GitLab

Summary

Location of documentation issue: GHC Error Message

Given this example:

class MyClass a where
  myFunc :: a -> Int

data MyExample = MyExample {myBool :: Bool, myInt :: Int}
  deriving (Show, Eq, MyClass)

GHC (correctly) rejects this with this error:

example.hs:5:23: error:
    • Can't make a derived instance of ‘MyClass MyExample’:
        ‘MyClass’ is not a stock derivable class (Eq, Show, etc.)
        Try enabling DeriveAnyClass
    • In the data declaration for ‘MyExample’
  |
5 |   deriving (Show, Eq, MyClass)
  |    

However, it's suggestion to Try enabling DeriveAnyClass is not great, because when one does that:

{-# LANGUAGE DeriveAnyClass #-}

class MyClass a where
  myFunc :: a -> Int

data MyExample = MyExample {myBool :: Bool, myInt :: Int}
  deriving (Show, Eq, MyClass)

GHC now (incorrectly, IMHO), accepts this with the following warning: (At least there is a warning, there didn't use to be, right?)

example.hs:7:23: warning: [-Wmissing-methods]
    • No explicit implementation for
        ‘myFunc’
    • In the instance declaration for ‘MyClass MyExample’
  |
7 |   deriving (Show, Eq, MyClass)
  |  

However, this fails at runtime (!):

*Main> myFunc (MyExample {myBool =True,myInt =5})
*** Exception: example.hs:7:23-29: No instance nor default method for class operation myFunc

Proposed improvements or changes

Don't suggest enabling DeriveAnyClass. Optional: Give a much more comprehensive recommendation that doesn't lead users to write code that throws runtime exceptions in pure code.