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
Hi @Margaret ,
On page VII the book tells us the example and snippets will be all using Elixir version 1.11
But on page 3 almost the en...
New
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 ...
New
The book has the same “Problem space/Solution space” diagram on page 18 as is on page 17. The correct Problem/Solution space diagrams ar...
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
“The ProductLive.Index template calls a helper function, live_component/3, that in turn calls on the modal component. ”
Excerpt From: Br...
New
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
@mfazio23
I’m following the indications of the book and arriver ad chapter 10, but the app cannot be compiled due to an error in the Bas...
New
Docker-Machine became part of the Docker Toolbox, which was deprecated in 2020, long after Docker Desktop supported Docker Engine nativel...
New
root_layout: {PentoWeb.LayoutView, :root},
This results in the following following error:
no “root” html template defined for PentoWeb...
New
Other popular topics
@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...
New
I know that these benchmarks might not be the exact picture of real-world scenario, but still I expect a Rust web framework performing a ...
New
I know that -t flag is used along with -i flag for getting an interactive shell. But I cannot digest what the man page for docker run com...
New
I’ve been hearing quite a lot of comments relating to the sound of a keyboard, with one of the most desirable of these called ‘thock’, he...
New
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
“Finding the Boundaries” Hero’s Journey with Noel Rappin @noelrappin
Even when you’re ultimately right about what the future ho...
New
Not sure if following fits exactly this thread, or if we should have a hobby thread…
For many years I’m designing and building model air...
New
Think Again 50% Off Sale »
The theme of this sale is new perspectives on familiar topics.
Enter coupon code ThinkAgain2021 at checkout t...
New
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
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...
New
Categories:
Sub Categories:
Popular Portals
- /elixir
- /rust
- /ruby
- /wasm
- /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
- /php
- /deepseek
- /html
- /zig
- /scala
- /textmate
- /sublime-text
- /lisp
- /nixos
- /debian
- /react-native
- /agda
- /kubuntu
- /arch-linux
- /django
- /ubuntu
- /revery
- /manjaro
- /spring
- /nodejs
- /diversity
- /lua
- /slackware
- /c
- /julia
- /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'll move you from confusion to "Aha, now I get it!"