如何实际使用 package 的已安装版本，或者找出为什么它不可能？

Question

Cabal-newstyle 有一个习惯，偶尔会无缘无故地从头开始安装所有依赖项。 对于使用lens和其他通用包的项目，这可能需要 ½ 小时以上，这很烦人，尤其是当目的只是快速将一个小更改编译为仅在一个月前构建良好的可执行文件时。

Cabal 确实提供了一个选项来指定一个约束，比如应该使用已经在全球范围内安装的lens版本。 但是，每当我尝试时，我都会得到

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

尽管有很多镜头版本可以在

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

那么为什么不使用这些呢？ 如何找出 Cabal 甚至认为“已安装的版本”，以及如何防止已安装的版本不再可用？

Answer 1

如评论中所述，使用cabal freeze创建“冻结文件”是避免“更新流失”的一种方法：

cabal freeze 写出一个冻结文件，该文件记录解算器在当前索引和标志下选择的所有版本和标志。

如果您以前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，并希望在不考虑该日期之后发布的项目依赖项的任何版本的情况下生成冻结文件。