What characters are permitted for Haskell operators?

Question

Is there a complete list of allowed characters somewhere, or a rule that determines what can be used in an identifier vs an operator?

Answer 1

From the Haskell 2010 Report §2.4:

Operator symbols are formed from one or more symbol characters...

§2.2 defines symbol characters as being any of !#$%&*+./<=>?@\^|-~: or "any [non-ascii] Unicode symbol or punctuation".

NOTE: User-defined operators cannot begin with a : as, quoting the language report, "An operator symbol starting with a colon is a constructor."