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!

Where Next?

Popular Pragmatic Bookshelf topics Top

jimmykiang
This test is broken right out of the box… — FAIL: TestAgent (7.82s) agent_test.go:77: Error Trace: agent_test.go:77 agent_test.go:...
New
rmurray10127
Title: Intuitive Python: docker run… denied error (page 2) Attempted to run the docker command in both CLI and Powershell PS C:\Users\r...
New
jskubick
I’m running Android Studio “Arctic Fox” 2020.3.1 Patch 2, and I’m embarrassed to admit that I only made it to page 8 before running into ...
New
Charles
In general, the book isn’t yet updated for Phoenix version 1.6. On page 18 of the book, the authors indicate that an auto generated of ro...
New
oaklandgit
Hi, I completed chapter 6 but am getting the following error when running: thread 'main' panicked at 'Failed to load texture: IoError(O...
New
dsmith42
Hey there, I’m enjoying this book and have learned a few things alredayd. However, in Chapter 4 I believe we are meant to see the “&gt;...
New
AufHe
I’m a newbie to Rails 7 and have hit an issue with the bin/Dev script mentioned on pages 112-113. Iteration A1 - Seeing the list of prod...
New
taguniversalmachine
Hi, I am getting an error I cannot figure out on my test. I have what I think is the exact code from the book, other than I changed “us...
New
tkhobbes
After some hassle, I was able to finally run bin/setup, now I have started the rails server but I get this error message right when I vis...
New
dachristenson
I just bought this book to learn about Android development, and I’m already running into a major issue in Ch. 1, p. 20: “Update activity...
New

Other popular topics Top

Devtalk
Reading something? Working on something? Planning something? Changing jobs even!? If you’re up for sharing, please let us know what you’...
1040 20280 387
New
brentjanderson
Bought the Moonlander mechanical keyboard. Cherry Brown MX switches. Arms and wrists have been hurting enough that it’s time I did someth...
New
AstonJ
Curious to know which languages and frameworks you’re all thinking about learning next :upside_down_face: Perhaps if there’s enough peop...
New
New
AstonJ
There’s a whole world of custom keycaps out there that I didn’t know existed! Check out all of our Keycaps threads here: https://forum....
New
PragmaticBookshelf
Rails 7 completely redefines what it means to produce fantastic user experiences and provides a way to achieve all the benefits of single...
New
AstonJ
Was just curious to see if any were around, found this one: I got 51/100: Not sure if it was meant to buy I am sure at times the b...
New
First poster: bot
zig/http.zig at 7cf2cbb33ef34c1d211135f56d30fe23b6cacd42 · ziglang/zig. General-purpose programming language and toolchain for maintaini...
New
CommunityNews
A Brief Review of the Minisforum V3 AMD Tablet. Update: I have created an awesome-minisforum-v3 GitHub repository to list information fo...
New
AstonJ
If you’re getting errors like this: psql: error: connection to server on socket “/tmp/.s.PGSQL.5432” failed: No such file or directory ...
New

Sub Categories: