Questions about prefix-independence for data files

[danidiaz]

I've been following the documentation for the Paths_pkgname and data-files mechanism and managed to get a simple example going.

However, I don't quite understand the documentation for prefix-independence:

On Windows it is possible to obtain the pathname of the running program.

Does that mean that prefix-independence is only for Windows? Perhaps it should be explicitly stated.

The executable can find its auxiliary files by finding its own path and knowing the location of the other files relative to $bindir. Prefix-independence is particularly useful: it means the user can choose the install location (i.e. the value of $prefix) at install-time

Here by "installing", do we mean something like copying the executable to another location, maybe across machines?

If the executable finds the other files relative to $bindir, why is $prefix needed at all?

In order to achieve this, we require that for an executable on Windows, all of $bindir, $libdir, $dynlibdir, $datadir and $libexecdir begin with $prefix.

Is this the default, or do I have to do tweak something in order to achieve it? Does "beginning with $prefix" mean "beginning with the literal text '$prefix'" or "beginning with the value of $prefix"?

System information

• Cabal 3.6.2.0

[Mikolaj]

This may be a problem not only with documentation. I've heard from @phadej the data files feature is not fully working or not for all OSes and there is no obvious way forward. I guess studying open issues about that might indicate why. Indeed, if would be great to at least document what is failing.

[danidiaz]

Working on Windows, and looking at my generated Paths_ module, I see code like:

getDataFileName :: FilePath -> IO FilePath
getDataFileName name = do
  dir <- getDataDir
  return (dir `joinFileName` name)

getDataDir    = catchIO (getEnv "dataexp_datadir")    (\_ -> return datadir)

datadir    = "C:\\Users\\MyUser\\.local\\cabal\\x86_64-windows-ghc-8.10.7\\dataexp-0.1.0.0"

So it seems that the data dir is hardwired, but it can be overridden using an environment variable.

I don't see anything related to finding the data dir by asking the OS for the path of the executable (as implied by the prefix-independence docs) but perhaps it would exist if I had used some non-default compilation option.

[danidiaz]

In the Cabal source code, the file templates\Paths_pkg.template.hs does have code like the following:

{% if relocatable %}

getPrefixDirReloc :: FilePath -> IO FilePath
getPrefixDirReloc dirRel = do
  exePath <- getExecutablePath
  let (dir,_) = splitFileName exePath
  return ((dir `minusFileName` {{ bindir }}) `joinFileName` dirRel)

But how to enable relocatable?

Apparently, there's a relocatable field in cabal.project, since Cabal 1.22. It's set to False by default. I tried setting it to True but it doesn't seem to do anything; getPrefixDirReloc still doesn't appear in my Paths_ file.

[Mikolaj]

Uhoh, so the feature is not only undocumented and not quite working, but there are also bits and pieces of its carcass laying around and tripping contributors? That definitely requires an intervention.

[jneira]

it seems to be a bug, as the hardwired path is wrong
"C:\\Users\\MyUser\\.local\\cabal\\x86_64-windows-ghc-8.10.7\\dataexp-0.1.0.0" should be "C:\\Users\\MyUser\\.local\\cabal\\packages\\xxxx\\x86_64-windows-ghc-8.10.7\\dataexp-0.1.0.0" or something alike (will check the exact path)

I did some chackes with:

module Main where
import Paths_datadirs


main :: IO ()
main = do
  dir <- getDataDir
  putStrLn $ "Datadir: " ++ dir

PS D:\dev\ws\haskell\issues\datadirs> cabal run
Datadir: D:\dev\ws\haskell\issues\datadirs\.\.
PS D:\dev\ws\haskell\issues\datadirs> cabal install
....
Copying 'datadirs.exe' to 'D:\bin\datadirs.exe'
PS D:\dev\ws\haskell\issues\datadirs> datadirs
Datadir: D:\cabal\store\ghc-8.10.7\datadirs-0.1.0.0-8741e1327c97f40d292496f63ccc5862cd3e0d46\share

Of course you need data-files filed filled to make cabal copy the files