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<p>> <span style="color: rgb(33, 33, 33); font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 16px; background-color: rgb(255, 255, 255);">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?</span><br>
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<p>Oops yes, sorry for omitting this context.<br>
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<p>> <span style="color: rgb(33, 33, 33); font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 16px; background-color: rgb(255, 255, 255);">What is the intent of what got implemented, anyway? Are there example use cases?</span><br>
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<p>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)<br>
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<p>Edward Kmett described a concrete motivating use case at <a href="https://github.com/haskell/cabal/issues/7290#issuecomment-783540208">https://github.com/haskell/cabal/issues/7290#issuecomment-783540208</a> although his use case is a little difficult to
understand.<br>
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<p>Edward<br>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" color="#000000" style="font-size:11pt"><b>From:</b> Bryan Richter <b@chreekat.net><br>
<b>Sent:</b> Thursday, February 25, 2021 10:06 AM<br>
<b>To:</b> Edward Z Yang<br>
<b>Cc:</b> cabal-devel@haskell.org; ekmett@gmail.com<br>
<b>Subject:</b> Re: [RFC] Qualified module renamings</font>
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<div dir="auto">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?
<div dir="auto"><br>
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<div dir="auto">What is the intent of what got implemented, anyway? Are there example use cases?</div>
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<div dir="ltr" class="gmail_attr">Den tors 25 feb. 2021 18:14Edward Z Yang <<a href="mailto:ezyang@mit.edu" target="_blank" rel="noreferrer">ezyang@mit.edu</a>> skrev:<br>
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<p>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.<br>
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<p><a href="https://github.com/haskell/cabal/pull/7303" rel="noreferrer noreferrer" target="_blank">https://github.com/haskell/cabal/pull/7303</a> 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 (<a href="https://github.com/ghc-proposals/ghc-proposals/pull/283" rel="noreferrer noreferrer" target="_blank">https://github.com/ghc-proposals/ghc-proposals/pull/283</a>)
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.<br>
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<p>There are two primary axes which I am looking for feedback:<br>
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<p>* 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.<br>
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<p>* 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 <a href="https://github.com/haskell/cabal/issues/7290#issue-812744575" rel="noreferrer noreferrer" target="_blank">https://github.com/haskell/cabal/issues/7290#issue-812744575</a><br>
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<p>Thanks Oleg for reminding me to send this RFC to this mailing list.<br>
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<p>Cheers,<br>
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<p>Edward<br>
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