Question

i have this

data Something = Something Integer deriving (MyClass, Show)

class MyClass a where   
    hello :: MyClass a => a -> a

instance MyClass Integer where
    hello i = i + 1

main = print . hello $ Something 3

but MyClass isn’t derivable. Why?

Answer 1

GHC cannot magically derive instances for arbitrary data types. However, it
can make use of the fact that newtype declarations create a new name for the
same underlying type to derive instances for those using the
GeneralizedNewtypeDeriving extension. So, you could do something like this:

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

newtype Something = Something Integer deriving (MyClass, Show)

class MyClass a where
    hello :: MyClass a => a -> a

instance MyClass Integer where
    hello i = i + 1

main = print . hello $ Something 3

The reason GHC cannot derive the new instance is that it does not know what the instance
should be. Even if your data type only has one field, it may not necessarily be the
same as that field. The ability to derive instances for newtypes is convenient, since they
are usually used to provide different behaviours for certain typeclasses or as a way to
use the type system to separate things that have the same type but different uses in your code.