Thanks for the context! By intent, I meant an example like "Here's what it would like now, and here's what it would look like with this implemented. For a much longer example with rationale, see [Kmett's comment]"
Den tors 25 feb. 2021 23:01Edward Z Yang <ezy...@mit.edu> skrev: > > It took me about five minutes to arrive at the guess that this is about > the syntax in Cabal files for using backpack - is that right? > > > Oops yes, sorry for omitting this context. > > > > What is the intent of what got implemented, anyway? Are there example > use cases? > > > I'm not exactly sure what you mean by intent. But a common pattern in > Backpack is to instantiate a library multiple time with different > requirements, and if you want them all in scope you have to rename them. > Right now, this has to be done one-by-one for each provided module, which > can be a bit annoying. For example, let say you parametrized parsec by > string type, and you wanted a bytestring version and a text version, it > would be convenient to be able to unconditionally rename every > Text.Parsec.* module to Text.Parsec.*.ByteString (for example) > > > Edward Kmett described a concrete motivating use case at > https://github.com/haskell/cabal/issues/7290#issuecomment-783540208 > although his use case is a little difficult to understand. > > > Edward > > > ------------------------------ > *From:* Bryan Richter <b...@chreekat.net> > *Sent:* Thursday, February 25, 2021 10:06 AM > *To:* Edward Z Yang > *Cc:* cabal-devel@haskell.org; ekm...@gmail.com > *Subject:* Re: [RFC] Qualified module renamings > > It took me about five minutes to arrive at the guess that this is about > the syntax in Cabal files for using backpack - is that right? > > What is the intent of what got implemented, anyway? Are there example use > cases? > > Den tors 25 feb. 2021 18:14Edward Z Yang <ezy...@mit.edu> skrev: > >> Today, using the 'mixins' field you can rename modules that come from >> other packages by manually expressing a renaming one-by-one. In some >> Backpack use cases, you may have a lot of modules that you would like to >> mechanically rename into some subnamespace; today, you have manually list >> each renaming one by one. >> >> >> https://github.com/haskell/cabal/pull/7303 contains an implementation of >> one possible way to extend mixin syntax to support qualified renaming; the >> implementation is very simple. The syntax here is based off of Richard >> Eisenberg's local modules proposal ( >> https://github.com/ghc-proposals/ghc-proposals/pull/283) which supports >> the qualified keyword before module exports/imports which has the same >> effect (bring the module into scope under a sub-module namespace). However, >> the PR isn't really meant to be an end all to the discussion: it's just to >> show that it's pretty simple to implement this functionality. >> >> >> There are two primary axes which I am looking for feedback: >> >> >> * Expressivity. The current PoC implementation only permits >> unconditionally prefix-ing all modules that would have been brought into >> scope by the mixin; e.g., transforming module A to Prefix.A. Edward Kmett >> has expressed that in some cases, he would like it if you could implement >> the import as a suffix. One could also imagine allowing arbitrary string >> transformations. Opinions on where to draw the line for expressivity are >> solicited. >> >> >> * Syntax. The current syntax is "pkgname qualified Prefix" as it is >> symmetric with "pkgname hiding (A, B)" and it was simple to implement. But >> I am not particularly attached to this syntax, and am open to other >> suggestions. If we permit suffixing, a wildcard based syntax like "pkgname >> (* as *.Suffix)" may be preferable (though modestly more complex to specify >> and implement; for example, is the glob recursive over dots?). Edward Kmett >> has offered some other possibilities at >> https://github.com/haskell/cabal/issues/7290#issue-812744575 >> >> >> Thanks Oleg for reminding me to send this RFC to this mailing list. >> >> >> Cheers, >> >> Edward >> _______________________________________________ >> cabal-devel mailing list >> cabal-devel@haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/cabal-devel >> >
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