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
mikecargal
PragProg Customers

Hands-On Rust: Wrong multiplier for 8x8 console in main.rs
Title: Hands-On Rust (Chap 8 (Adding a Heads Up Display) It looks like ​.with_simple_console_no_bg​(SCREEN_WIDTH*2, SCREEN_HEIGHT*2...
#errata/book-hands-on-rust
9 2026 5
New
HarryDeveloper
PragProg Customers

Programming Kotlin: Difference between SupervisorJob() and supervisorScope (324, 325)
Hi @venkats, It has been mentioned in the description of ‘Supervisory Job’ title that 2 things as mentioned below result in the same eff...
#question/book-programming-kotlin
0 2242 2
New
brian-m-ops
PragProg Customers

Python Testing with pytest, Second Edition: cards pip install error (page 12)
#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 ...
#errata/book-python-testing-with-pytest-second-edition
4 1191 3
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
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
hazardco
PragProg Customers

Agile Web Development with Rails 7: Example code does not work delete method (page 78)
On page 78 the following code appears: <%= link_to ‘Destroy’, product, class: ‘hover:underline’, method: :delete, data: { confirm...
#suggestion/book-agile-web-development-with-rails-7
1 1266 2
New
jonmac
PragProg Customers

Kotlin and Android Development featuring Jetpack: allprojects should be moved to settings.gradle (page 245)
The allprojects block listed on page 245 produces the following error when syncing gradle: “org.gradle.api.GradleScriptException: A prob...
#suggestion/book-kotlin-and-android-development-featuring-jetpack
1 2304 1
New
mert
PragProg Customers

Agile Web Development with Rails 7: page 152, page 153:
AWDWR 7, page 152, page 153: Hello everyone, I’m a little bit lost on the hotwire part. I didn’t fully understand it. On page 152 @rub...
#question/book-agile-web-development-with-rails-7
0 1230 4
New
jwandekoken
PragProg Customers

Programming Phoenix LiveView: Live Upload Match Error
Book: Programming Phoenix LiveView, page 142 (157/378), file lib/pento_web/live/product_live/form_component.ex, in the function below: d...
#errata/book-programming-phoenix-liveview
7 1158 10
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 22283 402
New
PragmaticBookshelf
Game Dev>Learning Resources

Hands-on Rust: Effective Learning through 2D Game Development and Play
Rust is an exciting new programming language combining the power of C with memory safety, fearless concurrency, and productivity boosters...
pragprog.com
#pragprog/rust#published-book/book-hands-on-rust
117 10879 30
New
AstonJ
General Dev>Hardware

BIIP MT3 Extended 2048 Custom Keycap Set (Drop)
This looks like a stunning keycap set :orange_heart: A LEGENDARY KEYBOARD LIVES ON When you bought an Apple Macintosh computer in the e...
/keyboards#apple#keycaps#mechanical-keyboards
14 6713 7
New
Margaret
General Dev>Sales

Get 50% off these PragProg books during our Think Again Sale
Think Again 50% Off Sale » The theme of this sale is new perspectives on familiar topics. Enter coupon code ThinkAgain2021 at checkout t...
#community#pragprog/elixir/book-a-common-sense-guide-to-data-structures-and-algorithms-second-edition/book-programming-machine-learning/book-the-ray-tracer-challenge/book-forge-your-future-with-open-source/book-software-design-x-rays#algorithms/book-testing-elixir#machine-learning#sale/book-concurrent-data-processing-in-elixir/book-intuitive-python
8 6415 2
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
PragmaticBookshelf
Community>In The Spotlight

Spotlight: Mike Riley (Author) Interview and AMA!
Author Spotlight Mike Riley @mriley This month, we turn the spotlight on Mike Riley, author of Portable Python Projects. Mike’s book ...
#author-spotlight/python#iot/book-portable-python-projects#internet-of-things
62 7035 19
New
PragmaticBookshelf
Community>In The Spotlight

Spotlight: Rebecca Skinner (Author) Interview and AMA!
Author Spotlight Rebecca Skinner @RebeccaSkinner Welcome to our latest author spotlight, where we sit down with Rebecca Skinner, auth...
#author-spotlight/haskell/book-effective-haskell
106 11719 28
New
sir.laksmana_wenk
Game Dev>Questions

I want to learn how make a game, but where should I start?
I’m able to do the “artistic” part of game-development; character designing/modeling, music, environment modeling, etc. However, I don’t...
#game-dev
15 4965 9
New
Fl4m3Ph03n1x
AI>Questions

What are the best text-to-speech ai generation tools that you can run locally?
Background Lately I am in a quest to find a good quality TTS ai generation tool to run locally in order to create audio for some videos I...
#ai#text-to-speech
6 8411 3
New
NewsBot
Backend>Official News

Ash v3.23.0 released!
Ash v3.23.0 has been released. Link: https://github.com/ash-project/ash/releases/tag/v3.23.0
github.com
#official-news/ash
0 0 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!"

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