molleweide
Effective Haskell: no `Show expr` arising from print
Hi @RebeccaSkinner,
In for example the Calculator segment of chapter 4, if I run the examples in GHCI this error is returned:
λ: Add (Lit 5) (Lit 4)
<interactive>:159:1: error:
• No instance for (Show Expr) arising from a use of ‘print’
• In a stmt of an interactive GHCi command: print it
I understand what it means but personally I am not sure how to make all examples go past this. It seems by your examples in the book also that you don’t have this issue?
First Post!
RebeccaSkinner
Author of Effective Haskell
Thanks for catching this! I’ll take a look and push a fix in the next update!
In the mean time, you can work around this bug by adding deriving Show after your definition of Expr. For example:
data Expr = Lit Int
| Sub Expr Expr
| Add Expr Expr
| Mul Expr Expr
| Div Expr Expr
deriving Show
Popular Pragmatic Bookshelf topics
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...
New
In Chapter 3, the source for index introduces Config on page 31, followed by more code including tests; Config isn’t introduced until pag...
New
Many tasks_proj/tests directories exist in chapters 2, 3, 5 that have tests that use the custom markers smoke and get, which are not decl...
New
I ran this command after installing the sample application:
$ cards add do something --owner Brian
And got a file not found error:
Fil...
New
@noelrappin
Running the webpack dev server, I receive the following warning:
ERROR in tsconfig.json
TS18003: No inputs were found in c...
New
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, ...
New
When running tox for the first time, I got the following error:
ERROR: InterpreterNotFound: python3.10
I realised that I was running ...
New
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...
New
Title: Agile Web Development with Rails 7: (page 70)
I am running windows 11 pro with rails 7.0.3 and ruby 3.1.2p20 (2022-04-12 revision...
New
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
Other popular topics
A thread that every forum needs!
Simply post a link to a track on YouTube (or SoundCloud or Vimeo amongst others!) on a separate line an...
New
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...
New
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
Inspired by this post from @Carter, which languages, frameworks or other tech or tools do you think is killing it right now? :upside_down...
New
Saw this on TikTok of all places! :lol:
Anyone heard of them before?
Lite:
New
Hi folks,
I don’t know if I saw this here but, here’s a new programming language, called Roc
Reminds me a bit of Elm and thus Haskell. ...
New
This is going to be a long an frequently posted thread.
While talking to a friend of mine who has taken data structure and algorithm cou...
New
I am trying to crate a game for the Nintendo switch, I wanted to use Java as I am comfortable with that programming language. Can you use...
New
Programming Ruby is the most complete book on Ruby, covering both the language itself and the standard library as well as commonly used t...
New
Develop, deploy, and debug BEAM applications using BEAMOps: a new paradigm that focuses on scalability, fault tolerance, and owning each ...
New
Categories:
Sub Categories:
Popular Portals
- /elixir
- /rust
- /wasm
- /ruby
- /erlang
- /phoenix
- /keyboards
- /rails
- /js
- /python
- /security
- /go
- /swift
- /vim
- /clojure
- /emacs
- /haskell
- /java
- /onivim
- /svelte
- /typescript
- /crystal
- /kotlin
- /c-plus-plus
- /tailwind
- /gleam
- /ocaml
- /react
- /elm
- /flutter
- /vscode
- /ash
- /opensuse
- /centos
- /html
- /php
- /deepseek
- /zig
- /scala
- /textmate
- /lisp
- /sublime-text
- /nixos
- /debian
- /react-native
- /agda
- /kubuntu
- /arch-linux
- /django
- /revery
- /ubuntu
- /manjaro
- /spring
- /nodejs
- /diversity
- /lua
- /julia
- /slackware
- /c
- /neovim
Devtalk Sponsors
Practical resources that improve the lives of professional developers.
We build trusted fault-tolerant systems that scale to billions of users.
Real-time error tracking, performance insights, and observability for devs.
Develop your skills with books, videos, and courses.
Catch errors, track performance, monitor hosts and more.
Enabling companies to succeed by building software people love.
Courses that move you from confusion to "Aha, now I get it!"