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

abtin
page 20: … protoc command… I had to additionally run the following go get commands in order to be able to compile protobuf code using go...
New
cro
I am working on the “Your Turn” for chapter one and building out the restart button talked about on page 27. It recommends looking into ...
New
gilesdotcodes
In case this helps anyone, I’ve had issues setting up the rails source code. Here were the solutions: In Gemfile, change gem 'rails' t...
New
Chrichton
Dear Sophie. I tried to do the “Authorization” exercise and have two questions: When trying to plug in an email-service, I found the ...
New
adamwoolhether
When trying to generate the protobuf .go file, I receive this error: Unknown flag: --go_opt libprotoc 3.12.3 MacOS 11.3.1 Googling ...
New
nicoatridge
Hi, I have just acquired Michael Fazio’s “Kotlin and Android Development” to learn about game programming for Android. I have a game in p...
New
New
creminology
Skimming ahead, much of the following is explained in Chapter 3, but new readers (like me!) will hit a roadblock in Chapter 2 with their ...
New
dtonhofer
@parrt In the context of Chapter 4.3, the grammar Java.g4, meant to parse Java 6 compilation units, no longer passes ANTLR (currently 4....
New
New

Other popular topics Top

Devtalk
Hello Devtalk World! Please let us know a little about who you are and where you’re from :nerd_face:
New
PragmaticBookshelf
Brace yourself for a fun challenge: build a photorealistic 3D renderer from scratch! In just a couple of weeks, build a ray tracer that r...
New
AstonJ
Or looking forward to? :nerd_face:
498 13326 269
New
AstonJ
I ended up cancelling my Moonlander order as I think it’s just going to be a bit too bulky for me. I think the Planck and the Preonic (o...
New
PragmaticBookshelf
Tailwind CSS is an exciting new CSS framework that allows you to design your site by composing simple utility classes to create complex e...
New
PragmaticBookshelf
Create efficient, elegant software tests in pytest, Python's most powerful testing framework. Brian Okken @brianokken Edited by Kat...
New
PragmaticBookshelf
Author Spotlight: Peter Ullrich @PJUllrich Data is at the core of every business, but it is useless if nobody can access and analyze ...
New
First poster: bot
zig/http.zig at 7cf2cbb33ef34c1d211135f56d30fe23b6cacd42 · ziglang/zig. General-purpose programming language and toolchain for maintaini...
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
Fl4m3Ph03n1x
Background Lately I am in a quest to find a good quality TTS ai generation tool to run locally in order to create audio for some videos I...
New

Sub Categories: