For a quite involved custom Setup library, I need to manipulate source file location directories:

• I need to read out hsSourceDirs and then read files from those directories, both directly from Setup.hs and also indirectly by passing the filenames as string arguments to external programs
• I need to inject new directories containing generated source files into hsSourceDirs

The problem I am facing is that the type of hsSourceDirs is [SymbolicPath PackageDir SourceDir], and I just can’t get my head around how to manipulate these SymbolicPaths. Looking at Distribution.Utils.Path, the datatype SymbolicPath from to is exported abstractly only, and the combinators available are:

• getSymbolicPath :: SymbolicPath from to -> FilePath, marked as “avoid using this in new code”
• sameDirectory :: _ => SymbolicPath from to
• unsafeMakeSymbolicPath :: FilePath -> SymbolicPath from to

This leaves me with the only choice of converting immediately from SymbolicPath PackageDir SourceDir into FilePath using getSymbolicPath (that apparently I’m not supposed to be using), doing all file access in terms of that FilePath, and then producing a new FilePath starting from e.g. buildDir, using FilePath combinators like </>, ending with a call to unsafeMakeSymbolicPath. This doesn’t feel ideal.

Is there a way to, instead:

• Get, perhaps in IO, perhaps using whatever Cabal context is available in a custom Setup.hs, the physical FilePath from a SymbolicPath PackageDir SourceDir

• Create, perhaps in IO, perhaps using whatever Cabal context is available in a custom Setup.hs, a new SymbolicPath that points to a new subdirectory (either named explicitly by me, or generated randomly afresh by Cabal) under the build directory

?