#262 iterative tests for foreign dependencies

Optimize for succesful case. First try all libs and includes in one command, 
proceed with further tests only if the first test fails. The same goes for libs 
and headers: look for an offending one only when overall test fails.

Distribution/Simple/Configure.hs

@@ -125,7 +125,7 @@ import qualified Distribution.Simple.Hugs as Hugs
 import Control.Monad
     ( when, unless, foldM, filterM )
 import Data.List
-    ( nub, partition, isPrefixOf, maximumBy )
+    ( nub, partition, isPrefixOf, maximumBy, inits )
 import Data.Maybe
     ( fromMaybe, isNothing )
 import Data.Monoid
@@ -660,24 +660,55 @@ configCompiler (Just hcFlavor) hcPath hcPkg conf verbosity = do
 
 checkForeignDeps :: PackageDescription -> ProgramConfiguration -> Verbosity -> IO ()
 checkForeignDeps pkg progCfg verbosity = do
-  missingHdrs <- findMissing PD.includes headerExists
-  missingLibs <- findMissing PD.extraLibs libExists
-  explainErrors missingHdrs missingLibs
+  ifBuildsWith allHeaders (commonCppArgs ++ makeLdArgs allLibs) -- I'm feeling lucky
+           (return ())
+           (do missingLibs <- findMissingLibs
+               missingHdr  <- findOffendingHdr 
+               explainErrors missingHdr missingLibs)
       where
-        findMissing field p = filterM (\v -> not `fmap` p v) (collectField field)
-
-        headerExists hdr = compilesAndLinks cppArgs $
-                           "#include \""  ++ hdr ++ "\"\n" ++
-                           "int main(int argc, char** argv) { return 0; }\n"
-            where cppArgs = [ "-I" ++ dir | dir <- collectField PD.includeDirs ]
-
-        libExists lib = compilesAndLinks ldArgs "int main(int argc, char** argv) { return 0; }\n"
-            where ldArgs = ("-l"++lib):[ "-L" ++ dir | dir <- collectField PD.extraLibDirs ]
-
-        bi = allBuildInfo pkg
+        allHeaders = collectField PD.includes
+        allLibs    = collectField PD.extraLibs
+
+        ifBuildsWith headers args success failure = do
+            ok <- builds (makeProgram headers) args
+            if ok then success else failure
+
+        -- NOTE: if some package-local header has errors,
+        -- we will report that this header is missing. 
+        -- Maybe additional tests for local headers are needed 
+        -- for better diagnostics
+        findOffendingHdr =
+            ifBuildsWith allHeaders cppArgs
+                         (return Nothing)
+                         (go . tail . inits $ allHeaders)
+            where
+              go [] = return Nothing       -- cannot happen
+              go (hdrs:hdrsInits) = do
+                    ifBuildsWith hdrs cppArgs
+                                 (go hdrsInits)
+                                 (return . Just . last $ hdrs) 
+
+              cppArgs = "-c":commonCppArgs -- don't try to link
+
+        findMissingLibs = ifBuildsWith [] (makeLdArgs allLibs)
+                                       (return [])
+                                       (filterM (fmap not . libExists) allLibs)
+                       
+        libExists lib = builds (makeProgram []) (makeLdArgs [lib])
+
+        commonCppArgs = [ "-I" ++ dir | dir <- collectField PD.includeDirs ]
+        commonLdArgs  = [ "-L" ++ dir | dir <- collectField PD.extraLibDirs ]
+        
+        makeLdArgs libs = [ "-l"++lib | lib <- libs ] ++ commonLdArgs
+
+        makeProgram hdrs = unlines $
+                           [ "#include \""  ++ hdr ++ "\"" | hdr <- hdrs ] ++
+                           ["int main(int argc, char** argv) { return 0; }"]
+        
         collectField f = nub $ concatMap f bi
+            where bi = allBuildInfo pkg
 
-        compilesAndLinks args program = do
+        builds program args = do
             tempDir <- getTemporaryDirectory
             withTempFile tempDir ".c" $ \fname hd ->
               do
@@ -691,8 +722,10 @@ checkForeignDeps pkg progCfg verbosity = do
         -- TODO: make error messages more user-friendly
         -- (guess the most probable causes of the problems,
         -- e.g. suggest that *-dev package is probably missing etc)
-        explainErrors hdrs libs = do
-            mapM_ (warn verbosity) $ map ("Required C header not found: " ++) hdrs
+        explainErrors hdr libs = do
+            case hdr of
+              Just h -> warn verbosity $ "Required C header not found: " ++ h
+              _      -> return ()
             mapM_ (warn verbosity) $ map ("Required C library not found: " ++) libs
 
 -- | Output package check warnings and errors. Exit if any errors.