haskell - 当 cabal 构建软件包时， "legacy fallback"是什么意思？

Question

使用 cabal 时要构建一个 Haskell 包，它似乎将一些包标记为 legacy fallback :

$ cabal build
Resolving dependencies...
Build profile: -w ghc-9.0.1 -O1
In order, the following will be built (use -v for more details):
 - appar-0.1.8 (lib:appar) (requires build)
 - auto-update-0.1.6 (lib) (requires build)
 - base-compat-0.11.2 (lib) (requires build)
...
Building     base-orphans-0.8.4 (lib)
Building     appar-0.1.8 (all, legacy fallback)
Downloaded   memory-0.16.0
Downloading  cryptonite-0.29
Installing   base-orphans-0.8.4 (lib)
Downloaded   cryptonite-0.29
Downloading  some-1.0.3
...

您可以看到，对于某些库，它们被专门标记为 (lib) ，但其他库标记为 (all, legacy fallback) .
这些有什么区别？ legacy fallback 是什么意思意思是？

我正在使用 cabal-install版本 3.4.0.0:

$ cabal --version
cabal-install version 3.4.0.0
compiled using version 3.4.0.0 of the Cabal library 

Answer 1

我深入研究了源代码。错误消息来自 here :

    dispname = case elabPkgOrComp pkg of
        ElabPackage _ -> prettyShow pkgid
            ++ " (all, legacy fallback)"
        ElabComponent comp -> prettyShow pkgid
            ++ " (" ++ maybe "custom" prettyShow (compComponentName comp) ++ ")"

于是我开始寻找 ElabPackage 的地方被构造。我找到了 this :

    elaborateSolverToPackage
        ...
      where
        ...
        elab1 = elab0 {
                elabUnitId = newSimpleUnitId pkgInstalledId,
                elabComponentId = pkgInstalledId,
                elabLinkedInstantiatedWith = Map.empty,
                elabPkgOrComp = ElabPackage $ ElaboratedPackage {..},
                elabModuleShape = modShape
            }

这又被使用 here :

    elaborateSolverToComponents mapDep spkg@(SolverPackage _ _ _ deps0 exe_deps0)
        = case mkComponentsGraph (elabEnabledSpec elab0) pd of
           Right g -> do
            ...
            let not_per_component_reasons = why_not_per_component src_comps
            if null not_per_component_reasons
                then return comps
                else do checkPerPackageOk comps not_per_component_reasons
                        return [elaborateSolverToPackage spkg g $
                                comps ++ maybeToList setupComponent]

现在 why_not_per_component非常有趣，因为该函数决定了何时使用遗留后备。它被定义为 here :

        -- You are eligible to per-component build if this list is empty
        why_not_per_component g
            = cuz_buildtype ++ cuz_spec ++ cuz_length ++ cuz_flag ++ cuz_coverage

在下面的代码中，我们可以看到它可能是由以下原因引起的:

构建类型为自定义或配置

cabal 版本小于 1.8

没有可构建的组件

你通过了--disable-per-component旗帜。

计划覆盖已启用

所以对于 appar库是因为 cabal 版本是 1.6，低于 1.8，见 https://github.com/kazu-yamamoto/appar/blob/v0.1.8/appar.cabal#L10 .