[Haskell-cafe] What is the meaning of tilde ("~") symbol

Hi, I'm reading the following subject : http://www.haskell.org/pipermail/haskell-cafe/2007-July/028227.html In the sample code, we can see : instance ReadAsAnyOf () ex where readAsAnyOf ~() = mzero And, I've search the meaning of the symbol "~", but I've found nothing about this (note that's not easy to search "~" on google ...) Could you give me a link about this, or explain me it ? Thx in advance.