    {-# LANGUAGE CPP #-}

    -- | Note [Base Dir]
    -- ~~~~~~~~~~~~~~~~~
    -- GHC's base directory or top directory containers miscellaneous settings and
    -- the package database.  The main compiler of course needs this directory to
    -- read those settings and read and write packages. ghc-pkg uses it to find the
    -- global package database too.
    --
    -- In the interest of making GHC builds more relocatable, many settings also
    -- will expand `${top_dir}` inside strings so GHC doesn't need to know it's on
    -- installation location at build time. ghc-pkg also can expand those variables
    -- and so needs the top dir location to do that too.

    module GHC.BaseDir where

    import Prelude -- See Note [Why do we import Prelude here?]

    import Data.List (stripPrefix)
    import Data.Maybe (listToMaybe)
    import System.FilePath

    -- Windows
    #if defined(mingw32_HOST_OS)
    import System.Environment (getExecutablePath)
    -- POSIX
    #elif defined(darwin_HOST_OS) || defined(linux_HOST_OS) || defined(freebsd_HOST_OS) || defined(openbsd_HOST_OS) || defined(netbsd_HOST_OS)
    import System.Environment (getExecutablePath)
    #endif

    -- | Expand occurrences of the @$topdir@ interpolation in a string.
    expandTopDir :: FilePath -> String -> String
    expandTopDir :: FilePath -> FilePath -> FilePath
    expandTopDir = FilePath -> FilePath -> FilePath -> FilePath
    expandPathVar FilePath
    "topdir"

    -- | @expandPathVar var value str@
    --
    --   replaces occurrences of variable @$var@ with @value@ in str.
    expandPathVar :: String -> FilePath -> String -> String
    expandPathVar :: FilePath -> FilePath -> FilePath -> FilePath
    expandPathVar FilePath
    var FilePath
    value FilePath
    str
      | Just FilePath
    str' <- FilePath -> FilePath -> Maybe FilePath
    forall a. Eq a => [a] -> [a] -> Maybe [a]
    stripPrefix (Char
    '$'Char -> FilePath -> FilePath
    forall a. a -> [a] -> [a]
    :FilePath
    var) FilePath
    str
      , Bool -> (Char -> Bool) -> Maybe Char -> Bool
    forall b a. b -> (a -> b) -> Maybe a -> b
    maybe Bool
    True Char -> Bool
    isPathSeparator (FilePath -> Maybe Char
    forall a. [a] -> Maybe a
    listToMaybe FilePath
    str')
      = FilePath
    value FilePath -> FilePath -> FilePath
    forall a. [a] -> [a] -> [a]
    ++ FilePath -> FilePath -> FilePath -> FilePath
    expandPathVar FilePath
    var FilePath
    value FilePath
    str'
    expandPathVar FilePath
    var FilePath
    value (Char
    x:FilePath
    xs) = Char
    x Char -> FilePath -> FilePath
    forall a. a -> [a] -> [a]
    : FilePath -> FilePath -> FilePath -> FilePath
    expandPathVar FilePath
    var FilePath
    value FilePath
    xs
    expandPathVar FilePath
    _ FilePath
    _ [] = []

    -- | Calculate the location of the base dir
    getBaseDir :: IO (Maybe String)
    #if defined(mingw32_HOST_OS)
    getBaseDir = Just . (\p -> p </> "lib") . rootDir <$> getExecutablePath
      where
        -- locate the "base dir" when given the path
        -- to the real ghc executable (as opposed to symlink)
        -- that is running this function.
        rootDir :: FilePath -> FilePath
        rootDir = takeDirectory . takeDirectory . normalise
    #elif defined(darwin_HOST_OS) || defined(linux_HOST_OS) || defined(freebsd_HOST_OS) || defined(openbsd_HOST_OS) || defined(netbsd_HOST_OS)
    -- on unix, this is a bit more confusing.
    -- The layout right now is something like
    --
    --   /bin/ghc-X.Y.Z <- wrapper script (1)
    --   /bin/ghc       <- symlink to wrapper script (2)
    --   /lib/ghc-X.Y.Z/bin/ghc <- ghc executable (3)
    --   /lib/ghc-X.Y.Z <- $topdir (4)
    --
    -- As such, we first need to find the absolute location to the
    -- binary.
    --
    -- getExecutablePath will return (3). One takeDirectory will
    -- give use /lib/ghc-X.Y.Z/bin, and another will give us (4).
    --
    -- This of course only works due to the current layout. If
    -- the layout is changed, such that we have ghc-X.Y.Z/{bin,lib}
    -- this would need to be changed accordingly.
    --
    getBaseDir :: IO (Maybe FilePath)
    getBaseDir = FilePath -> Maybe FilePath
    forall a. a -> Maybe a
    Just (FilePath -> Maybe FilePath)
    -> (FilePath -> FilePath) -> FilePath -> Maybe FilePath
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . (\FilePath
    p -> FilePath
    p FilePath -> FilePath -> FilePath
    </> FilePath
    "lib") (FilePath -> FilePath)
    -> (FilePath -> FilePath) -> FilePath -> FilePath
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . FilePath -> FilePath
    takeDirectory (FilePath -> FilePath)
    -> (FilePath -> FilePath) -> FilePath -> FilePath
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . FilePath -> FilePath
    takeDirectory (FilePath -> Maybe FilePath) -> IO FilePath -> IO (Maybe FilePath)
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$> IO FilePath
    getExecutablePath
    #else
    getBaseDir = return Nothing
    #endif