在堆栈项目中添加MaybeT依赖项时遇到错误 | 在堆栈项目中添加MaybeT依赖项时遇到错误

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问题描述

我正在学习本教程到 MaybeT 变压器提供的变压器包装在。

也就是说，请注意，版本不匹配涉及 base 。如果您实际需要使用 MaybeT 包，则必须将堆栈解析器切换为诸如，它使用 base-4.8 或更低（即GHC 7.10.3或更低）。 base 版本与GHC版本相关联，而其他所有内容都依赖于 base ，所以没有简单的方法将 base 切换到旧版本，因为它可能是有可能的，因为有些运气，还有其他软件包。

I am following this tutorial webScrapper using HXT.I am using Stack here.

While adding dependency for MaybeT, it asked to:

Now doing so to ended up with this message on running stack build:

Being new to Stack, I am unable to solve the problem, in my cabal file thethe dependency:

  build-depends:       base >= 4.7 && < 5
                    ,  hxt
                    ,  url
                    ,  HTTP
                    ,  MaybeT

In stack.yaml

resolver: lts-7.12
extra-deps: [MaybeT-0.1.2]

What is it that I have missed. How can I solve this?

解决方案

The solution for your concrete problem is switching from the outdated MaybeT package to the MaybeT transformer offered by the transformers package in Control.Monad.Trans.Maybe.

That said, note that the version mismatch involves base. If you actually needed to use the MaybeT package, you would have to switch the Stack resolver to a snapshot such as lts-6.26, which uses base-4.8 or below (i.e. GHC 7.10.3 or below). base versions are coupled to GHC versions, and everything else depends on base, so there is no easy way of switching just base to an older version, as it might be possible, given some luck, with other packages.

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