melston

melston

Effective Haskell: Discussion of map (page 58 B2)

In the discussion of map on page 58 you present the example:

map ($ 10) [(+1), (*3), (`div` 5)]

This confuses me a lot. You introduces the function application operator ($) back on page 15 as a way of forcing validation of arguments prior to the evaluation of the function. In that case the $ comes after the function but before the arguments.

Here we have the $ 10 as the function. I look at this example and it seems to be completely backward. I know what you are doing but I just cannot make sense of how it works. In fact, the ‘div’ example is a great example of where I go off the rails trying to understand this. If I execute

div 5 $ 10 

in ghci I get the result 0. And, of course, this doesn’t work at all:

$ 10 `div` 5 // or even $ 10 div 5

So what this does is

div 10 5

But, for the life of me, I cannot figure out how that happens from the given map command.

Where Next?

Popular Pragmatic Bookshelf topics Top

johnp
Running the examples in chapter 5 c under pytest 5.4.1 causes an AttributeError: ‘module’ object has no attribute ‘config’. In particula...
New
ianwillie
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...
New
jdufour
Hello! On page xix of the preface, it says there is a community forum "… for help if your’re stuck on one of the exercises in this book… ...
New
adamwoolhether
When trying to generate the protobuf .go file, I receive this error: Unknown flag: --go_opt libprotoc 3.12.3 MacOS 11.3.1 Googling ...
New
AndyDavis3416
@noelrappin Running the webpack dev server, I receive the following warning: ERROR in tsconfig.json TS18003: No inputs were found in c...
New
Charles
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...
New
jonmac
The allprojects block listed on page 245 produces the following error when syncing gradle: “org.gradle.api.GradleScriptException: A prob...
New
rainforest
Hi, I’ve got a question about the implementation of PubSub when using a Phoenix.Socket.Transport behaviour rather than channels. Before ...
New
bjnord
Hello @herbert ! Trying to get the very first “Hello, Bracket Terminal!" example to run (p. 53). I develop on an Amazon EC2 instance runn...
New
davetron5000
Hello faithful readers! If you have tried to follow along in the book, you are asked to start up the dev environment via dx/build and ar...
New

Other popular topics Top

Devtalk
Reading something? Working on something? Planning something? Changing jobs even!? If you’re up for sharing, please let us know what you’...
1050 21151 394
New
AstonJ
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...
New
AstonJ
poll poll Be sure to check out @Dusty’s article posted here: An Introduction to Alternative Keyboard Layouts It’s one of the best write-...
New
AstonJ
Thanks to @foxtrottwist’s and @Tomas’s posts in this thread: Poll: Which code editor do you use? I bought Onivim! :nerd_face: https://on...
New
dimitarvp
Small essay with thoughts on macOS vs. Linux: I know @Exadra37 is just waiting around the corner to scream at me “I TOLD YOU SO!!!” but I...
New
husaindevelop
Inside our android webview app, we are trying to paste the copied content from another app eg (notes) using navigator.clipboard.readtext ...
New
New
AnfaengerAlex
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...
New
AstonJ
This is cool! DEEPSEEK-V3 ON M4 MAC: BLAZING FAST INFERENCE ON APPLE SILICON We just witnessed something incredible: the largest open-s...
New
RobertRichards
Hair Salon Games for Girls Fun Girls Hair Saloon game is mainly developed for kids. This game allows users to select virtual avatars to ...
New

Sub Categories: