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
Dear Committe,
Proposal: https://github.com/tweag/ghc-proposals/blob/local-do/proposals/0000-local-do...
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@joachim-breitner.de http://www.joachim-breitner.de/
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