add arg1 arg2 -> eval' ( + ) arg1 arg2
    Sub arg1 arg2 -> eval' ( - ) arg1 arg2
    Mul arg1 arg2 -> eval' ( * ) arg1 arg2
    Div arg1 arg2 -> eval' div arg1 arg2
    where
      eval' :: (Int -> Int -> Int) -> Expr -> Expr -> Int
      eval' operator arg1 arg2 = operator (eval arg1) (eval arg2)

parse :: String -> Either String Expr
parse str =
  case parse' (words str) of
    Left err          -> Left err
    Right (e, [])     -> Right e
    Right (_, rest)   -> Left $ "Found extra tokens: " <> (unwords rest)

parse' :: [String] -> Either String (Expr, [String])
parse' [] = Left "unexpected end of expression"
parse' (token:rest) =
  case token of
    "+" -> parseBinary Add rest
    "*" -> parseBinary Mul rest
    "-" -> parseBinary Sub rest
    "/" -> parseBinary Div rest
    lit ->
      case readEither of
        Left err -> Left err
        Right lit' -> Right (Lit lit', rest)

parseBinary :: (Expr -> Expr -> Expr) -> [String] -> Either String (Expr, [String])
parseBinary exprConstructor args =
  case parse' args of
    Left err -> Left err
    Right (firstArg, rest') ->
      case parse' rest' of
        Left err -> Left err
        Right (secondArg, rest'') ->
          Right $ (exprConstructor firstArg secondArg, rest'')

run :: String -> String
run expr =
  case parse expr of
    Left err -> "Error: " <> err
    Right expr' ->
      let answer = show $ eval expr'
      in "The answer is: " <> answer

First Post!

RebeccaSkinner

RebeccaSkinner

Author of Effective Haskell

Hi, it looks like you’re missing the lit parameter to readEither in the parse' function. The correct version of the function is:

parse' :: [String] -> Either String (Expr, [String])
parse' [] = Left "unexpected end of expression"
parse' (token:rest) =
  case token of
    "+" -> parseBinary Add rest
    "*" -> parseBinary Mul rest
    "-" -> parseBinary Sub rest
    "/" -> parseBinary Div rest
    lit ->
      case readEither lit of
        Left err -> Left err
        Right lit' -> Right (Lit lit', rest)

It looks like this is fixed in the most recent version- thanks for the catch and sorry for the trouble!

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