molleweide

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

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

Where Next?

Popular Pragmatic Bookshelf topics Top

herminiotorres
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
cro
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
swlaschin
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
leonW
I ran this command after installing the sample application: $ cards add do something --owner Brian And got a file not found error: Fil...
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
jgchristopher
“The ProductLive.Index template calls a helper function, live_component/3, that in turn calls on the modal component. ” Excerpt From: Br...
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
a.zampa
@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
redconfetti
Docker-Machine became part of the Docker Toolbox, which was deprecated in 2020, long after Docker Desktop supported Docker Engine nativel...
New
gorkaio
root_layout: {PentoWeb.LayoutView, :root}, This results in the following following error: no “root” html template defined for PentoWeb...
New

Other popular topics Top

wolf4earth
@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
DevotionGeo
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
DevotionGeo
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
AstonJ
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
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
PragmaticBookshelf
“Finding the Boundaries” Hero’s Journey with Noel Rappin @noelrappin Even when you’re ultimately right about what the future ho...
New
Rainer
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
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
Fl4m3Ph03n1x
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

Sub Categories: