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

abtin
PragProg Customers

Distributed Services With Go - gogoproto on linux
page 20: … protoc command… I had to additionally run the following go get commands in order to be able to compile protobuf code using go...
#errata/book-distributed-services-with-go
14 2953 7
New
GilWright
PragProg Customers

Apple Game Frameworks and Technologies: Screen image may be wrong (p 13)
Working through the steps (checking that the Info,plist matches exactly), run the demo game and what appears is grey but does not fill th...
#errata/book-apple-game-frameworks-and-technologies
11 1081 5
New
yulkin
PragProg Customers

Programming Flutter: Image.toByteData() does not exist (page40)
your book suggests to use Image.toByteData() to convert image to bytes, however I get the following error: "the getter ‘toByteData’ isn’t...
/book-programming-flutter#question
1 4102 2
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
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
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
taguniversalmachine
PragProg Customers

Real-Time Phoenix: ProductChannelTest - undefined function describe/2 (pg 203)
Hi, I am getting an error I cannot figure out on my test. I have what I think is the exact code from the book, other than I changed “us...
#question/book-real-time-phoenix
0 1775 6
New
kolossal
PragProg Customers

Hands on Rust: Need help with error - thread 'main' panicked at 'Failed to load texture
Hi, I need some help, I’m new to rust and was learning through your book. but I got stuck at the last stage of distribution. Whenever I t...
#question/book-hands-on-rust
3 1989 5
New
SlowburnAZ
PragProg Customers

Machine Learning in Elixir:Error installing EXLA dependency - Page 25
Getting an error when installing the dependencies at the start of this chapter: could not compile dependency :exla, "mix compile" failed...
#errata/book-machine-learning-in-elixir
1 1159 10
New
dachristenson
PragProg Customers

Kotlin and Android Development featuring Jetpack: End Ch. 11 app runs but consistently crashes
I’ve got to the end of Ch. 11, and the app runs, with all tabs displaying what they should – at first. After switching around between St...
#question/book-kotlin-and-android-development-featuring-jetpack
0 936 8
New

Other popular topics Top

PragmaticBookshelf
Backend>Learning Resources

Web Development with Clojure, Third Edition
Stop developing web apps with yesterday’s tools. Today, developers are increasingly adopting Clojure as a web-development platform. See f...
pragprog.com
#pragprog#web-development/clojure#published-book/book-web-development-with-clojure-third-edition
5 4584 1
New
PragmaticBookshelf
Game Dev>Learning Resources

The Ray Tracer Challenge
Brace yourself for a fun challenge: build a photorealistic 3D renderer from scratch! In just a couple of weeks, build a ray tracer that r...
pragprog.com
#pragprog#published-book/book-the-ray-tracer-challenge#algorithms
3 6115 0
New
AstonJ
General Dev>Hardware

Custom keyboard keycaps
There’s a whole world of custom keycaps out there that I didn’t know existed! Check out all of our Keycaps threads here: https://forum....
#hardware/keyboards#keycaps#mechanical-keyboards
15 11086 19
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
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
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
AstonJ
Data Science

Can AI/ML predict a lottery win?
Biggest jackpot ever apparently! :upside_down_face: I don’t (usually) gamble/play the lottery, but working on a program to predict the...
#ai#machine-learning
19 3939 10
New
foxtrottwist
General Dev>Dev Chat

Warp—The blazingly fast, Rust-based terminal
A few weeks ago I started using Warp a terminal written in rust. Though in it’s current state of development there are a few caveats (tab...
/rust#terminal
52 6785 22
New
First poster: bot
General Dev>In The News

Zig now has built-in HTTP server and client in std
zig/http.zig at 7cf2cbb33ef34c1d211135f56d30fe23b6cacd42 · ziglang/zig. General-purpose programming language and toolchain for maintaini...
github.com
/zig#http
0 5624 0
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
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