How to programmatically retrieve GHC package information?

More specifically, given an arbritary package name I need to retrieve the same library-dirs field that can be obtained with the ghc-pkg describe command from inside a running Haskell program.

Solution

Here's what I could come up with by peeking into the ghc-pkg source code.

The getPkgInfos function returns the package definitions for all installed packages (hopefully including user-installed packages). With this in your hands, you can retrieve the library directories and other package information. See the documentation for details.

The GHC_PKGCONF variable needs to point to the global package config file for systems where it isn't located at the usual place. ghc-pkg solves this problem by receiving a command line flag via a wrapper script in Ubuntu, for instance.

import qualified Config
import qualified System.Info
import Data.List
import Distribution.InstalledPackageInfo
import GHC.Paths
import System.Directory
import System.Environment
import System.FilePath
import System.IO.Error

getPkgInfos :: IO [InstalledPackageInfo]
getPkgInfos = do
    global_conf <-
        catch (getEnv "GHC_PKGCONF")
              (\err ->  if isDoesNotExistError err
                            then do let dir = takeDirectory $ takeDirectory ghc_pkg
                                        path1 = dir </> "package.conf"
                                        path2 = dir </> ".." </> ".." </> ".."
                                                    </> "inplace-datadir"
                                                    </> "package.conf"
                                    exists1 <- doesFileExist path1
                                    exists2 <- doesFileExist path2
                                    if exists1 then return path1
                                       else if exists2 then return path2
                                       else ioError $ userError "Can't find package.conf"
                            else ioError err)

    let global_conf_dir = global_conf ++ ".d"
    global_conf_dir_exists <- doesDirectoryExist global_conf_dir
    global_confs <-
        if global_conf_dir_exists
            then do files <- getDirectoryContents global_conf_dir
                    return  [ global_conf_dir ++ '/' : file
                            | file <- files
                            , isSuffixOf ".conf" file]
            else return []

    user_conf <-
        try (getAppUserDataDirectory "ghc") >>= either
            (\_ -> return [])
            (\appdir -> do
                let subdir = currentArch ++ '-':currentOS ++ '-':ghcVersion
                    user_conf = appdir </> subdir </> "package.conf"
                user_exists <- doesFileExist user_conf
                return (if user_exists then [user_conf] else []))

    let pkg_dbs = user_conf ++ global_confs ++ [global_conf]
    return.concat =<< mapM ((>>= return.read).readFile) pkg_dbs

currentArch = System.Info.arch
currentOS = System.Info.os
ghcVersion = Config.cProjectVersion

I wrote this code myself, but it was largely inspired by ghc-pkg (with some pieces copied verbatim). The original code was licensed under a BSD-style license, I think this can be distributed under the cc-wiki license all Stackoverflow content is under, but I'm not really sure. Anyway, as anything else, I did some initial testing and it seems to work, but use it at your own risk.