Re: a monadic if or case?

Here's another way to sugar if-then-else that works like C's ?: and Lisp's cond: import Monad (liftM3) import Directory (doesFileExist, doesDirectoryExist) infix 1 ?, ?? (?) :: Bool -> a -> a -> a (c ? t) e = if c then t else e (??) :: (Monad m) => m Bool -> m a -> m a -> m a (??) = liftM3 (?) main = do print $ 1>2 ? 1 $ 2 print =<< fileType "foo" fileType :: String -> IO String fileType name = doesDirectoryExist name ?? return "dir" $ doesFileExist name ?? return "file" $ return "nothing" -- Dean Keith Wansbrough wrote:

...

Not with the syntactic sugar of 'if'. But you can write [warning: untested code ahead]

ifM :: IO Bool -> IO a -> IO a -> IO a ifM test yes no = do b <- test if b then yes else no

There is a little trick that allows you to "sort-of" get the syntactic sugar. It goes like this [warning: also untested code]:

data Then_ = Then_ data Else_ = Else_ then_ = Then_ else_ = Else_

ifM :: IO Bool -> Then_ -> IO a -> Else_ -> IO a -> IO a ifM test Then_ yes Else_ no = do b <- test if b then yes else no

and then you can write

ifM (doesDirectoryExist f) then_ (return "dir") else_ (ifM (doesFileExist f) then_ (return "file") else_ (return "nothing"))

Note that this doesn't save you any parentheses, sadly, although there may be tricky ways to do that.

References:

LATOS uses this syntax; see http://www.dsse.ecs.soton.ac.uk/techreports/97-1.html [Pieter H. Hartel, 1997, LATOS - A Lightweight Animation Tool for Operational Semantics]

http://www.eecs.usma.edu/Personnel/okasaki/pubs.html#hw02 [Chris Okasaki, Haskell Workshop 2002, Techniques for embedding postfix languages in Haskell]

Enjoy!

--KW 8-) -- Keith Wansbrough http://www.cl.cam.ac.uk/users/kw217/ University of Cambridge Computer Laboratory.