Why are nested brackets disallowed?

[Richard Eisenberg]

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 <matthewtpickering at 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 <rae at 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 <ben at smart-cactus.org> wrote:
>>> 
>>> Matthew Pickering <matthewtpickering at 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
>>> 
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>>