[Haskell-beginners] Syntax of Complex

Hi, The Data.Complex package defines the new data type 'Complex' as ---------- data Complex a = !a :+ !a ------- Where ':+' is an infix operator. I don't however understand the usage of '!' in front of the type variable 'a'. What exactly is the purpose of '!' ? Any help would be appreciated. Thanks, Shishir Srivastava