On Jun 21, 2009, at 11:52 AM, Andrew Coppin wrote:
In a similar vein:
d1x <- doesDirectoryExist d1
if d1x
then do
f1x <- doesFileExist (d1 </> f1)
if f1x
then do
d2x <- doesDirectoryExist d2
if d2x
then do
f2x <- doesFileExist (d2 </> f2)
if f2x
then do_stuff d1 d2 f1 f2
else hPutStrLn stderr $ "File " ++ f2 ++ " not found."
else hPutStrLn stderr $ "Directory " ++ d2 ++ " not found."
else hPutStrLn stderr $ "File " ++ f1 ++ " not found."
else hPutStrLn stderr $ "Directory " ++ d1 ++ " not found."
using Control.Monad.Error:
either (hPutStrLn stderr) return =<< runErrorT $
do d1x <- lift $ doesDirectoryExist d1
unless d1x $ fail "Directory " ++ d1 ++ " not found."
f1x <- lift $ doesFileExist (d1 </> f1)
unless f1x $ fail "File " ++ f1 ++ " not found."
d2x <- lift $ doesDirectoryExist d2
unless d2x $ fail "Directory " ++ d2 ++ " not found."
f2x <- lift $ doesFileExist (d2 </> f2)
unless f2x $ fail "File " ++ f2 ++ " not found."
lift $ doStuff d1 d2 f1 f2
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
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.)
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