Is there a complete list of allowed characters somewhere, or a rule that determines what can be used in an identifier vs an operator?
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 -> ! | # | $ | % | & | * | + | . | / | < | = | > | ? | @
\ | ^ | | | - | ~
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.