On 30/11/2022 20:37, Joachim Breitner wrote:
Hi,
Am Mittwoch, dem 30.11.2022 um 19:28 +0000 schrieb Adam Gundry:
What do you think?
my initial feeling about `language … where …` is that it is a modifer of sorts, however * with a syntax that may not scale well (hard to target anything but a whole set of declarations) * looks like it could support any kind of language extension, when it probably doesn’t make sense for all of them. so may not gain much over implementing (parts) of the modifier syntax.
Well, I find it hard to imagine really needing to enable an extension for anything smaller than a declaration group. On the other hand, I not infrequently want to enable particular extensions only for a few specific definitions (AllowAmbiguousTypes comes to mind). As I understand it, modifiers need to be type-checked before they have meaning assigned. This presumably means they cannot change the behaviour of the parser, whereas an explicit "language ... where ..." construct could do so. And I don't think modifiers can scope over a declaration group, only a single declaration? I agree that we wouldn't necessarily support *all* language extensions locally, but I think the list for which this fundamentally does not make sense is relatively short (the main ones that come to mind are import-related extensions such as ExplicitNamespaces). Others might be hard to specify/implement (e.g. Safe Haskell seems tricky) but we could simply not support them locally.
...
Or we revive local modules, and use that as a then natural way of scoping language pragmas…
There's clearly a relationship to local modules, but that seems like more complexity than we need for the problem at hand. I don't see why we shouldn't add "language ... where ..." now, then potentially later support local (or top-level!) modules with language Blah where module M where ... After all, {-# LANGUAGE #-} pragmas violate the principle that pragmas shouldn't change semantics. ;-) Cheers, Adam -- Adam Gundry, Haskell Consultant Well-Typed LLP, https://www.well-typed.com/ Registered in England & Wales, OC335890 27 Old Gloucester Street, London WC1N 3AX, England