Hi Luke Thank you ! I got it :) Best Regards Chunye Wang ________________________________ From: ext Luke Palmer [mailto:lrpalmer@gmail.com] Sent: Tuesday, January 06, 2009 3:44 PM To: Wang, Chunye (NSN - CN/Beijing) Cc: Haskell-Cafe@haskell.org Subject: Re: [Haskell-cafe] Can I destructive rebind a local variable in haskell? 2009/1/6 Wang, Chunye (NSN - CN/Beijing) Dear haskeller, Can I destructive rebind a local variable like this import System.Directory test filename = do is_dir <- doesDirectoryExist filename let filename = if not is_dir then filename else filename Nope. The "filename" on the right side of the = is the same as the "filename" on the left, so you're making an infinite loop, the same way: let x = x in x is an infinite loop. However you can make a new name as you are trying, you just can't reference the old one. e.g.: let filename = 42 Here would be just fine. However, for cases like this, it is useful that single quote is a valid identifier, so you can say: let filename' = if not is_dir then filename else filename (read "filename prime") Luke

Hi Evan,

You can also reuse the name exactly by using bind+return instead of let: test filename = do is_dir <- doesDirectoryExist filename filename <- return $ if not is_dir then filename else filename

I'm not a huge fan of the prime thing because it's tiny and easy to miss and if you forget it you probably won't get a type error, you'll just get a bug, possibly a subtle one. Besides, what's the next step? filename''? filename'''?

Nice. Good solution. ``imperative style'' is not a bad idea when I'm not used to the ``pure functional style'' E.g. filename <- return $ combine filename "Makefile" Similar to the other imperative language filename = joinPath(filename,"Makefile") and I often write similar code in EmacsLisp also. It is not so buggy, because we live in "imperitive programming" for long time Best Regards Chunye Wang