I added this to the PR! Hopefully it is what you are looking for.
________________________________
From: Bryan Richter
Sent: Thursday, February 25, 2021 10:44 PM
To: Edward Z Yang
Cc: cabal-devel@haskell.org; ekmett@gmail.com
Subject: Re: [RFC] Qualified module renamings
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 mailto:ezyang@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 mailto:b@chreekat.net>
Sent: Thursday, February 25, 2021 10:06 AM
To: Edward Z Yang
Cc: cabal-devel@haskell.orgmailto:cabal-devel@haskell.org; ekmett@gmail.commailto:ekmett@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 mailto:ezyang@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
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