[ghc-steering-committee] #216: Qualified Do, recommendation: accept

[Richard Eisenberg]

Sorry to say that I do not like this proposal in its current form, as I elaborate on GitHub: 
https://github.com/ghc-proposals/ghc-proposals/pull/216#issuecomment-598191928 <https://github.com/ghc-proposals/ghc-proposals/pull/216#issuecomment-598191928>

Richard

> On Mar 12, 2020, at 8:16 AM, Vitaly Bragilevsky <bravit111 at gmail.com> wrote:
> 
> I support this proposal in the current form. I am especially happy
> with the idea that the 'M.do' qualifier does not affect the 'return'
> function. It's just fine to affect implicit constructions exclusively.
> 
> Vitaly
> 
> ср, 11 мар. 2020 г. в 21:37, Joachim Breitner <mail at joachim-breitner.de>:
>> 
>> Dear Committe,
>> 
>> Proposal:
>> https://github.com/tweag/ghc-proposals/blob/local-do/proposals/0000-local-do.rst
>> 
>> Discussion: https://github.com/ghc-proposals/ghc-proposals/pull/216
>> 
>> 
>> Summary:
>> 
>> last June we looked at a “local do” proposal that would be a more
>> targetted variant of RebindableSyntax, just for “do”. After an initial
>> acceptance recommendation from me we had some good discussion, and
>> eventually sent it back for revision. One problem, among many, was that
>> the story around type inference wasn’t as good as with normal “do”.
>> 
>> All attempts to give meaning to “do” via a local value (e.g. a record,
>> or type classes etc.) were unsatisfying in one way or another. (Often
>> something about mumble impredicative types mumble.)
>> 
>> Anyways, Facundo Domínguez joined the author team and the discussion
>> steered towards a less ambitious, but simpler and cleaner proposal:
>> Qualified do. The idea is simply to treat a module qualifier on the do
>> keyword as a qualified on all the implicit :
>> 
>>  M.do { x <- u; stmts }  =  (M.>>=) u (\x -> M.do { stmts })
>> 
>> and similar for the other monad desugaring rules.
>> 
>> 
>> Recommendation:
>> 
>> This seems simple, convenient and easy to understand, so I recommend
>> accepting the proposal.
>> 
>> 
>> Comment:
>> 
>> There was some discussion if
>> 
>>  M.do { return x } = M.return x
>> 
>> but it seems that changing the meaning of more than `do` it self may be
>> more confusing than helpful, so the proposal as it stands does _not_
>> propose it (see Alternatives). I wish we had a better story here, and
>> welcome ideas from the committee.
>> 
>> 
>> WDYAT?
>> 
>> 
>> Cheers,
>> Joachim
>> 
>> 
>> --
>> Joachim Breitner
>>  mail at joachim-breitner.de
>>  http://www.joachim-breitner.de/
>> 
>> 
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