Proposal: Automatic derivation of Lift

Andres Loeh mail at
Tue Sep 8 11:59:42 UTC 2015

I don't think there's any fundamental reason why unboxed fields
prevent a Generic instance, as long as we're happy that unboxed values
will be re-boxed in the generic representation. It simply seems as if
nobody has thought of implementing this. As an example, consider the
following hand-written example which works just fine:

{-# LANGUAGE MagicHash, KindSignatures, PolyKinds, TypeOperators,
TypeFamilies #-}
module GenUnboxed where

import GHC.Exts
import GHC.Generics
import Generics.Deriving.Eq

data UPair = UPair Int# Char#

instance Generic UPair where
  type Rep UPair = K1 R Int :*: K1 R Char
  from (UPair x y) = K1 (I# x) :*: K1 (C# y)
  to (K1 (I# x) :*: K1 (C# y)) = UPair x y

instance GEq UPair

test :: Bool
test = let p = UPair 3# 'x'# in geq p p


On Mon, Sep 7, 2015 at 10:02 PM, Ryan Scott < at> wrote:
> Unlifted types can't be used polymorphically or in instance
> declarations, so this makes it impossible to do something like
>     instance Generic Int#
> or store an Int# in one branch of a (:*:), preventing generics from
> doing anything in #-land. (unless someone has found a way to hack
> around this).
> I would be okay with implementing a generics-based approach, but we'd
> have to add a caveat that it will only work out-of-the-box on GHC 8.0
> or later, due to TH's need to look up package information. (We could
> give users the ability to specify a package name manually as a
> workaround.)
> If this were added, where would be the best place to put it? th-lift?
> generic-deriving? template-haskell? A new package (lift-generics)?
> Ryan S.
> On Mon, Sep 7, 2015 at 3:10 PM, Matthew Pickering
> <matthewtpickering at> wrote:
>> Continuing my support of the generics route. Is there a fundamental
>> reason why it couldn't handle unlifted types? Given their relative
>> paucity, it seems like a fair compromise to generically define lift
>> instances for all normal data types but require TH for unlifted types.
>> This approach seems much smoother from a maintenance perspective.
>> On Mon, Sep 7, 2015 at 5:26 PM, Ryan Scott < at> wrote:
>>> There is a Lift typeclass defined in template-haskell [1] which, when
>>> a data type is an instance, permits it to be directly used in a TH
>>> quotation, like so
>>>     data Example = Example
>>>     instance Lift Example where
>>>       lift Example = conE (mkNameG_d "<package-name>" "<module-name>" "Example")
>>>     e :: Example
>>>     e = [| Example |]
>>> Making Lift instances for most data types is straightforward and
>>> mechanical, so the proposal is to allow automatic derivation of Lift
>>> via a -XDeriveLift extension:
>>>     data Example = Example deriving Lift
>>> This is actually a pretty a pretty old proposal [2], dating back to
>>> 2007. I wanted to have this feature for my needs, so I submitted a
>>> proof-of-concept at the GHC Trac issue page [3].
>>> The question now is: do we really want to bake this feature into GHC?
>>> Since not many people opined on the Trac page, I wanted to submit this
>>> here for wider visibility and to have a discussion.
>>> Here are some arguments I have heard against this feature (please tell
>>> me if I am misrepresenting your opinion):
>>> * We already have a th-lift package [4] on Hackage which allows
>>> derivation of Lift via Template Haskell functions. In addition, if
>>> you're using Lift, chances are you're also using the -XTemplateHaskell
>>> extension in the first place, so th-lift should be suitable.
>>> * The same functionality could be added via GHC generics (as of GHC
>>> 7.12/8.0, which adds the ability to reify a datatype's package name
>>> [5]), if -XTemplateHaskell can't be used.
>>> * Adding another -XDerive- extension places a burden on GHC devs to
>>> maintain it in the future in response to further Template Haskell
>>> changes.
>>> Here are my (opinionated) responses to each of these:
>>> * th-lift isn't as fully-featured as a -XDerive- extension at the
>>> moment, since it can't do sophisticated type inference [6] or derive
>>> for data families. This is something that could be addressed with a
>>> patch to th-lift, though.
>>> * GHC generics wouldn't be enough to handle unlifted types like Int#,
>>> Char#, or Double# (which other -XDerive- extensions do).
>>> * This is a subjective measurement, but in terms of the amount of code
>>> I had to add, -XDeriveLift was substantially simpler than other
>>> -XDerive extensions, because there are fewer weird corner cases. Plus,
>>> I'd volunteer to maintain it :)
>>> Simon PJ wanted to know if other Template Haskell programmers would
>>> find -XDeriveLift useful. Would you be able to use it? Would you like
>>> to see a solution other than putting it into GHC? I'd love to hear
>>> feedback so we can bring some closure to this 8-year-old feature
>>> request.
>>> Ryan S.
>>> -----
>>> [1]
>>> [2]
>>> [3]
>>> [4]
>>> [5]
>>> [6]
>>> _______________________________________________
>>> ghc-devs mailing list
>>> ghc-devs at
> _______________________________________________
> ghc-devs mailing list
> ghc-devs at

More information about the ghc-devs mailing list