API Annotations and HsSCC / HsTickPragma

[Alan & Kim Zimmerman]

Ok, that makes sense.

Thanks
  Alan

On Wed, Dec 3, 2014 at 1:30 PM, Simon Peyton Jones <simonpj at microsoft.com>
wrote:

>  Please don’t do (1).  That would horribly clutter HsPar.
>
>
>
> I suggest (2).  Actually I suggest you don’t have the Bool at all.
> Instead, in the desugarer, if you come across a HsScc, discard it unless it
> is active.   We only need the Bool to cache the “am I active” question if
> there are zillions of places where we need to know.  But I bet there is
> only one, namely the desugarer.
>
>
>
> The spirit of the front end is: leave the source code entirely undisturbed
> until desugaring. Throwing away HsScc and turning them in HsPar is against
> this spirit
>
>
>
> SImon
>
>
>
> *From:* ghc-devs [mailto:ghc-devs-bounces at haskell.org] *On Behalf Of *Alan
> & Kim Zimmerman
> *Sent:* 02 December 2014 20:19
> *To:* ghc-devs at haskell.org
> *Subject:* API Annotations and HsSCC / HsTickPragma
>
>
>
> I am in the process of working the shiny new API annotations through into
> a practical example in ghc-exactprint [1], but I have hit a snag.
>
> A SCC annotation appears in the source as
>
>   {-# SCC "name" #-} <expression>
>
>
>
> and if enabled via -prof results in the expression being wrapped in
>
>   HsSCC FastString (LHsExpr id)
>
>
> BUT, if not enabled, it appears as
>
>   HsPar (LHsExpr id)
>
> From the parser/annotation point of view, the appropriate annotations are
> generated, and can be used to distinguish the two cases. The problem is
> that the annotations only capture the SrcSpan of the thing being annotated,
> so in the HsPar case the contents of the FastString is lost.
>
>
>
> A similar situation exists for HsTickPragma,
>
>   HsTickPragma                        -- A pragma introduced tick
>      (FastString,(Int,Int),(Int,Int))   -- external span for this tick
>      (LHsExpr id)
>
> which also degrades to HsPar
>
>
>
> I see a number of possible solutions
>
>   1. Add the missing information to HsPar in a Maybe
>
>
>      HsPar (Maybe (FastString,(Int,Int),(Int,Int))) (LHsExpr id)
>
>
>
>   2. Modify HsSCC / HsTickPragma to have a Bool indicating whether they
> are active or not.
>
>
>
>   3. Introduce an additional annotation type to carry the missing
> information.
>
>   I welcome advice on the best way forward.
>
>
>
>   Alan
>
>
>
> [1] https://github.com/alanz/ghc-exactprint/tree/wip
>
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