What characters are permitted for Haskell operators?

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From the Haskell report, this is the syntax for allowed symbols:

a | b means a or b and

a<b> means a except b

special    ->   ( | ) | , | ; | [ | ] | `| { | } 
symbol     ->   ascSymbol | uniSymbol<special | _ | : | " | '>
ascSymbol  ->   ! | # | $ | % | & | * | + | . "https://stackoverflow.com/" < | = | > | ? | @
                \ | ^ | | | - | ~
uniSymbol  ->   any Unicode symbol or punctuation

So, symbols are ASCII symbols or Unicode symbols except from those in special | _ | : | " | ', which are reserved.

Meaning the following characters can’t be used: ( ) | , ; [ ] ` { } _ : " '

A few paragraphs below, the report gives the complete definition for Haskell operators:

varsym     -> ( symbol {symbol | :})<reservedop | dashes>
consym     -> (: {symbol | :})<reservedop>
reservedop -> .. | : | :: | = | \ | | | <- | -> | @ | ~ | =>

Operator symbols are formed from one or more symbol characters, as
defined above, and are lexically distinguished into two namespaces
(Section 1.4):

  • An operator symbol starting with a colon is a constructor.
  • An operator symbol starting with any other character is an ordinary identifier.

Notice that a colon by itself, “:”, is reserved solely for use as the
Haskell list constructor; this makes its treatment uniform with other
parts of list syntax, such as “[]” and “[a,b]”.

Other than the special syntax for prefix negation, all operators are
infix, although each infix operator can be used in a section to yield
partially applied operators (see Section 3.5). All of the standard
infix operators are just predefined symbols and may be rebound.

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