Re: [Haskell-cafe] Can I destructive rebind a local variable in haskell?

2009/1/6 Luke Palmer :

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

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'''?