PragProg Customers

Effective Haskell: list-comprehension version of partyBudget has an error if food is duplicated (page 90)

kimwallmark

kimwallmark

Effective Haskell: list-comprehension version of partyBudget has an error if food is duplicated (page 90)

(I may be overthinking corner cases for a little example program)

If you pass in a list of guests with a duplicated food, like [(“Alice”, “hamburger”), (“Bob”, “ice cream”), (“Chris”, “ice cream”)], and a willEat where both Bob and Chris will eat ice cream, then the total number of ice creams bought is 4, not 2, because there are two entries for “ice cream” in the food list.

A way to fix it would be to change the line of the program to:
food ← nub $ map snd guests

2 comments
#errata/book-effective-haskell
2 429 2

Most Liked

RebeccaSkinner

RebeccaSkinner

Author of Effective Haskell

Another good catch. Some of these are tricky because if you dig too much into the edge cases the example starts becoming dominated by edge case handling rather than the idea that’s being explained, but I think there’s probably a good way to handle this one.

Unfortunately, nub is probably best avoided since it’s very slow, and using it on even moderately sized lists can be really bad for performance, so it’s probably best to avoid teaching people to use it.

1
Post #2

Where Next?

View thread on forum
View Effective Haskell's book portal
Post errata for this book
Post a suggestion for this book
Post a question about this book
Post a review about this book
Home PragProg Customers
#errata/book-effective-haskell
2 429 2
Last post

Popular Pragmatic Bookshelf topics Top

jon
PragProg Customers

The Pragmatic Programmer, 20th Anniversary Edition errata
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...
#errata/book-the-pragmatic-programmer-20th-anniversary-edition
10 2636 9
New
jeffmcompsci
PragProg Customers

Design and Build Great Web APIs: typo (page, 19, third bullet in URL list)
Title: Design and Build Great Web APIs - typo “https://company-atk.herokuapp.com/2258ie4t68jv” (page 19, third bullet in URL list) Typo:...
#pragprog#errata/book-design-and-build-great-web-apis
8 1941 7
New
yulkin
PragProg Customers

Programming Flutter: Image.toByteData() does not exist (page40)
your book suggests to use Image.toByteData() to convert image to bytes, however I get the following error: "the getter ‘toByteData’ isn’t...
/book-programming-flutter#question
1 4102 2
New
jskubick
PragProg Customers

Kotlin and Android Development featuring Jetpack: SwitchCompat's thumbTint and trackTint being ignored (page 76)
I think I might have found a problem involving SwitchCompat, thumbTint, and trackTint. As entered, the SwitchCompat changes color to hol...
#errata/book-kotlin-and-android-development-featuring-jetpack
0 2749 2
New
brunogirin
PragProg Customers

Python Testing with pytest, Second Edition: InterpreterNotFound error when running tox (page 150)
When running tox for the first time, I got the following error: ERROR: InterpreterNotFound: python3.10 I realised that I was running ...
#errata/book-python-testing-with-pytest-second-edition
0 1306 1
New
oaklandgit
PragProg Customers

Hands-on Rust: Failed to Load Texture error (page 121)
Hi, I completed chapter 6 but am getting the following error when running: thread 'main' panicked at 'Failed to load texture: IoError(O...
/book-hands-on-rust
2 1310 3
New
Henrai
PragProg Customers

The Ray Tracer Challenge: Cannot get rid of acne When implement Shadows in Chapter 8.
Hi, I’m working on the Chapter 8 of the book. After I add add the point_offset, I’m still able to see acne: In the image above, I re...
#question/book-the-ray-tracer-challenge
1 1052 4
New
gorkaio
PragProg Customers

Programming Phoenix LiveView: PentoWeb.LayoutView errata (56)
root_layout: {PentoWeb.LayoutView, :root}, This results in the following following error: no “root” html template defined for PentoWeb...
#errata/book-programming-phoenix-liveview
8 1094 4
New
mcpierce
PragProg Customers

Kotlin and Android Development featuring Jetpack: Unique constraint error when running chapter 5 code changes
@mfazio23 I’ve applied the changes from Chapter 5 of the book and everything builds correctly and runs. But, when I try to start a game,...
/book-kotlin-and-android-development-featuring-jetpack
0 1180 10
New
dachristenson
PragProg Customers

Kotlin and Android Development featuring Jetpack: End Ch. 11 app runs but consistently crashes
I’ve got to the end of Ch. 11, and the app runs, with all tabs displaying what they should – at first. After switching around between St...
#question/book-kotlin-and-android-development-featuring-jetpack
0 936 8
New

Other popular topics Top

Devtalk
General Dev>Dev Chat

What dev-related stuff have you been up to?
Reading something? Working on something? Planning something? Changing jobs even!? If you’re up for sharing, please let us know what you’...
#community
1052 21915 398
New
AstonJ
General Dev>Hardware

Which keyboard do you have?
If it’s a mechanical keyboard, which switches do you have? Would you recommend it? Why? What will your next keyboard be? Pics always w...
#hardware/keyboards#sticky#mechanical-keyboards
144 9115 50
New
PragmaticBookshelf
General Dev>Learning Resources

A Common-Sense Guide to Data Structures and Algorithms, Second Edition
Algorithms and data structures are much more than abstract concepts. Mastering them enables you to write code that runs faster and more e...
pragprog.com
#pragprog/python/ruby#published-book/book-a-common-sense-guide-to-data-structures-and-algorithms-second-edition#math#algorithms/js
19 6022 5
New
Exadra37
Linux>Chat

RancherOS is in end of life
Oh just spent so much time on this to discover now that RancherOS is in end of life but Rancher is refusing to mark the Github repo as su...
#linux#rancheros
10 6358 6
New
PragmaticBookshelf
Backend>Learning Resources

Programming Phoenix LiveView
Build highly interactive applications without ever leaving Elixir, the way the experts do. Let LiveView take care of performance, scalabi...
pragprog.com
#pragprog/elixir/phoenix#published-book/book-programming-phoenix-liveview
79 10732 24
New
AstonJ
Data Science

Can AI/ML predict a lottery win?
Biggest jackpot ever apparently! :upside_down_face: I don’t (usually) gamble/play the lottery, but working on a program to predict the...
#ai#machine-learning
19 3939 10
New
AstonJ
General Dev>Code Editors

Doom-Emacs: Can't find emacs in your PATH
If you get Can't find emacs in your PATH when trying to install Doom Emacs on your Mac you… just… need to install Emacs first! :lol: bre...
#macos/emacs#doom-emacs
4 5837 0
New
DevotionGeo
General Dev>Questions

Do you prefer regular mechanical keyboards or low profile mechanical keyboards and why?
I have always used antique keyboards like Cherry MX 1800 or Cherry MX 8100 and almost always have modified the switches in some way, like...
/keyboards#mechanical-keyboards
27 3877 9
New
First poster: AstonJ
General Dev>In The News

Jan: An open source alternative to ChatGPT that runs on the desktop
Jan | Rethink the Computer. Jan turns your computer into an AI machine by running LLMs locally on your computer. It’s a privacy-focus, l...
jan.ai
#desktop#chatgpt
4 5652 4
New
RobertRichards
Game Dev>Chat

Hair Salon Games for Girls Fun
Hair Salon Games for Girls Fun Girls Hair Saloon game is mainly developed for kids. This game allows users to select virtual avatars to ...
#ios#android#unity#fun
2 3556 1
New
Back to top

Latest in Effective Haskell

Effective Haskell Portal ❯

PragProg Customers

Pragmatic Bookshelf PragProg Customers ❯

Latest on Devtalk

Devtalk ❯

We ❤️ helpful members!

We reward our most helpful members via our MOTM scheme - by giving away a whopping 25 books per year!

Member of the Month

Devtalk Sponsors

Pragmatic Bookshelf

Practical resources that improve the lives of professional developers.

Erlang Solutions

We build trusted fault-tolerant systems that scale to billions of users.

Honeybadger

Real-time error tracking, performance insights, and observability for devs.

EEF

Supporting innovation across the BEAM ecosystem.

coders51

We build reliable cloud platforms for business-critical systems.

Manning Publications

Develop your skills with books, videos, and courses.

AppSignal

Catch errors, track performance, monitor hosts and more.

WyeWorks

Enabling companies to succeed by building software people love.

Pragmatic Studio

Courses that move you from confusion to "Aha, now I get it!"

Linux New Media

Publishing Linux Magazine, ADMIN, and MakerSpace for 25+ years.

View all our sponsors ❯

Categories:

Sub Categories:

Popular Portals

Devtalk Sponsors

Pragmatic Bookshelf

Practical resources that improve the lives of professional developers.

Erlang Solutions

We build trusted fault-tolerant systems that scale to billions of users.

Honeybadger

Real-time error tracking, performance insights, and observability for devs.

EEF

Supporting innovation across the BEAM ecosystem.

coders51

We build reliable cloud platforms for business-critical systems.

Manning Publications

Develop your skills with books, videos, and courses.

AppSignal

Catch errors, track performance, monitor hosts and more.

WyeWorks

Enabling companies to succeed by building software people love.

Pragmatic Studio

Courses that move you from confusion to "Aha, now I get it!"

Linux New Media

Publishing Linux Magazine, ADMIN, and MakerSpace for 25+ years.

View all our sponsors ❯

We're in Beta

About us Mission Statement See our Roadmap