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<p class="MsoNormal" style="margin-left:36.0pt">If I'm reading this correctly, the proposal then would be to have cabal automatically hide packages (as oppose to unregister them) to arrive at a world where all exposed packages are consistent. Extrapolating
for the case you mention above<o:p></o:p></p>
<p class="MsoNormal" style="margin-left:36.0pt"><o:p> </o:p></p>
<p class="MsoNormal" style="margin-left:36.0pt">* if I installed P and then Q, I'd end up with containers-3.1 and Q exposed, and containers-2.9 and P hidden<o:p></o:p></p>
<p class="MsoNormal" style="margin-left:36.0pt">* if I installed Q and then P, I'd end up with containers-2.9 and P exposed, and containers-3.1 and Q hidden<o:p></o:p></p>
<p class="MsoNormal" style="margin-left:36.0pt"><o:p> </o:p></p>
<p class="MsoNormal" style="margin-left:36.0pt"><b>But either way, all four package/versions would be available, and cabal would be able to select an appropriate subset of packages when configuring</b>. Does that sound about right?<o:p></o:p></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Correct, esp the bit that I have emboldened. For the former bullets, perhaps Cabal might ask you want you want to do.
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">That still leaves open questions. What happens if you say “ghc –package P –package Q Foo.hs”? Should GHC complain that you’ve chosen an inconsistent set? Perhaps!<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">(NB if P and Q use containers only internally, and do not expose any types from containers, then arguably it’s ok; but I’d argue for jumping that bridge when we come
to it.)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif"> Michael Snoyman [mailto:michael@snoyman.com]
<br>
<b>Sent:</b> 23 March 2015 09:58<br>
<b>To:</b> Simon Peyton Jones; cabal-devel@haskell.org<br>
<b>Cc:</b> haskell-platform@projects.haskell.org; haskell-infrastructure@community.galois.com; Haskell Libraries; ghc-devs@haskell.org<br>
<b>Subject:</b> Re: Cabal and simultaneous installations of the same package<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<o:p> </o:p></p>
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On Mon, Mar 23, 2015 at 11:53 AM Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com">simonpj@microsoft.com</a>> wrote:<o:p></o:p></p>
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It's already a huge source of confusion for people using GHCi what they get messages about "ByteString is not ByteString."<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Calibri",sans-serif">Reading your blog post [1] it seems that we are addressing different questions:</span><o:p></o:p></p>
<p><span style="font-family:Symbol">·</span><span style="font-size:7.0pt">
</span><span style="font-family:"Calibri",sans-serif">My proposal is only that the act of *<b>installing</b>* a package does not break existing installed package, and is not rejected because it risks doing so.</span><o:p></o:p></p>
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<p class="MsoNormal">Thank you for the clarification, I had misread that. On that front: I agree. <o:p></o:p></p>
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<p><span style="font-family:Symbol">·</span><span style="font-size:7.0pt">
</span><span style="font-family:"Calibri",sans-serif">You agree that the confusing behaviour you describe can’t happen with Cabal. In any one build, Cabal can ensure that only one version of each package is used in the build, so such a message could never
show up.</span><o:p></o:p></p>
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<p class="MsoNormal">I've seen people discussing exactly such a change to Cabal's behavior, so I mistakenly took your comments to be heading in that direction. While I think there *might* be some future where we could expose that functionality, it could be
incredibly confusing. I'd feel much better starting off with simply the act of installing. <o:p></o:p></p>
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<p><span style="font-family:Symbol">·</span><span style="font-size:7.0pt">
</span><span style="font-family:"Calibri",sans-serif">What you want is for the confusing behaviour to be true of GHCi too. Well that’s simple enough: ensure that the set of
<b>exposed</b> packages (ie the ones you say ‘import M’ for), is consistent in the same way. The point is that I may need to install a bunch of packages to build a program. If I’m using Cabal, none of those newly installed packages need be exposed; I simply
need them there so I can compile my program (using Cabal). But at the moment I can’t do that.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Calibri",sans-serif"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Calibri",sans-serif">That leaves open the following question. Suppose</span><o:p></o:p></p>
<p><span style="font-family:Symbol">·</span><span style="font-size:7.0pt">
</span><span style="font-family:"Calibri",sans-serif">I want to install <b>and expose</b> package P and Q</span><o:p></o:p></p>
<p><span style="font-family:Symbol">·</span><span style="font-size:7.0pt">
</span><span style="font-family:"Calibri",sans-serif">But alas, P depends on containers 2.9 and Q depends on containers 3.1</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Calibri",sans-serif">Now I’m stuck. But there is a good reason for being stuck, and one that is explicable.
</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Calibri",sans-serif"> </span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">If I'm reading this correctly, the proposal then would be to have cabal automatically hide packages (as oppose to unregister them) to arrive at a world where all exposed packages are consistent. Extrapolating for the case you mention above<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">* if I installed P and then Q, I'd end up with containers-3.1 and Q exposed, and containers-2.9 and P hidden<o:p></o:p></p>
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<p class="MsoNormal">* if I installed Q and then P, I'd end up with containers-2.9 and P exposed, and containers-3.1 and Q hidden<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">But either way, all four package/versions would be available, and cabal would be able to select an appropriate subset of packages when configuring. Does that sound about right?<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Michael<o:p></o:p></p>
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