[ghc-steering-committee] Proposal #270: Support pun-free code. Recommendation: accept

[Simon Peyton Jones]

>
>    {-# LANGUAGE ExplicitNamespaces #-}
>    module N where
>      import M (T(type MkT)) -- NB "type" import of a data constructor
>      v = MkT                -- usage at term level
>

Crumbs.  I had not realised the proposal is to allow *nested* uses of
'type' in import lists, as you show above.

In general, I don't feel the extensions to ExplicitNamespaces included
> in the proposal are very clearly specified.
>

Actually isn't the proposal pretty clear on this, namely the first bullet
of proposed change spec
<https://github.com/hithroc/ghc-proposals/blob/master/proposals/0000-support-pun-free-code.md#2-proposed-change-specification>.
It only covers
import M *type *
import M *data *as MD
where I have emboldened the new bits.  Nothing about the contents of import
lists.   Why did you think your example is covered by the proposal?

Simon


On Mon, 12 Dec 2022 at 09:15, Adam Gundry <adam at well-typed.com> wrote:

> Actually, reading https://gitlab.haskell.org/ghc/ghc/-/issues/22581 I
> realised I'm unclear how the proposed extensions to ExplicitNamespaces
> are supposed to work. The existing situation is apparently that for a
> (non-punned) data constructor, it is possible to use either a pattern or
> type qualifier in an import list (presumably because DataKinds means the
> constructor is in scope at both the term and type levels), and the
> imported constructor is then usable in both contexts.
>
> For example, the following is accepted at present:
>
>     module M where
>       data T = MkT
>
>    {-# LANGUAGE ExplicitNamespaces #-}
>    module N where
>      import M (T(type MkT)) -- NB "type" import of a data constructor
>      v = MkT                -- usage at term level
>
> The present proposal says "With type specified in the import, only
> identifiers belonging to the type namespace will be brought into the
> scope." I'm not exactly sure how to interpret this, does it mean the
> following alternative will be accepted or rejected?
>
>    module N where
>      import M type (MkT)
>      v = MkT
>
> I'm worried we will end up with a situation where ExplicitNamespaces
> does subtly different things depending on the position of the keyword.
>
> In general, I don't feel the extensions to ExplicitNamespaces included
> in the proposal are very clearly specified. Given the discussion about
> exactly which parts belong to ExplicitNamespaces/PatternSynonyms versus
> separate extensions, perhaps we should accept the parts relating to
> -Wpuns/-Wpun-bindings, but ask for the ExplicitNamespaces changes to be
> proposed separately?
>
> Cheers,
>
> Adam
>
>
> On 09/12/2022 11:11, Adam Gundry wrote:
> > I'm broadly in favour of accepting the proposal. I realise the history
> > is complex here, so I don't think we should ask anyone to rewrite things
> > further, though in general it would be nicer to have separate proposals
> > for -Wpuns/-Wpun-bindings (which is unambiguously fine) and for the
> > changes to imports (which as Joachim points out raise issues).
> >
> > I'm a bit concerned that the proposal does not motivate or specify
> > -Wpattern-namespace-qualified very well.
> >
> >
> > On 08/12/2022 08:33, Joachim Breitner wrote:
> >> ...
> >>
> >> This gives us (at least) these options:
> >>
> >> 1. Leave ExplicitNamespaces alone, add ExplicitNamespaces to GHC2023,
> >>     introduce one or two new extensions for the newer changes.
> >> 2. Extend ExplicitNamespaces, and don’t add it already to GHC2023,
> >>     disregarding issue #551.
> >> 3. Add ExplicitNamespaces to GHC2023, and still add it to GHC2023,
> >>     arguing that GHC20xx allows more liberal (backward-compatibile)
> >>     changes than, say, Haskell2010 would allow.
> >>
> >> Certainly 1 is the least bold move. I am not sure what the best way
> >> forwards is, and welcome other opinions.
> >
> > I would prefer a variant of 1: allow "data" as a keyword in import lists
> > under ExplicitNamespaces, but make the other changes under other
> > extensions.
> >
> > As I've said previously, I have a general preference for multiple small,
> > orthogonal extensions rather than changing existing extensions to add
> > unrelated features that happen to be in similar territory. I realise
> > this is controversial, of course.
> >
> > 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
>
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