Haskell / cabal 包的解决方法受到 Nix 和 Cabal 的限制？

我最近开始开发反射平台 https://github.com/reflex-frp/reflex-platform，有一些额外的配置类似于优秀的反射项目骨架 https://github.com/ElvishJerricco/reflex-project-skeleton.

现在我遇到了无法解决的包冲突。 我使用与reflex-project-skeleton，这会调用nix-shell与 cabal 命令处于非交互模式。

If I try

./cabal new-build --allow-newer all

I get

these derivations will be built:
  /nix/store/d6ji516i7pry5l6gv18y6hpj9k1bvgg5-heist-1.0.1.0.drv
  /nix/store/zj1clks7mzq8gn91ahhwa3nvpi5rwra9-snap-1.0.0.2.drv
  /nix/store/mx861972jnjabn7yxyr3y3q1yhf25jfq-snaplet-acid-state-0.2.7.drv
  /nix/store/grhp4dhavmpi6bgns5a6vdzg8ny2bsf8-hoogle-local-0.1.drv
  /nix/store/y3sszsj58f6ad3r06540w0mlr1pncd59-ghc-8.0.2-with-packages.drv

...

Configuring heist-1.0.1.0...
Setup: Encountered missing dependencies:
aeson >=0.6 && <1.2

builder for '/nix/store/d6ji516i7pry5l6gv18y6hpj9k1bvgg5-heist-1.0.1.0.drv' failed with exit code 1
cannot build derivation '/nix/store/y3sszsj58f6ad3r06540w0mlr1pncd59-ghc-8.0.2-with-packages.drv': 1 dependencies couldn't be built

有问题的包似乎是snaplet-acid-state，所以我尝试从 nix-shell 内部安装它：

cabal install snaplet-acid-state
Resolving dependencies...
cabal: Could not resolve dependencies:
trying: snaplet-acid-state-0.2.7 (user goal)
trying: base-4.9.1.0/installed-4.9... (dependency of snaplet-acid-state-0.2.7)
next goal: mtl (dependency of snaplet-acid-state-0.2.7)
rejecting: mtl-2.2.1/installed-BLK... (conflict: mtl =>
transformers==0.5.2.0/installed-0.5..., snaplet-acid-state =>
transformers>=0.3.0.0 && <0.5)
trying: mtl-2.2.2
next goal: transformers (dependency of snaplet-acid-state-0.2.7)
rejecting: transformers-0.5.2.0/installed-0.5..., transformers-0.5.5.0,
transformers-0.5.4.0, transformers-0.5.2.0, transformers-0.5.1.0,
transformers-0.5.0.1, transformers-0.5.0.0 (conflict: snaplet-acid-state =>
transformers>=0.3.0.0 && <0.5)
rejecting: transformers-0.4.3.0, transformers-0.4.2.0 (conflict:
base==4.9.1.0/installed-4.9..., transformers => base>=2 && <4.9)
rejecting: transformers-0.4.1.0 (conflict: base==4.9.1.0/installed-4.9...,
transformers => base>=2 && <4.8 || >=1.0 && <2)
rejecting: transformers-0.3.0.0 (conflict: mtl => transformers>=0.4 && <0.6)
rejecting: transformers-0.2.2.1, transformers-0.2.1.0, transformers-0.2.0.0,
transformers-0.1.4.0, transformers-0.1.3.0, transformers-0.1.1.0,
transformers-0.1.0.1, transformers-0.0.1.0, transformers-0.0.0.0,
transformers-0.5.3.1, transformers-0.5.3.0, transformers-0.5.0.2 (conflict:
snaplet-acid-state => transformers>=0.3.0.0 && <0.5)
rejecting: transformers-0.4.0.0 (conflict: base==4.9.1.0/installed-4.9...,
transformers => base>=2 && <4.8 || >=1.0 && <2)
rejecting: transformers-0.2.2.0, transformers-0.1.0.0 (conflict:
snaplet-acid-state => transformers>=0.3.0.0 && <0.5)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: transformers, snap, base,
snaplet-acid-state, mtl

尝试在 cabal 文件中指定版本约束等，但没有效果。

有什么线索吗？我什至不确定确切的问题是什么。为什么第一次安装 aeson 失败，第二次又出现 snaplet-acid-state 和 mtl 冲突？

这会发生什么cabal2nix工作流程基本上是依赖关系求解器被迫只考虑 Nixpkgs 提供的包，或者在本例中是 reflex-platform + Nixpkgs。通过剥夺选择多个版本之一的自由，Cabal 依赖项求解器被简化为依赖项checker.

现在，如果版本限制太严格，您可以考虑 Nixpkgs 中所谓的“越狱”：从 cabal 文件中删除所有版本限制。另一种选择是更改软件包版本。

加布里埃尔·冈萨雷斯写道很好地解释了 Nixpkgs 的 Haskell 基础设施的版本管理 https://github.com/Gabriel439/haskell-nix/blob/master/project4/README.md。您还可以查看这个线程 https://github.com/Gabriel439/haskell-nix/issues/6。虽然有点啰嗦，但是讨论了callHackage and callCabal2nix非常有帮助的功能。