[Haskell-cafe] Nested tests [Code walking off the right edge of the screen]

[Andrew Coppin]

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?