Exact-print info in the the HsSyn syntax tree

[Simon Peyton Jones]

Thanks Jakob -- but would it be possible to comment *on the ticket*, not
here?   (I should have said that more clearly.)

Simon

On Thu, 13 Jul 2023 at 13:31, Jakob Brünker <jakob.bruenker at gmail.com>
wrote:

> Having written the MonadicBang plugin somewhat recently, where I don't
> care about the exact-print annotations, I do have some snippets in my code
> that look like this:
>
>     AsPat xa name tok pat -> do
>       tellName name
>       AsPat xa name tok <$> traverse (liftMaybeT . evacPats) pat
>
> where the `tok` variable only exists to pass along the exact-print
> annotations unchanged. So in that context, I would have a slight preference
> for the exact-print annotations being hidden away in the extension points.
> However, I think this also illustrates that the cost to clients is quite
> manageable. Adding this one variable doesn't make the code unreadable - of
> course, that's assuming exact-print annotations remain special and not just
> the first in a long list of properties to eventually be added to each node.
>
> Of course, the cost *is* multiplied by the number of pattern matches on
> AST nodes you have, which could be a lot given the amount of constructors
> the types have. In my case, it was very few, because I was able to handle
> the vast majority of constructors generically via the Data.Data instance.
>
> Jakob
>
> On Thu, Jul 13, 2023 at 1:23 PM Simon Peyton Jones <
> simon.peytonjones at gmail.com> wrote:
>
>> Dear GHC developers
>>
>> Could you please look at #23447 Where should "tokens" live
>> <https://gitlab.haskell.org/ghc/ghc/-/issues/23447>?
>>
>> In brief, the question is whether we want to have:
>>
>> data HsExpr p = ....
>>    | HsLet (XLet p) (HsLocalBinds p) (LHsExpr p)
>> or
>>
>> data HsExpr p = ....
>>    | HsLet (XLet p) (HsToken "let" p)
>>            (HsLocalBinds p) (HsToken "in" p) (LHsExpr p)
>>
>>
>> In the former, if a client wants HsTokes to track the precise source
>> locations of the "let" and "in" keywords, they'd have to put it in the TTG
>> extension field; in the latter, this information is in *every* syntax
>> tree.
>>
>> At the moment we have some of each, which is not satisfactory. We need to
>> decide a policy and stick to it.  If you use HsSyn, HsExpr, HsPat etc, in
>> any way, you should have an opinion.  Please do express it. At the moment
>> we have only a few voices so we risk deciding without enough evidence and
>> use-cases.
>>
>> Comments with specific use-cases and examples would be particularly
>> helpful.
>>
>> Thanks!
>>
>> Simon
>>
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>>
>
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