On Jun 21, 2009, at 2:36 PM, Sebastian Fischer wrote:
On Jun 21, 2009, at 11:52 AM, Andrew Coppin wrote: ...
When using
failUnless boolAction message = lift boolAction >>= (`unless`fail message)
the code becomes
either (hPutStrLn stderr) return =<< runErrorT $ do failUnless (doesDirectoryExist d1) $ "Directory " ++ d1 ++ " not found." failUnless (doesFileExist (d1 f1)) $ "File " ++ f1 ++ " not found." failUnless (doesDirectoryExist d2) $ "Directory " ++ d2 ++ " not found." failUnless (doesFileExist (d2 f2)) $ "File " ++ f2 ++ " not found." lift $ doStuff d1 d2 f1 f2
Or make is somewhat prettier (imho) using infix: orError boolAction message = lift boolAction >>= (`unless`fail message) either (hPutStrLn stderr) return =<< runErrorT $ do doesDirectoryExist d1 `orError` ("Directory " ++ d1 ++ " not found.") doesFileExist (d1 f1) `orError` ("File " ++ f1 ++ " not found.") doesDirectoryExist d2 `orError` ("Directory " ++ d2 ++ " not found.") doesFileExist (d2 f2) `orError` ("File " ++ f2 ++ " not found.") lift $ doStuff d1 d2 f1 f2
It's similar to Claus's proposal to use MaybeT with additional support for error messages.
Sebastian
-- Underestimating the novelty of the future is a time-honored tradition. (D.G.)
-- Sebastiaan Visser