Automatic instance deriving after declaration

14
votes

In Haskell, when defining a data type you can choose to automatically derive some instances, but can I defer the automatic deriving, perhaps even put it in another library?

Here is an example:

Automatic deriving in Haskell is a real time saver!

module MoneyModule where

data Money = Money Int
  deriving Show

Now I wish to use the MoneyModule, but I also want a Read instance for Money:

module ExternalModule where

instance Read Money where
  read = error "Can't this be done automatically instead?"

But I would really have preferred for it to be derived automatically, which I know ghc could have done if only the MoneyModule author had auto-derived the Read instance.

I know that:

  • It's better to fix the problem in the actual MoneyModule by patching it with the missing instance.
  • That it's considered bad to have orphan instances. Instance declarations are preferably put in the module where either the type class or the data type was defined.

In my case I can't follow best practices since the type class is unrelated to the data type. I doubt that the type class module nor the data type module wants to hold the instance, so therefore I'm creating a third library because in some applications you need the instance declaration.

2
haskelltypeclass

2 Answers

21
votes

GHC has the StandaloneDeriving extension, with that, you can

{-# LANGUAGE StandaloneDeriving #-}
import MoneyModule

deriving instance Read Money

derive instances for many classes.

4
votes

To solve the problem about stand alone deriving, see daniels solution. But as you mentioned in your question, orphan instances is not best practice and ghc will generate warnings. To surpress the ghc orphan warnings, you can use the flag -fno-warn-orphans. You can also add it to your cabal file:

...
library
  exposed-modules: ...
  ...
  ghc-options: -fno-warn-orphans
...