Making cabalBuildInfo pure.

Cabal.hs

@@ -38,7 +38,7 @@ fromCabal :: [String] -> IO ([String], [FilePath], [String])
 fromCabal ghcOptions = do
     (owdir,cdir,cfile) <- getDirs
     cabal <- cabalParseFile cfile
-    binfo@BuildInfo{..} <- cabalBuildInfo cabal
+    let binfo@BuildInfo{..} = cabalBuildInfo cabal
     let exts = map (("-X" ++) . display) $ usedExtensions binfo
         lang = maybe "-XHaskell98" (("-X" ++) . display) defaultLanguage
         libs = map ("-l" ++) extraLibs

CabalApi.hs

@@ -24,9 +24,8 @@ cabalParseFile :: FilePath -> IO GenericPackageDescription
 cabalParseFile =  readPackageDescription silent
 
 -- Causes error, catched in the upper function.
-cabalBuildInfo :: GenericPackageDescription -> IO BuildInfo
-cabalBuildInfo pd =
-    return . fromJust $ fromLibrary pd <|> fromExecutable pd
+cabalBuildInfo :: GenericPackageDescription -> BuildInfo
+cabalBuildInfo pd = fromJust $ fromLibrary pd <|> fromExecutable pd
   where
     fromLibrary c     = libBuildInfo . condTreeData <$> condLibrary c
     fromExecutable c  = buildInfo . condTreeData . snd <$> toMaybe (condExecutables c)

test/CabalApiSpec.hs

@@ -13,6 +13,6 @@ spec = do
 
     describe "cabalBuildInfo" $ do
         it "extracts build info" $ do
-            info <- cabalParseFile "test/data/cabalapi.cabal" >>= cabalBuildInfo
+            info <- cabalBuildInfo <$> cabalParseFile "test/data/cabalapi.cabal"
             let infoStr = show info
             infoStr `shouldBe` "BuildInfo {buildable = True, buildTools = [], cppOptions = [], ccOptions = [], ldOptions = [], pkgconfigDepends = [], frameworks = [], cSources = [], hsSourceDirs = [], otherModules = [ModuleName [\"Browse\"],ModuleName [\"CabalApi\"],ModuleName [\"Cabal\"],ModuleName [\"CabalDev\"],ModuleName [\"Check\"],ModuleName [\"ErrMsg\"],ModuleName [\"Flag\"],ModuleName [\"GHCApi\"],ModuleName [\"GHCChoice\"],ModuleName [\"Gap\"],ModuleName [\"Info\"],ModuleName [\"Lang\"],ModuleName [\"Lint\"],ModuleName [\"List\"],ModuleName [\"Paths_ghc_mod\"],ModuleName [\"Types\"]], defaultLanguage = Nothing, otherLanguages = [], defaultExtensions = [], otherExtensions = [], oldExtensions = [], extraLibs = [], extraLibDirs = [], includeDirs = [], includes = [], installIncludes = [], options = [(GHC,[\"-Wall\"])], ghcProfOptions = [], ghcSharedOptions = [], customFieldsBI = [], targetBuildDepends = []}"