getModificationTime gives only second-level resolution

The System.Directory.getModificationTime function only returns time with second-level granularity. This is no good for applications like ghc --make where we need to compare file timestamps.

The fix should be simple, here's a patch (compiles but not tested) that should fix it for Unix, for Windows we will need a different fix, probably using the Win32 API directly, rather than the mingw C api.

diff --git a/System/Directory.hs b/System/Directory.hs
index cfe7cd9..f27bfc4 100644
--- a/System/Directory.hs
+++ b/System/Directory.hs
@@ -995,14 +995,14 @@ The operation may fail with:
 getModificationTime :: FilePath -> IO UTCTime
 getModificationTime name = do
 #ifdef mingw32_HOST_OS
- -- ToDo: use Win32 API
+ -- ToDo: use Win32 API so we can get sub-second resolution
  withFileStatus "getModificationTime" name $ \ st -> do
  modificationTime st
 #else
   stat <- Posix.getFileStatus name
-  let mod_time :: Posix.EpochTime
-      mod_time = Posix.modificationTime stat
-  return $ posixSecondsToUTCTime $ realToFrac mod_time
+  let mod_time :: POSIXTime
+      mod_time = Posix.modificationTimeHiRes stat
+  return $! posixSecondsToUTCTime mod_time
 #endif
 
 #endif /* __GLASGOW_HASKELL__ */

Trac metadata

Trac field	Value
Version	7.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/directory
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