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

[Adam Gundry]

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
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