Deniz Dogan wrote:
I (too) often find myself writing code such as this:
if something then putStrLn "howdy there!" else if somethingElse then putStrLn "howdy ho!" else ...
I recall reading some tutorial about how you can use the Maybe monad if your code starts looking like this, but as you can see, that doesn't really apply here. "something" and "somethingElse" are simply booleans and each of them have different actions to take if either of them is True.
So how do I make code like this prettier?
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." Obviously, this is nausiating. Surely we can do better somehow? The above is a simple example. I might need to check file permissions, invoke external programs, parse files, all sorts of things. And it might matter which order the tests happen in. Later tests might use information gained during earlier tests. But the recurring pattern is "check this thing; if it passes, continue; if it fails, stop here and emit an error message". Can we abstract this somehow?