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?

score 69 · Answer 1 · edited Oct 02 '21 at 13:58

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.

Should probably be citing the [haskell2010 report](http://www.haskell.org/onlinereport/haskell2010/haskellch2.html#x7-180002.4) instead of the haskell98 report these days (although in this case they say the same thing, as far as I can see). — Ben Millwood, May 11 '12 at 10:00
FWIW, tryhaskell.org currently gives a lexical error for trying to use one of the mathematical bracketing symbols, e.g. `let a ⟬ b = 1` — rampion, May 14 '19 at 13:58

score 35 · Answer 2 · edited Mar 10 '20 at 00:54

35

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."

edited Mar 10 '20 at 00:54

Sridhar Ratnakumar

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answered May 11 '12 at 08:53

dave4420

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Interesting that you can use arbitrary Unicode. So, for instance, λ or ⊗ would be valid Haskell operators? – Chris Taylor May 11 '12 at 08:57
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No, `λ` is a Unicode letter, not a Unicode symbol or a Unicode punctuation character. So you can't use it as part of an operator name (but you can use it as part of an ordinary identifier). – dave4420 May 11 '12 at 08:59
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I expect you could use `⊗` as a Haskell operator, but I don't know for sure. – dave4420 May 11 '12 at 09:01
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You can. Its `generalCategory` is `MathSymbol` (just to make sure, I actually defined an operator `(⊗)` in ghci, and it was accepted). – Daniel Fischer May 11 '12 at 09:19

Peter Hall · Accepted Answer · 2019-08-30T13:42:29.320

What I was looking for was the complete list of characters. Based on the other answers, the full list is;

Unicode Punctuation:

Unicode Symbols:

But excluding the following characters with special meaning in Haskell:

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

A : is only permitted as the first character of the operator, and denotes a constructor (see An operator symbol starting with a colon is a constructor).

This is absolutely insane! It is great that using any Unicode symbols is possible, but unfortunately they are usually very hard to type on current keyboards. — Qqwy, Jul 08 '16 at 14:18
@Qqwy - Haskell is designed for use in a literate programming environment. You could be producing a document designed for typesetting with code fragments, and have those code fragments actually executable in your source document. The ability to define unicode operators is invaluable for that purpose. — Jules, Apr 11 '17 at 03:07

What characters are permitted for Haskell operators?

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