Proposal: GHC.Generics marked UNSAFE for SafeHaskell

[Ryan Newton]

But what stops the user from defining their own instances if they in fact
did not derive it?

The explicit "False" in Pedro's formulation seems to serve this purpose.



On Mon, Oct 14, 2013 at 11:20 PM, Nicolas Frisby
<nicolas.frisby at gmail.com>wrote:

> The formulation as a type family seems to conflict with the open-world
> principle. Would a class-based encoding instead be sufficient?
>
> -- the proposed, special wired-in class
> class Derives (t :: k1) (c :: k2)
>
> -- GHC would infer these from Pedro's example's declarations
> instance Derives MyData Eq
> instance Derives MyData Generic
> instance Derives MyData Show
>
> NB that there is no instance Derives MyData Ord, but standalone deriving
> could yield one "later"
>
> On Mon, Oct 14, 2013 at 10:02 PM, Ryan Newton <rrnewton at gmail.com> wrote:
>
>> Hey, that's an awesome formulation!  Thanks Pedro.
>>
>> Any idea how much work this would be to implement in GHC, if it did
>> garner approval?
>>
>>
>> On Tue, Oct 8, 2013 at 3:48 AM, José Pedro Magalhães <dreixel at gmail.com>wrote:
>>
>>> Hi,
>>>
>>> On Mon, Oct 7, 2013 at 10:32 AM, Dag Odenhall <dag.odenhall at gmail.com>wrote:
>>>
>>>> Here‘s a thought: doesn’t Generic already have an unused phantom type
>>>> that's only there “just in case we need to add something in the future”?
>>>>
>>> No, it doesn't.
>>>
>>> Just a thought: what if we had a type family
>>>
>>> type family Derives (t :: k1) (c :: k2) :: Bool
>>>
>>> which would automatically be instantiated by GHC appropriately? E.g., if
>>> the user had the following code:
>>>
>>> data MyData = MyData deriving (Eq, Generic)
>>> deriving instance Show MyData
>>> instance Ord MyData
>>>
>>> GHC would automatically instantiate:
>>>
>>> type instance Derives MyData Eq      = True
>>> type instance Derives MyData Generic = True
>>> type instance Derives MyData Show    = True
>>> type instance Derives MyData Ord     = False
>>>
>>> Would this be something Ryan could use for detecting safe instances for
>>> LVish?
>>>
>>>
>>> Cheers,
>>> Pedro
>>>
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>>>
>>>
>>
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>
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