andre

andre

Effective Haskell: Observation - Chapter 15 - associated data families (B9 - PDF version)

I’m hopeful for someone that could explain what GHCI does to differentiate an IO action returning a value that has a Show instance versus another IO action returning a value that does not.

Is this difference in behavior related to using associated data families ?

Here is the context that brought this question.

Chapter 15, in the section "Associated Data Families.

On p.574, we find the instance definition of ListDirectory.
Note that the associated data family has a deriving clause.

instance ShellCommand ListDirectory where
    data ShellOutput ListDirectory = 
        DirectoryListing 
          { containingDirectory :: FilePath
          , filenamesInListing :: [FilePath]
          } deriving (Show, Eq)

At the ghci prompt, when you run either of these commands, you get some output.
With the first command, you get a list of values. With the second command, you get a DirectoryListing. Make sense, we have derived a Show instance.

λ> directoryListingWithParent <$> runShellCommand (ListDirectory ".")
[ "./app", "./CHANGELOG.md", "./dist-newstyle",  ... ]

λ> runShellCommand (ListDirectory ".")
DirectoryListing
    { containingDirectory = "."
    , filenamesInListing = [ "app" , "CHANGELOG.md", "dist-newstyle", ...  ]
    }


Moving on to the following page, we find the instance definition of Grep.
Note that the associated data family does NOT have a deriving clause

instance ShellCommand Grep where
    newtype ShellOutput Grep =
        ListOfGrepMatches { getListOfGrepMatches :: [GrepMatch] }

At the GHCI prompt, when you run this command, you do NOT get any output
This makes sense since we have no Show instance. However, why not throwing an error ?

λ> runShellCommand (Grep "version" ["./effectiveHaskell.cabal", "./CHANGELOG.md"])

I’m not sure if what I’m about to say is correct. I suspect that GHCI understands that there is no Show instance.

λ> :t runShellCommand (Grep "version" ["./effectiveHaskell.cabal", "./CHANGELOG.md"])
runShellCommandV3 (Grep "version" ["./effectiveHaskell.cabal", "./CHANGELOG.md"])
  :: IO (ShellOutput Grep)
λ> putStrLn . show <$> runShellCommand (Grep "version" ["./effectiveHaskell.cabal", "./CHANGELOG.md"])

    <interactive>:45:12: error: [GHC-39999]
        • No instance for ‘Show (ShellOutput Grep)’
            arising from a use of ‘show’

If you add the deriving clause, the command returns some output.

instance ShellCommand Grep where
    newtype ShellOutput Grep =
        ListOfGrepMatches { getListOfGrepMatches :: [GrepMatch] }
           deriving Show

-- Same type as the above example, which produces not output
λ> :t (runShellCommand (Grep "version" ["./effectiveHaskell.cabal", "./CHANGELOG.md"]))
(runShellCommandV3 (Grep "version" ["./effectiveHaskell.cabal", "./CHANGELOG.md"]))
  :: IO (ShellOutput Grep)


λ> runShellCommand (Grep "version" ["./effectiveHaskell.cabal", "./CHANGELOG.md"])`

    ListOfGrepMatches
        { getListingOfGrepMatches =
            [ GrepMatch
                { grepMatchingFileName = "./effectiveHaskell.cabal"
                , grepMatchingLineNumber = 1
                , grepMatchingLineContents = "cabal-version:      2.4"
                }
   << rest of output truncated >>

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
mikecargal
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...
New
lirux
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: ...
New
mikecargal
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...
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
New
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
mcpierce
@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,...
New

Other popular topics Top

Devtalk
Hello Devtalk World! Please let us know a little about who you are and where you’re from :nerd_face:
New
AstonJ
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
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
axelson
I’ve been really enjoying obsidian.md: It is very snappy (even though it is based on Electron). I love that it is all local by defaul...
New
AstonJ
I have seen the keycaps I want - they are due for a group-buy this week but won’t be delivered until October next year!!! :rofl: The Ser...
New
PragmaticBookshelf
Tailwind CSS is an exciting new CSS framework that allows you to design your site by composing simple utility classes to create complex e...
New
PragmaticBookshelf
Create efficient, elegant software tests in pytest, Python's most powerful testing framework. Brian Okken @brianokken Edited by Kat...
New
rustkas
Intensively researching Erlang books and additional resources on it, I have found that the topic of using Regular Expressions is either c...
New
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

Sub Categories: