阴谋集团:无法安装软件包的特定版本(带有消息“只能使用已安装的实例”)

Question

我一直在尝试安装一些软件包：

PS C:\Users\Neo> cabal install template-haskell-2.15.0.0
Resolving dependencies...
cabal-3.6.2.0.exe: Could not resolve dependencies:
[__0] next goal: template-haskell (user goal)
[__0] rejecting: template-haskell-2.19.0.0, template-haskell-2.18.0.0,
template-haskell-2.17.0.0, template-haskell-2.16.0.0 (constraint from user
target requires ==2.15.0.0)
[__0] rejecting: template-haskell-2.15.0.0 (only already installed instances
can be used)
[__0] rejecting: template-haskell-2.14.0.0, template-haskell-2.13.0.0,
template-haskell-2.12.0.0, template-haskell-2.11.1.0,
template-haskell-2.11.0.0, template-haskell-2.10.0.0,
template-haskell-2.9.0.0, template-haskell-2.8.0.0, template-haskell-2.7.0.0,
template-haskell-2.6.0.0, template-haskell-2.5.0.0, template-haskell-2.4.0.1,
template-haskell-2.4.0.0, template-haskell-2.3.0.1, template-haskell-2.3.0.0,
template-haskell-2.2.0.0 (constraint from user target requires ==2.15.0.0)
[__0] fail (backjumping, conflict set: template-haskell)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: template-haskell

“只能使用已安装的实例”的消息实际上意味着什么？

Answer 1

与大多数软件包不同，template-haskell不能与GHC单独安装、升级或降级。如果你真的需要降级到template-haskell-2.15.0.0，那么你需要降级到8.8.4。https://gitlab.haskell.org/ghc/ghc/-/wikis/commentary/libraries/version-history有一个图表，可以让您检查哪个GHC版本对应于哪个库版本。