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

belgoros
PragProg Customers

Effective Testing with RSpec 3: Running Sequel migrations fails (page 84)
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...
#errata/book-effective-testing-with-rspec-3
4 1152 1
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
jdufour
PragProg Customers

The Cucumber for Java Book:
Hello! On page xix of the preface, it says there is a community forum "… for help if your’re stuck on one of the exercises in this book… ...
#question/book-the-cucumber-for-java-book
3 1143 1
New
herminiotorres
PragProg Customers

Programming Phoenix LiveView: Typo
Hi! I know not the intentions behind this narrative when called, on page XI: mount() |> handle_event() |> render() but the correc...
#errata/book-programming-phoenix-liveview
3 1307 17
New
fynn
PragProg Customers

Build a Weather Station with Elixir and Nerves: SGP30 vs SGP40
This is as much a suggestion as a question, as a note for others. Locally the SGP30 wasn’t available, so I ordered a SGP40. On page 53, ...
#suggestion/book-build-a-weather-station-with-elixir-and-nerves
0 1292 2
New
nicoatridge
PragProg Customers

Kotlin and Android Development: Could not get unknown property ‘kotlin_version’…
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...
/book-kotlin-and-android-development-featuring-jetpack
0 8095 6
New
brunogirin
PragProg Customers

Python Testing with pytest, Second Edition: error when running tox in parallel (page 151)
When trying to run tox in parallel as explained on page 151, I got the following error: tox: error: argument -p/–parallel: expected one...
#errata/book-python-testing-with-pytest-second-edition
0 1111 1
New
kevinjyh
PragProg Customers

Python Testing with pytest, Second Edition:Abou epub format
Is the book’s epub format available to read on Google Play Books?
#question/book-python-testing-with-pytest-second-edition
0 1126 3
New
dsmith42
PragProg Customers

iOS Unit Testing by Example: 54 Don't seem to be getting the print out on tests (Xcode 13.2)
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 “>...
#question/book-ios-unit-testing-by-example
0 1963 3
New
taguniversalmachine
PragProg Customers

Programming Phoenix LiveView: B6.0 pg 56 - current-user not set
It seems the second code snippet is missing the code to set the current_user: current_user: Accounts.get_user_by_session_token(session["...
#errata/book-programming-phoenix-liveview
0 1898 8
New

Other popular topics Top

ohm
Science/Tech>Tech Chat

Games! Which do you play?
Which, if any, games do you play? On what platform? I just bought (and completed) Minecraft Dungeons for my Nintendo Switch. Other than ...
#games
246 6097 101
New
AstonJ
Backend>Chat

How to install Ruby 3 with ASDF
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...
/ruby#asdf
11 5961 4
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
General Dev>Hardware

CharaChorder - type at the speed of thought?
Saw this on TikTok of all places! :lol: Anyone heard of them before? Lite:
/keyboards#charachorder
13 4703 4
New
AstonJ
Science/Tech>Health & Diet

David Sinclair's new Lifespan podcast
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...
#health#podcasts#bio-hackers#david-sinclair
87 6790 49
New
PragmaticBookshelf
Community>In The Spotlight

Spotlight: Dmitry Zinoviev (Author) Interview and AMA!
Author Spotlight Dmitry Zinoviev @aqsaqal Today we’re putting our spotlight on Dmitry Zinoviev, author of Data Science Essentials in ...
#author-spotlight/python/book-complex-network-analysis-in-python/book-data-science-essentials-in-python/book-resourceful-code-reuse/book-pythonic-programming
33 5211 14
New
Help
Game Dev>Questions

Can I use Java to program a game for Nintendo switch?
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...
/java#nintendo
8 4771 3
New
PragmaticBookshelf
Community>In The Spotlight

Spotlight: Bruce Tate (Author) Interview and AMA!
Author Spotlight: Bruce Tate @redrapids Programming languages always emerge out of need, and if that’s not always true, they’re defin...
/elixir/ruby/phoenix/book-seven-more-languages-in-seven-weeks/book-seven-languages-in-seven-weeks#liveview/book-programming-phoenix-liveview
54 5678 23
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
NewsBot
Backend>Official News

Node.js v22.14.0 released!
Node.js v22.14.0 has been released. Link: Release 2025-02-11, Version 22.14.0 'Jod' (LTS), @aduh95 · nodejs/node · GitHub
github.com
/nodejs#official-news
0 4251 0
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!"

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!"

View all our sponsors ❯

We're in Beta

About us Mission Statement See our Roadmap