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

[kg]

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.