That definition typechecks but doesn't do what you intend I think. If you `lift (lift Bool)` then result of splicing this is of type `Exp` and not `Q Exp` as you intended so you can't splice it again.
That is why my definition is more complicated and involves `return`. Cheers, Matt On Fri, Jan 25, 2019 at 12:50 PM Richard Eisenberg <r...@cs.brynmawr.edu> wrote: > > This typechecks: > > instance Lift a => Lift (Q a) where > lift :: Q a -> Q Exp > lift x = join (fmap lift x) > > And it looks correct. Do you agree? > > In general, it sounds like you've thought this through. To me, this doesn't > rise to the need of a ghc-proposal; I would say to go for it. > > Richard > > > On Jan 25, 2019, at 6:56 AM, Matthew Pickering > > <matthewtpicker...@gmail.com> wrote: > > > > I don't think that cross stage persistence will work as it is > > currently implemented which is probably why the check exists. > > > > 1. The normal case > > > > foo x = [| x |] ===> > > foo x = [| $(lift x) |] > > > > 2. x is defined at stage 0, and used at stage 2. > > > > One option is: > > foo x = [| [| x |] |] ===> > > foo x = [| [| $($(lift (lift x))) |] |] > > or > > foo x = [| [| x |] |] ===> > > foo x = [| let x' = $(lift x) in [| $(lift [| x' |]) |] > > > > We need to think a bit how to `lift` something of type `Q Exp` because > > of the `Q` monad. Lifting an `Exp` seems trivial as it's a normal ADT > > (I tested and this works after deriving 40 instances). > > > > You can define `lift2` which lifts an expression twice as follows. > > > > ``` > > lift2 :: Lift a => a -> Q Exp > > lift2 a = lift a >>= \e -> [| return $ $(lift e) |] > > ``` > > > > 3. x is defined at stage 1 and used in stage 2 > > > > foo = [| \x -> [| x |] |] ===> > > foo = [| \x -> [| $(lift x) |] |] > > > > Desugared with a single call to `lift` like normal. > > > > 4. x is defined in stage 2 and used in stage 1 > > > > foo = [| [| \x -> $(x) |] |] > > > > Rejected just like usual. `x` won't be bound when the splice is run. > > > > It seems that with some suitable care that things will work out when > > lifting across multiple levels but that is the point where care needs > > to be taken. > > > > Matt > > > > > > > > On Thu, Jan 24, 2019 at 5:46 PM Richard Eisenberg <r...@cs.brynmawr.edu> > > wrote: > >> > >> I think Geoff was primarily concerned with typed Template Haskell, not the > >> untyped variety. > >> > >> I, too, have wondered if there was a technical reason behind this > >> restriction, or if merely it was assumed that nested brackets were not > >> worthwhile. > >> > >> One question: how would staging work between nesting levels of brackets? > >> > >> Richard > >> > >>> On Jan 24, 2019, at 12:42 PM, Ben Gamari <b...@smart-cactus.org> wrote: > >>> > >>> Matthew Pickering <matthewtpicker...@gmail.com> writes: > >>> > >>>> There is a check in `RnSplice` which errors on the following program > >>>> with nested brackets. > >>>> > >>> It might be good to explicitly include Geoff Mainland in this thread. > >>> I'm not sure he'll see it otherwise. > >>> > >>> Cheers, > >>> > >>> - Ben > >>> > >>> _______________________________________________ > >>> ghc-devs mailing list > >>> ghc-devs@haskell.org > >>> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs > >> > _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
