molleweide
Effective Haskell: Calculator throws errors in parse'
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
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!
Popular Pragmatic Bookshelf topics
Some minor things in the paper edition that says “3 2020” on the title page verso, not mentioned in the book’s errata online:
p. 186 But...
New
Following the steps described in Chapter 6 of the book, I’m stuck with running the migration as described on page 84:
bundle exec sequel...
New
Title: Design and Build Great Web APIs - typo “https://company-atk.herokuapp.com/2258ie4t68jv” (page 19, third bullet in URL list)
Typo:...
New
I can’t setup the Rails source code. This happens in a working directory containing multiple (postgres) Rails apps.
With:
ruby-3.0.0
s...
New
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
Hello! Thanks for the great book.
I was attempting the Trie (chap 17) exercises and for number 4 the solution provided for the autocorre...
New
#book-python-testing-with-pytest-second-edition
Hi. Thanks for writing the book. I am just learning so this might just of been an issue ...
New
I’m new to Rust and am using this book to learn more as well as to feed my interest in game dev. I’ve just finished the flappy dragon exa...
New
I got this error when executing the plot files on macOS Ventura 13.0.1 with Python 3.10.8 and matplotlib 3.6.1:
programming_ML/code/03_...
New
Hello @herbert ! Trying to get the very first “Hello, Bracket Terminal!" example to run (p. 53). I develop on an Amazon EC2 instance runn...
New
Other popular topics
You might be thinking we should just ask who’s not using VSCode :joy: however there are some new additions in the space that might give V...
New
My first contact with Erlang was about 2 years ago when I used RabbitMQ, which is written in Erlang, for my job. This made me curious and...
New
In case anyone else is wondering why Ruby 3 doesn’t show when you do asdf list-all ruby :man_facepalming: do this first:
asdf plugin-upd...
New
Intensively researching Erlang books and additional resources on it, I have found that the topic of using Regular Expressions is either c...
New
We’ve talked about his book briefly here but it is quickly becoming obsolete - so he’s decided to create a series of 7 podcasts, the firs...
New
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
I am trying to crate a game for the Nintendo switch, I wanted to use Java as I am comfortable with that programming language. Can you use...
New
There appears to have been an update that has changed the terminology for what has previously been known as the Taskbar Overflow - this h...
New
A Brief Review of the Minisforum V3 AMD Tablet.
Update: I have created an awesome-minisforum-v3 GitHub repository to list information fo...
New
Hello,
I’m a beginner in Android development and I’m facing an issue with my project setup. In my build.gradle.kts file, I have the foll...
New
Categories:
Sub Categories:
Popular Portals
- /elixir
- /rust
- /wasm
- /ruby
- /erlang
- /phoenix
- /keyboards
- /python
- /js
- /rails
- /security
- /go
- /swift
- /vim
- /clojure
- /emacs
- /haskell
- /java
- /svelte
- /onivim
- /typescript
- /kotlin
- /c-plus-plus
- /crystal
- /tailwind
- /react
- /gleam
- /ocaml
- /elm
- /flutter
- /vscode
- /ash
- /html
- /opensuse
- /zig
- /centos
- /deepseek
- /php
- /scala
- /react-native
- /lisp
- /textmate
- /sublime-text
- /nixos
- /debian
- /agda
- /django
- /deno
- /kubuntu
- /arch-linux
- /nodejs
- /ubuntu
- /revery
- /spring
- /manjaro
- /lua
- /diversity
- /julia
- /markdown
- /slackware








