PragProg Customers

Effective Haskell - inaccurate description of filter example (page 61)

tusktenon

tusktenon

Effective Haskell - inaccurate description of filter example (page 61)

@RebeccaSkinner

On p.61 in Chapter 2, the following is described as finding “the sum of the first ten odd numbers”:

λ (foldr (+) 0 . filter odd) [0..10]
25

In fact, this code sums the odd numbers between 0 and 10 – that is, the first five odd numbers. For the sum of the first ten odd numbers, you’d probably want something like

λ (foldr (+) 0 . map (\x -> 2 * x + 1)) [0..9]
100
2 comments
#errata/book-effective-haskell
1 300 2

Most Liked

tusktenon

tusktenon

It’s actually not a bad example of the differing use cases for map and filter: if the problem is to “sum the odd numbers between m and n,” then filter is perfect; if the problem is to “sum the first n odd numbers,” map is more appropriate.

I’ve only made it to chapter 3 so far, but I’m really enjoying the book!

1
Post #3

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
1 300 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 3547 1
New
ianwillie
PragProg Customers

Build Websites with Hugo: Code will not run in a working Hugo setup
Hello Brian, I have some problems with running the code in your book. I like the style of the book very much and I have learnt a lot as...
#question/book-build-websites-with-hugo
0 1500 3
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 1619 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 1451 5
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 1160 5
New
cro
PragProg Customers

Programming Phoenix LiveView: P27, live_path confusion
I am working on the “Your Turn” for chapter one and building out the restart button talked about on page 27. It recommends looking into ...
#question/book-programming-phoenix-liveview
7 3487 14
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
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 1120 3
New
rainforest
PragProg Customers

Real-Time Phoenix: use PubSub with implementation of Phoenix.Socket.Transport rather than channels (page 37)
Hi, I’ve got a question about the implementation of PubSub when using a Phoenix.Socket.Transport behaviour rather than channels. Before ...
#question/book-real-time-phoenix#websockets#phoenixsocket#pubsub
0 1506 3
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 1160 10
New

Other popular topics Top

wolf4earth
Science/Tech>Health & Diet

How do you keep fit and healthy?
@AstonJ prompted me to open this topic after I mentioned in the lockdown thread how I started to do a lot more for my fitness. https://f...
#fitness#heath
193 4510 82
New
dasdom
General Dev>Dev Chat

Standing Desks
No chair. I have a standing desk. This post was split into a dedicated thread from our thread about chairs :slight_smile:
#workspace#opinions
177 8632 77
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 3579 14
New
Exadra37
Linux>Questions

What is the most minimalist Linux server distro?
I am asking for any distro that only has the bare-bones to be able to get a shell in the server and then just install the packages as we ...
#linux#servers
66 21694 24
New
PragmaticBookshelf
Backend>Learning Resources

Python Testing with pytest, Second Edition
Create efficient, elegant software tests in pytest, Python's most powerful testing framework. Brian Okken @brianokken Edited by Kat...
pragprog.com
#pragprog/python#published-book/book-python-testing-with-pytest-second-edition
16 3580 4
New
First poster: joeb
General Dev>In The News

Safari now supports File System Access API with private origin
The File System Access API with Origin Private File System. WebKit supports new API that makes it possible for web apps to create, open,...
webkit.org
#api#safari
43 3103 21
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 6351 19
New
AstonJ
macOS>Chat

How to block any website on Mac using Little Snitch
If you want a quick and easy way to block any website on your Mac using Little Snitch simply… File > New Rule: And select Deny, O...
#macos#how-to#littlesnitch
5 8045 3
New
First poster: CommunityNews
AI>In The News

How to fix the eyes in AI-generated images
aidemos.info
0 3038 0
New
PragmaticBookshelf
Community>In The Spotlight

Spotlight: Sophie DeBenedetto (Author) Interview and AMA!
Author Spotlight: Sophie DeBenedetto @SophieDeBenedetto The days of the traditional request-response web application are long gone, b...
/elixir/phoenix#liveview/book-programming-phoenix-liveview
37 3237 14
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.

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.

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