更新 mtl 后找不到模块“Control.Monad.State”

我想用Control.Monad.Except模块，但结果发现我有一个过时的 mtl 包（它导致了导入错误 - 我有一个过时的模块Control.Monad.Error）。所以我做了

sudo cabal install mtl

并且安装了2.2.2版本。但是，现在我安装了两个版本，2.1.2 和 2.2.2，仍然导致导入错误。我按照指示操​​作here https://stackoverflow.com/a/10587933/2266261 and did

sudo ghc-pkg unregister --force mtl-2.1.2

删除旧版本。但现在我得到一个错误：

Could not find module `Control.Monad.State'
It is a member of the hidden package `monads-tf-0.1.0.2'.
Use -v to see a list of the files searched for.

ghc-pkg check outputs

There are problems in package HTTP-4000.2.17:
  dependency "mtl-2.1.2-735d9c92b4f214d454fb5168bb1eb6ee" doesn't exist
There are problems in package fgl-5.5.0.1:
  dependency "mtl-2.1.2-735d9c92b4f214d454fb5168bb1eb6ee" doesn't exist
There are problems in package cgi-3001.1.8.5:
  dependency "mtl-2.1.2-735d9c92b4f214d454fb5168bb1eb6ee" doesn't exist
There are problems in package parsec-3.1.3:
  dependency "mtl-2.1.2-735d9c92b4f214d454fb5168bb1eb6ee" doesn't exist
There are problems in package regex-base-0.93.2:
  dependency "mtl-2.1.2-735d9c92b4f214d454fb5168bb1eb6ee" doesn't exist

The following packages are broken, either because they have a problem
listed above, or because they depend on a broken package.
HTTP-4000.2.17
fgl-5.5.0.1
cgi-3001.1.8.5
parsec-3.1.3
regex-base-0.93.2
network-2.4.1.2
regex-compat-0.95.1
regex-posix-0.95.2

我现在应该怎么做？

需要重新安装所有依赖的包mtl让他们及时了解新版本。 Cabal-install 应该能够自动执行此操作，如果在您的项目文件夹中包含yourproject.cabal文件，你做

$ cabal install --dependencies-only  

正如丹尼尔所说，可能还需要添加--reinstall，即如果 Cabal-install 没有正确捕获依赖项更改，并且需要手动勾选以重建它们。如果它认为重新安装本身可能会破坏其他软件包，--force-reinstalls可能也需要。

避免依赖地狱的另一种方法是使用新的 Nix 风格命令 https://www.haskell.org/cabal/users-guide/nix-local-build-overview.html，重新安装时不会破坏任何东西。