Add test to check package id extraction

This commit add a failing test to check extracting package id. The test will
fail with the following output for now:

1) Cradle.getPackageDbPackages find a config file and extracts packages with their ids
expected: [("template-haskell",Just "template-haskell-2.8.0.0-32d4f24abdbb6bf41272b183b2e23e9c")]
 but got: []

Language/Haskell/GhcMod/Cradle.hs

@@ -4,6 +4,7 @@ module Language.Haskell.GhcMod.Cradle (
     findCradle
   , findCradleWithoutSandbox
   , getPackageDbDir
+  , getPackageDbPackages
   ) where
 
 import Data.Char (isSpace)
@@ -141,3 +142,43 @@ extractGhcVer dir = ver
     (verStr1,_:left) = break (== '.') $ findVer file
     (verStr2,_)      = break (== '.') left
     ver = read verStr1 * 100 + read verStr2
+
+-- | Obtaining packages installed in a package db directory.
+getPackageDbPackages :: FilePath -> IO [Package]
+getPackageDbPackages cdir = (getPkgDb >>= listDbPackages) `E.catch` handler
+  where
+    getPkgDb = getPackageDbDir (cdir </> configFile)
+    handler :: SomeException -> IO [Package]
+    handler _ = return []
+
+listDbPackages :: FilePath -> IO [Package]
+listDbPackages pkgdir = do
+  files <- filter (".conf" `isSuffixOf`) <$> getDirectoryContents pkgdir
+  mapM extractPackage $ map (pkgdir </>) files
+
+extractPackage :: FilePath -> IO Package
+extractPackage pconf = do
+  contents <- lines <$> readFile pconf
+  -- Be strict to ensure that an error can be caught.
+  let !name = extractName $ parseName contents
+      !pid = extractId $ parseId contents
+  return (name, Just pid)
+  where
+    parseName = parse nameKey
+    extractName = extract nameKeyLength
+    parseId = parse idKey
+    extractId = extract idKeyLength
+    parse key = head . filter (key `isPrefixOf`)
+    extract keylen = fst . break isSpace . dropWhile isSpace . drop keylen
+
+nameKey :: String
+nameKey = "name:"
+
+idKey :: String
+idKey = "id:"
+
+nameKeyLength :: Int
+nameKeyLength = length nameKey
+
+idKeyLength :: Int
+idKeyLength = length idKey