如何实际使用已安装的软件包版本，或找出不可能的原因？

Question

Cabal-newstyle 习惯于偶尔无缘无故地从头开始安装所有依赖项。对于使用其他常见软件包的项目lens，这可能需要长达 \xc2\xbd 小时，这很烦人，尤其是当目的只是快速编译对一个月前构建良好的可执行文件的小更改时。

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Cabal 确实提供了指定作为约束的选项，例如lens应该使用已经全局安装的版本。然而，每当我尝试时，我就会得到

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$ cabal new-run --constraint \'lens installed\'\nResolving dependencies...\ncabal: Could not resolve dependencies:\n[__0] trying: my-example-project-0.1.0.0 (user goal)\n[__1] next goal: lens (dependency of dat103-Justus-lectures)\n[__1] rejecting: lens-5.2, lens-5.1.1, lens-5.1, lens-5.0.1, lens-5,\nlens-4.19.2, lens-4.19.1, lens-4.19, lens-4.18.1, lens-4.18, lens-4.17.1,\nlens-4.17, lens-4.16.1, lens-4.16, lens-4.15.4, lens-4.15.3, lens-4.15.2,\nlens-4.15.1, lens-4.15, lens-4.14, lens-4.13.2.1, lens-4.13.2, lens-4.13.1,\n...\nlens-0.4, lens-0.3, lens-0.2, lens-0.1 (constraint from command line flag\nrequires installed instance)\n[__1] fail (backjumping, conflict set: my-example-project, lens)\nAfter searching the rest of the dependency tree exhaustively\n

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尽管事实上可以在以下位置找到大量镜头版本：

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$ ls -d ~/.cabal/store/ghc-8.6.4/lens*\n/home/js/.cabal/store/ghc-8.6.4/lens-4.19.2-1392f624ae052009275d5902a574e3f1804a66406a28222f0221cc2211da4f78\n/home/js/.cabal/store/ghc-8.6.4/lens-4.19.2-4854f213f63b67c0e1ea871303590def7a79bce4cc742f658e41dc4c76b56022\n/home/js/.cabal/store/ghc-8.6.4/lens-4.19.2-4caff99c8da3869df1dcfb67181d1be33d98316c3f458f93caedae05279c15e5\n/home/js/.cabal/store/ghc-8.6.4/lens-4.19.2-76531bf275303c15a9e6ae5160ca29f8fb15782d230b2576724103d67a4020d9\n/home/js/.cabal/store/ghc-8.6.4/lens-4.19.2-76a66d74e9cb85066009b345289db97d78bcf441beb12e6b762eb4d197000f19\n/home/js/.cabal/store/ghc-8.6.4/lens-4.19.2-813faecc9a5593fc14ef1677e82b9b384b66de9fff7843635cd7f4e5f4993d16\n/home/js/.cabal/store/ghc-8.6.4/lens-4.19.2-8c724c6d1fcc65ea38dc13fde30d86c4856151b235ef63628b23cee76d2feac5\n/home/js/.cabal/store/ghc-8.6.4/lens-4.19.2-90ec9224b09b828c948a6a1a310a085edc28f92a269bb839a2ac575dc60d579a\n/home/js/.cabal/store/ghc-8.6.4/lens-4.19.2-a157954a3d903fbf6444c054f9b704cab3fc23b36debc16fdc9ee8c0f5c0d5bd\n/home/js/.cabal/store/ghc-8.6.4/lens-4.19.2-c3bf086b6e081a91c7085b3c645652de0c4097edc4c88069203c5f741b14b97f\n/home/js/.cabal/store/ghc-8.6.4/lens-4.19.2-d5dca17dd7dddf6f324313f363ef74ff15d5717011dd750f415ce39942fc703b\n/home/js/.cabal/store/ghc-8.6.4/lens-4.19.2-e4a1a8c9f3867cc6e6dc4dc059cc7fc0e4912d9aa4ce5476f63cdea9d3c8535e\n/home/js/.cabal/store/ghc-8.6.4/lens-5.0.1-0956efd5ac393e6034c76c4dcc30a40ad1b12ad2b7a708e0f980648bac6287f3\n/home/js/.cabal/store/ghc-8.6.4/lens-5.0.1-bfba2b0d59ebb884a2f32a960694c0e44af4fd29a21d8021ebdc42ce994fd055\n/home/js/.cabal/store/ghc-8.6.4/lens-5.0.1-ce6c5a7496a674d60f93fae50caa4eb58acc26c6536937cce0f5243238cd6215\n/home/js/.cabal/store/ghc-8.6.4/lens-5.0.1-d951ac24c700a8ae7f708a46a084e8fb4b0b008af29e5527f4d8986399248c62\n/home/js/.cabal/store/ghc-8.6.4/lens-5.1-08384468083ebbc9d39619bfb59a32b97333acbf703c24bcf3b0bd22357a0115\n...\n

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那么为什么不使用这些呢？如何找出 Cabal 甚至认为 \xe2\x80\x9c 安装版本 \xe2\x80\x9d 的内容，以及如何防止已安装版本停止可用？

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Answer 1

正如评论中提到的，使用创建“冻结文件”cabal freeze是避免“更新搅动”的一种方法：

cabal freeze 写出一个冻结文件，其中记录了求解器在当前索引和标志下选取的所有版本和标志。

如果您当时没有创建这样的文件怎么办，现在如何避免更新流失？cabal freeze似乎支持以下命令行选项：

 --index-state=STATE            Use source package index state as it existed
                                at a previous time. Accepts unix-timestamps
                                (e.g. '@1474732068'), ISO8601 UTC timestamps
                                (e.g. '2016-09-24T17:47:48Z'), or 'HEAD'
                                (default: 'HEAD').

因此，您可以提供大约一个月前的日期，并希望生成冻结文件时无需考虑该日期之后发布的项目依赖项的任何版本。