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

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
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
lirux
PragProg Customers

The Ray Tracer Challenge: "Computing the normal on a transformed sphere" ebook test
Hi Jamis, I think there’s an issue with a test on chapter 6. I own the ebook, version P1.0 Feb. 2019. This test doesn’t pass for me: ...
#errata/book-the-ray-tracer-challenge
10 1461 5
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
leonW
PragProg Customers

Python Testing with pytest, Second Edition: File not Found Error (page 43)
I ran this command after installing the sample application: $ cards add do something --owner Brian And got a file not found error: Fil...
#errata/book-python-testing-with-pytest-second-edition
5 4391 7
New
Charles
PragProg Customers

Programming Phoenix LiveView: book not updated to version 1.6 (p. 18)
In general, the book isn’t yet updated for Phoenix version 1.6. On page 18 of the book, the authors indicate that an auto generated of ro...
#errata/book-programming-phoenix-liveview
9 2051 12
New
AufHe
PragProg Customers

Agile Web Development with Rails 7: page 113 - Seeing the list of products
I’m a newbie to Rails 7 and have hit an issue with the bin/Dev script mentioned on pages 112-113. Iteration A1 - Seeing the list of prod...
#question/book-agile-web-development-with-rails-7
0 2631 9
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
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
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

Other popular topics Top

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
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
AstonJ
General Dev>Code Editors

SpaceVim vs SpaceMacs
SpaceVim seems to be gaining in features and popularity and I just wondered how it compares with SpaceMacs in 2020 - anyone have any thou...
/vim#spacevim#spacemacs/emacs#code-editors
30 4336 14
New
brentjanderson
General Dev>Hardware

Moonlander Keyboard (Mechanical) (Ergonomic) (Split) (Ortholinear)
Bought the Moonlander mechanical keyboard. Cherry Brown MX switches. Arms and wrists have been hurting enough that it’s time I did someth...
#hardware/keyboards#moonlander#mechanical-keyboards#ortholinear#ergonomic
212 17779 90
New
DevotionGeo
Backend>Chat

Would you use Erlang now when there is Elixir?
Why, if your answer is yes?
/elixir/erlang
167 4955 52
New
AstonJ
General Dev>Dev Chat

Which language or framework do you want to learn next?
Curious to know which languages and frameworks you’re all thinking about learning next :upside_down_face: Perhaps if there’s enough peop...
#community#learning
243 6639 97
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
80 9318 24
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
AstonJ
Backend>Questions

Psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such file or directory
If you’re getting errors like this: psql: error: connection to server on socket “/tmp/.s.PGSQL.5432” failed: No such file or directory ...
#macos/rails/postgresql
1 5553 1
New
AstonJ
AI>Chat

How to: Run DeepSeek on Mac, Windows, and Linux!
This is a very quick guide, you just need to: Download LM Studio: https://lmstudio.ai/ Click on search Type DeepSeek, then select the o...
#macs/deepseek#guides#lm-studio
14 9328 10
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