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

6 Jan 2009

      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