What characters are allowed for Haskell statements?

Question

Is there somewhere a complete list of valid characters or a rule that defines what can be used in an identifier or statement?

Answer 1

What I was looking for was a complete list of characters. Based on other answers, a complete list:

Unicode Punctuation:

• http://www.fileformat.info/info/unicode/category/Pc/list.htm
• http://www.fileformat.info/info/unicode/category/Pd/list.htm
• http://www.fileformat.info/info/unicode/category/Pe/list.htm
• http://www.fileformat.info/info/unicode/category/Pf/list.htm
• http://www.fileformat.info/info/unicode/category/Pi/list.htm
• http://www.fileformat.info/info/unicode/category/Po/list.htm
• http://www.fileformat.info/info/unicode/category/Ps/list.htm

Unicode characters:

• http://www.fileformat.info/info/unicode/category/Sc/list.htm
• http://www.fileformat.info/info/unicode/category/Sk/list.htm
• http://www.fileformat.info/info/unicode/category/Sm/list.htm
• http://www.fileformat.info/info/unicode/category/So/list.htm

But excluding the following characters with special meaning in Haskell:

(),;[]'{}_:"' 

A : allowed only as the first character of an operator and denotes a constructor (see The operator symbol starting with a colon is a constructor ).

Answer 2

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

a | b 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 

Thus, characters are ASCII characters or Unicode characters, with the exception of characters in special | _ | : | " | ' special | _ | : | " | ' special | _ | : | " | ' which are reserved.

This means that the following characters cannot be used: | , ; [ ] ' { } _ : " ' | , ; [ ] ' { } _ : " '

In a few paragraphs below, the report contains a complete definition of Haskell statements:

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

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

• An operator character starting with a colon is a constructor.
• An operator character starting with any other character is a regular identifier.

Note that the colon itself, ":" is reserved exclusively for use as a Haskell List Constructor; this makes its treatment consistent with other parts of the list syntax, such as "[]" and "[a, b]".

Except for the special syntax for negating a prefix, all infix operators, although each infix operator can be used in a section to obtain partially applicable operators (see section 3.5). All standard Infix operators are just predefined characters and can be a rebound.

Answer 3

From Haskell 2010 Report §2.4 :

Operator characters are formed from one or more character characters ...

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