[RFC] Qualified module renamings

[Bryan Richter]

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 <ezyang at 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 at chreekat.net>
> *Sent:* Thursday, February 25, 2021 10:06 AM
> *To:* Edward Z Yang
> *Cc:* cabal-devel at haskell.org; ekmett at 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 <ezyang at 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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>>
>
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