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
Alexandr
PragProg Customers

Programming Machine Learning: Incorrect formula
Hi everyone! There is an error on the page 71 in the book “Programming machine learning from coding to depp learning” P. Perrotta. You c...
#errata/book-programming-machine-learning
5 1341 4
New
mikecargal
PragProg Customers

Hands-on Rust: can_place logic is "inside out"
Title: Hands-On Rust (Chapter 11: prefab) Just played a couple of amulet-less games. With a bit of debugging, I believe that your can_p...
#errata/book-hands-on-rust
22 1628 20
New
alanq
PragProg Customers

Modern Front-End Development for Rails: Can't get TURBO_STREAM format in some cases
This isn’t directly about the book contents so maybe not the right forum…but in some of the code apps (e.g. turbo/06) it sends a TURBO_ST...
#question/book-modern-front-end-development-for-rails
1 1386 7
New
frasette
PragProg Customers

A Common-Sense Guide to Data Structures and Algorithms, Second Edition: erroneous extra space complexity (page400)
A Common-Sense Guide to Data Structures and Algorithms, Second Edition by Jay Wengrow @jaywengrow Hi, I have the paperback version of t...
#errata/book-a-common-sense-guide-to-data-structures-and-algorithms-second-edition
4 1180 2
New
adamwoolhether
PragProg Customers

Distributed Services with Go: receiving Unknown flag: --go_opt when compiling protobuf (page 20)
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 ...
/book-distributed-services-with-go
0 4924 3
New
patoncrispy
PragProg Customers

Hands-on Rust: Adequate docs for bracket_lib
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...
#question/book-hands-on-rust
5 1516 3
New
AndyDavis3416
PragProg Customers

Modern Front-End Development for Rails: ERROR in tsconfig.json Chapter_01/01
@noelrappin Running the webpack dev server, I receive the following warning: ERROR in tsconfig.json TS18003: No inputs were found in c...
/book-modern-front-end-development-for-rails
3 1438 3
New
jskubick
PragProg Customers

Kotlin and Android Development featuring Jetpack: Error in generated Game.java (page 136)
I’m under the impression that when the reader gets to page 136 (“View Data with the Database Inspector”), the code SHOULD be able to buil...
#question/book-kotlin-and-android-development-featuring-jetpack
2 1172 8
New
andreheijstek
PragProg Customers

Modern front-end development for Rails, second edition - Struggling to get the first chapter to work
After running /bin/setup, the first error was: The foreman' command exists in these Ruby versions: That was easy to fix: gem install fore...
#errata/book-modern-front-end-development-for-rails
1 1116 3
New

Other popular topics Top

Devtalk
General Dev>Dev Chat

HELLO WORLD (Introductions thread!)
Hello Devtalk World! Please let us know a little about who you are and where you’re from :nerd_face:
#community
482 7161 117
New
DevotionGeo
Backend>Chat

What is the reason behind Rust’s web framework, Rocket, not performing as well as expected in the Techempower benchmarks?
I know that these benchmarks might not be the exact picture of real-world scenario, but still I expect a Rust web framework performing a ...
#web-frameworks/rust
36 7463 11
New
AstonJ
General Dev>Hardware

Keyboard thock (sound)
I’ve been hearing quite a lot of comments relating to the sound of a keyboard, with one of the most desirable of these called ‘thock’, he...
/keyboards#mechanical-keyboards
14 11197 8
New
PragmaticBookshelf
Frontend>Learning Resources

Modern CSS with Tailwind
Tailwind CSS is an exciting new CSS framework that allows you to design your site by composing simple utility classes to create complex e...
pragprog.com
#pragprog/tailwind#published-book/book-modern-css-with-tailwind
12 5813 4
New
AstonJ
General Dev>Dev Chat

Languages Without Garbage Collection
Continuing the discussion from Thinking about learning Crystal, let’s discuss - I was wondering which languages don’t GC - maybe we can c...
#garbage-collection
21 5575 7
New
PragmaticBookshelf
Community>In The Spotlight

Spotlight: Jamis Buck (Author) Interview and AMA!
Author Spotlight Jamis Buck @jamis This month, we have the pleasure of spotlighting author Jamis Buck, who has written Mazes for Prog...
#author-spotlight/ruby/book-the-ray-tracer-challenge/book-mazes-for-programmers
21 6352 9
New
First poster: CommunityNews
AI>In The News

How to fix the eyes in AI-generated images
aidemos.info
0 4508 0
New
PragmaticBookshelf
Community>In The Spotlight

Spotlight: Peter Ullrich (Author) Interview and AMA!
Author Spotlight: Peter Ullrich @PJUllrich Data is at the core of every business, but it is useless if nobody can access and analyze ...
/elixir/phoenix/book-building-table-views-with-phoenix-liveview
72 4765 21
New
CommunityNews
General Dev>In The News

X can’t stop spread of explicit, fake AI Taylor Swift images
Will Swifties’ war on AI fakes spark a deepfake porn reckoning?
arstechnica.com
/swift
0 8379 0
New
AstonJ
AI>In The News

DeepSeek (671B) running on a cluster of 8 Mac Mini Pros with 64GB RAM each
This is cool! DEEPSEEK-V3 ON M4 MAC: BLAZING FAST INFERENCE ON APPLE SILICON We just witnessed something incredible: the largest open-s...
#ai#macs/deepseek
0 6695 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