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<p class="MsoNormal">That’s a tremendously helpful summary, thank you Iavor. And Michail’s summary was also very helpful.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Most of this is doubtless well-known to habitual cabal users, but it might be useful to explain the user model, in a way that covers these points, somewhere close to the Cabal home page.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Simon<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> Iavor Diatchki <iavor.diatchki@gmail.com>
<br>
<b>Sent:</b> 15 April 2019 23:39<br>
<b>To:</b> Simon Peyton Jones <simonpj@microsoft.com><br>
<b>Cc:</b> Brandon Allbery <allbery.b@gmail.com>; ghc-devs@haskell.org<br>
<b>Subject:</b> Re: Cabal woes<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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Hello,<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
in case it is useful, here is how I think about what's happening with cabal. At present, `cabal-install` supports two different modes of operation: the old style (aka `v1`) and the new style (aka `v2`) and---at least for me---the two require a slightly different
mental model of what is going on.<o:p></o:p></p>
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In the old model, there is a user package database, and users would use "cabal install" to install libraries their manually (e.g., using `cabal-install`). Later, when building various artifacts cabal would prefer using the packages installed in the user's
database. This database supported having multiple versions of a package, but NOT multiple builds of the same version of a package (e.g., against different dependencies). As a result, builds would sometimes fail, because the dependencies of packages would
clash with each other (the unfortunate "cabal hell").<o:p></o:p></p>
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<o:p> </o:p></p>
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With the new model, there is still a "user" level location where libraries are installed, but it is not really directly manipulated by the user---rather it acts as more of a "cache" containing all versions of all libraries every built and---crucially---it supports
having multiple builds of the same version of a package against different dependencies. When users build an artifact using the new style (aka "v2"), cabal automatically checks if a suitable version of the library is already built in its cache, and if not
it adds it there.<o:p></o:p></p>
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<o:p> </o:p></p>
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The important difference between the two (at least in my mind) is that with the new style, you never just install a library on its own. Rather, you install it as a part of a project, so Cabal can compute which version it should install so that you get a version
compatible with the rest of the project. Since in this model you never really install libraries directly, the `install` command defaults to installing executables, which is what the first error is trying to say.<o:p></o:p></p>
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<o:p> </o:p></p>
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So, if you want to try out `hspec` with the `v2` style of Cabal, you'd just add it as a dependencies in the `cabal` file of your project, and then use `cabal v2-build` to build the project, without having to install it manually first.<o:p></o:p></p>
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<o:p> </o:p></p>
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I hope this helps,<o:p></o:p></p>
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-Iavor<o:p></o:p></p>
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<o:p> </o:p></p>
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<o:p> </o:p></p>
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<o:p> </o:p></p>
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On Mon, Apr 15, 2019 at 3:01 PM Simon Peyton Jones via ghc-devs <<a href="mailto:ghc-devs@haskell.org">ghc-devs@haskell.org</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Thanks. But alas I have no clue about whether I want a v1-install or a v2-install, nor how to achieve them if I knew what they were. I just want to install ‘hspec’ so that I can
use it when compiling a program. How would I do that?<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">The instructions here
<a href="https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwiki.haskell.org%2FCabal-Install&data=02%7C01%7Csimonpj%40microsoft.com%7Ce83dcc16b5a542b8a9b008d6c1f32668%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636909647403187221&sdata=l5Ft4P6VmYcKSMACJWRjQs%2BxeA6LTAj74qTmCakhbPU%3D&reserved=0" target="_blank">
https://wiki.haskell.org/Cabal-Install</a> just say “cabal install hspec” which is what I tried. Those instructions are pointed to from here
<a href="https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwiki.haskell.org%2FCabal%2FHow_to_install_a_Cabal_package&data=02%7C01%7Csimonpj%40microsoft.com%7Ce83dcc16b5a542b8a9b008d6c1f32668%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636909647403197230&sdata=1uversp%2FCN3h2q0VVmp97WsdnuiHyAl%2B0UfrH0r%2FFek%3D&reserved=0" target="_blank">
https://wiki.haskell.org/Cabal/How_to_install_a_Cabal_package</a>, which in turn are pointed to from the main Cabal home page
<a href="https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.haskell.org%2Fcabal%2F&data=02%7C01%7Csimonpj%40microsoft.com%7Ce83dcc16b5a542b8a9b008d6c1f32668%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636909647403207238&sdata=1jW3cLEe144ky%2F40Lh4ObiTQQT22TSyhEB03V1Zi09E%3D&reserved=0" target="_blank">
https://www.haskell.org/cabal/</a>.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">I must be missing something.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Simon<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"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> Brandon Allbery <<a href="mailto:allbery.b@gmail.com" target="_blank">allbery.b@gmail.com</a>>
<br>
<b>Sent:</b> 15 April 2019 22:54<br>
<b>To:</b> Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com" target="_blank">simonpj@microsoft.com</a>><br>
<b>Cc:</b> <a href="mailto:ghc-devs@haskell.org" target="_blank">ghc-devs@haskell.org</a><br>
<b>Subject:</b> Re: Cabal woes</span><o:p></o:p></p>
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<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;margin-bottom:6.0pt">I think you wanted v1-install to install a library into the user package database, since your cabal is 3.x and the v2-* commands are now the default (that is, you did what used to be cabal
new-install or cabal v2-install). <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt">On Mon, Apr 15, 2019 at 5:47 PM Simon Peyton Jones via ghc-devs <<a href="mailto:ghc-devs@haskell.org" target="_blank">ghc-devs@haskell.org</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">I’m trying to install ‘hspec’ on my WSL (Windows subsystem for Linux) system.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">But I fail; see below.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">For some reason cabal complains about installing a library. (That seems peculiar – isn’t that what cabal is for?) But it helpfully suggests adding –lib.
<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Alas, cabal then crashes outright, which should never happen.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">So I’m stuck. What should I do?<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Thanks<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Simon<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="gmail-m-4734454596412187589gmail-m-1866220934865054569code">simonpj@MSRC-9870733:~$ cabal --version<o:p></o:p></p>
<p class="gmail-m-4734454596412187589gmail-m-1866220934865054569code">cabal-install version 3.0.0.0<o:p></o:p></p>
<p class="gmail-m-4734454596412187589gmail-m-1866220934865054569code">compiled using version 3.0.0.0 of the Cabal library
<o:p></o:p></p>
<p class="gmail-m-4734454596412187589gmail-m-1866220934865054569code">simonpj@MSRC-9870733:~$ cabal install hspec<o:p></o:p></p>
<p class="gmail-m-4734454596412187589gmail-m-1866220934865054569code">Resolving dependencies...<o:p></o:p></p>
<p class="gmail-m-4734454596412187589gmail-m-1866220934865054569code">Up to date<o:p></o:p></p>
<p class="gmail-m-4734454596412187589gmail-m-1866220934865054569code">Warning: You asked to install executables, but there are no executables in<o:p></o:p></p>
<p class="gmail-m-4734454596412187589gmail-m-1866220934865054569code">target: hspec. Perhaps you want to use --lib to install libraries instead.<o:p></o:p></p>
<p class="gmail-m-4734454596412187589gmail-m-1866220934865054569code">simonpj@MSRC-9870733:~$ cabal install --lib hspec<o:p></o:p></p>
<p class="gmail-m-4734454596412187589gmail-m-1866220934865054569code">Resolving dependencies...<o:p></o:p></p>
<p class="gmail-m-4734454596412187589gmail-m-1866220934865054569code">Up to date<o:p></o:p></p>
<p class="gmail-m-4734454596412187589gmail-m-1866220934865054569code">Distribution/Simple/GHC.hs:1959:5-56: Irrefutable pattern failed for pattern Just ghcPkgProg<o:p></o:p></p>
<p class="gmail-m-4734454596412187589gmail-m-1866220934865054569code"> <o:p></o:p></p>
<p class="gmail-m-4734454596412187589gmail-m-1866220934865054569code">simonpj@MSRC-9870733:~$ which ghc<o:p></o:p></p>
<p class="gmail-m-4734454596412187589gmail-m-1866220934865054569code">/opt/ghc/bin/ghc<o:p></o:p></p>
<p class="gmail-m-4734454596412187589gmail-m-1866220934865054569code">simonpj@MSRC-9870733:~$ which ghc-pkg<o:p></o:p></p>
<p class="gmail-m-4734454596412187589gmail-m-1866220934865054569code">/opt/ghc/bin/ghc-pkg<o:p></o:p></p>
<p class="gmail-m-4734454596412187589gmail-m-1866220934865054569code">simonpj@MSRC-9870733:~$<o:p></o:p></p>
<p class="gmail-m-4734454596412187589gmail-m-1866220934865054569code"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">_______________________________________________<br>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">brandon s allbery kf8nh<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><a href="mailto:allbery.b@gmail.com" target="_blank">allbery.b@gmail.com</a><o:p></o:p></p>
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<p class="MsoNormal">_______________________________________________<br>
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