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

johnp
PragProg Customers

Python Testing with pytest - Chapter 5 example c AttributeError: 'module' object has no attribute 'config'
Running the examples in chapter 5 c under pytest 5.4.1 causes an AttributeError: ‘module’ object has no attribute ‘config’. In particula...
#errata/book-python-testing-with-pytest
5 3582 1
New
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
mikecargal
PragProg Customers

Hands-on Rust: question about get_component
Title: Hands-on Rust: question about get_component (page 295) (feel free to respond. “You dug you’re own hole… good luck”) I have somet...
#question/book-hands-on-rust
1 1167 5
New
herminiotorres
PragProg Customers

Concurrent Data Processing in Elixir: various errata
Hi @Margaret , On page VII the book tells us the example and snippets will be all using Elixir version 1.11 But on page 3 almost the en...
#errata/book-concurrent-data-processing-in-elixir
15 1549 13
New
raul
PragProg Customers

Distributed Services with Go: Chapter 3 suggestions
Page 28: It implements io.ReaderAt on the store type. Sorry if it’s a dumb question but was the io.ReaderAt supposed to be io.ReadAt? ...
#errata/book-distributed-services-with-go
0 1302 3
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
leba0495
PragProg Customers

A Common-Sense Guide to Data Structures and Algorithms, Second Edition: (pg460)
Hello! Thanks for the great book. I was attempting the Trie (chap 17) exercises and for number 4 the solution provided for the autocorre...
#errata/book-a-common-sense-guide-to-data-structures-and-algorithms-second-edition
0 1157 3
New
adamwoolhether
PragProg Customers

Distributed Services With Go: Unable to pass readiness/liveness checks. (page 210, 215)
I’m not quite sure what’s going on here, but I’m unable to have to containers successfully complete the Readiness/Liveness checks. I’m im...
/book-distributed-services-with-go
5 1420 5
New
a.zampa
PragProg Customers

Kotlin and Android Development featuring Jetpack: error concerning function getCurrentStandings in BaseballDao.java
@mfazio23 I’m following the indications of the book and arriver ad chapter 10, but the app cannot be compiled due to an error in the Bas...
#question/book-kotlin-and-android-development-featuring-jetpack
1 3397 6
New
dachristenson
PragProg Customers

Kotlin and Android Development featuring Jetpack: No activity_main.xml anymore
I just bought this book to learn about Android development, and I’m already running into a major issue in Ch. 1, p. 20: “Update activity...
#errata/book-kotlin-and-android-development-featuring-jetpack
0 2172 6
New

Other popular topics Top

PragmaticBookshelf
General Dev>Learning Resources

Seven More Languages in Seven Weeks
Learn from the award-winning programming series that inspired the Elixir language, and go on a step-by-step journey through the most impo...
pragprog.com
#pragprog/elixir/julia/lua#published-book#factor/elm#minikanren/idris/book-seven-more-languages-in-seven-weeks
4 5862 0
New
PragmaticBookshelf
Backend>Learning Resources

Seven Languages in Seven Weeks
Ruby, Io, Prolog, Scala, Erlang, Clojure, Haskell. With Seven Languages in Seven Weeks, by Bruce A. Tate, you’ll go beyond the syntax—and...
pragprog.com
#pragprog/clojure/erlang/haskell/prolog/ruby/scala#published-book/book-seven-languages-in-seven-weeks
5 5730 1
New
siddhant3030
General Dev>Dev Chat

Which vertical monitor do you use?
I’m thinking of buying a monitor that I can rotate to use as a vertical monitor? Also, I want to know if someone is using it for program...
#monitors#programming
51 4892 20
New
AstonJ
General Dev>Code Editors

Poll: Which code editor do you use?
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...
#community#polls/vim/emacs#code-editors/vscode#notepad/sublime-text#atom/textmate#codespaces#brackets/onivim#geany
121 5796 61
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
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
PragmaticBookshelf
Backend>Learning Resources

Agile Web Development with Rails 8
Get the comprehensive, insider information you need for Rails 8 with the new edition of this award-winning classic. Sam Ruby @rubys ...
pragprog.com
#pragprog#web-development/ruby/rails#published-book/book-agile-web-development-with-rails-8
12 7404 7
New
AnfaengerAlex
Android>Questions

Unresolved Reference to android in build.gradle.kts – Beginner Issue
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...
#binding
0 7460 2
New
PragmaticBookshelf
Backend>Learning Resources

Ash Framework
Explore the power of Ash Framework by modeling and building the domain for a real-world web application. Rebecca Le @sevenseacat and ...
pragprog.com
#pragprog/elixir#published-book/ash/book-ash-framework
15 7555 9
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