Dynamically check `FsPath` invariants

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首先阅读 FsPath 的定义以及 pull request 72 中的设计讨论。将提议的 invariant、unsafeFsPathFromList 和 fsPathFromList API 与现有路径行为进行比较，并考虑 force/assert 之间的权衡。完成的标准是就是否需要动态检查达成一致，并定义出在各操作系统之间保持一致的已验证构造和逃生通道构造行为。

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描述

First suggested in https://github.com/input-output-hk/fs-sim/pull/72#issue-2337874221. FsPath has a number of invariants, and we currently rely on the user to satisfy them. It might be nice to check these invariants dynamically, while also providing an escape hatch for users in case they want to skip the check. For example:

newtype FsPath = UnsafeFsPath { fsPathToList :: [Strict.Text] }
  deriving (Eq, Ord, Generic)
  deriving newtype NFData

invariant :: FsPath -> Bool
invariant (UnsafeFsPath xs) = all p xs
 where p x = -- Paths are monotonic
             x /= ".."
          && -- Paths can not have empty directory/file names
             not (any Strict.null xs)
          && -- There are no path separators in individual names
             not (Text.any (`Text.elem` allPathSeparators) x)

allPathSeparators :: Text
allPathSeparators = Text.pack (Posix.pathSeparators ++ Windows.pathSeparators)

unsafeFsPathFromList :: [Strict.Text] -> FsPath
unsafeFsPathFromList xs = assert (invariant fsp) $ fsp
  where fsp = UnsafeFsPath (force xs)

fsPathFromList :: [Strict.Text] -> Maybe FsPath
fsPathFromList xs
  | invariant fsp = Just fsp
  | otherwise     = Nothing
  where fsp = UnsafeFsPath (force xs)

Note two things in the example above:

  • Both unsafeFsPathFromList and fsPathFromList ensure that the resulting FsPath contains no thunks my using force. Maybe we want to assert instead that the FsPath contains no thunks in unsafeFsPathFromList, and have the user ensure that fact.
  • To ensure similar behaviour across OS distributions, invariant checks do not depend on the underlying OS distribution, hence allPathSeparators.

We should also consider whether this design pattern is overkill: in practice, users are probably unlikely to use non-sensible path names

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