Automatically deriving Generic for every algebraic data type

Oleg Grenrus oleg.grenrus at
Thu Feb 4 13:17:34 UTC 2016


sometimes I want to use Generic derivation, but don’t want expose the Generic
instance outside the module.  The reason, is that for some types I want to
export only smart constructors / modifier lenses; yet the structure is probably
simple enough to benefit from `Generic`.

If it will be possible to restrict export of the Generic instance, then I don’t
see problem of auto-deriving it for everything possible though.

- Oleg

> On 04 Feb 2016, at 14:28, Wolfgang Jeltsch <g9ks157k at> wrote:
> Am Donnerstag, den 04.02.2016, 14:19 +0200 schrieb Wolfgang Jeltsch:
>> Hi,
>> if you do generic programming these days, you can use DeriveAnyClass to
>> write code like the following (where Serializable is a class with a
>> generic default implementation):
>>> data Tree a = Leaf | Branch (Tree a) a (Tree a)
>>>              deriving (Generic, Serializable)
>> It would be great, if you could just write the following instead:
>>> data Tree a = Leaf | Branch (Tree a) a (Tree a) deriving Serializable
>> This would correspond exactly to what you do when using standard Haskell
>> deriving. It could be made possible by letting the compiler instantiate
>> the Generic class automatically every time an algebraic data type is
>> declared. A potential downside of this would be that programmers would
>> not be able to define non-standard instances of Generics, but I actually
>> cannot see that this is very useful anyhow.
> I want to add that this would probably allow us to implement all the
> other deriving mechanisms (for standard classes, for Functor, etc.)
> entirely in libraries, using generic programming, without forcing users
> to change their code by adding deriving of Generic. Maybe a future
> standard Haskell would not even have deriving rules hardwired into the
> language anymore (which always felt somehow wrong to me). Wouldn’t this
> be great? ;-)
> All the best,
> Wolfgang
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