Proposal #229 specifies that operators can occur in one of four contexts: prefix (like a +1), suffix (like a+ 1), tight infix (like a+1), and loose infix (like a + 1). Various constructs steal syntax from prefix and suffix occurrences of operators (examples below). This proposal suggests warnings on prefix or suffix uses of operators, so that Haskellers are forewarned that the syntax might be stolen in the future.

There are currently a number of symbols which change meaning (with certain extensions enabled) depending on whether they occur prefix, suffix, or infix (adapted from #229):

Operator	Occurrence	Meaning override
!, ~	prefix	strictness annotation in types, bang/lazy pattern in term-level patterns
$, $$	prefix	untyped/typed Template Haskell splice
@	prefix	type application
@	tight infix	as-pattern
@	suffix	parse error
-	prefix	negation
%	prefix	syntax a %1 -> b is a linear function
#	prefix	overloaded label
#	suffix	the "magic hash"

I'm confident that, in time, we will want to add to this list. So we should start warning users not to write new code with prefix/suffix occurrences of operators. (Note that the only operator with a special tight infix meaning is @, which cannot be used as a user-definable operator.)

The very concrete motivation for this proposal is a question on a ticket about the implementation of the syntax for linear types. The question is how GHC should treat e.g. Int %1 -> Int if -XLinearTypes is not enabled. GHC could suggest enabling -XLinearTypes, or it could complain that no type operator % is in scope. If we were more confident that users would avoid prefix uses of %, then it would be a clearer decision of what to do: we should just suggest enabling -XLinearTypes.

Introduce a new warning -Woperator-spacing that will be reported whenever an operator is used in either a prefix or suffix position, in either types or terms.

The new warning will be part of the -Wall and -Wcompat warning groups, but not on by default.

The following will warn:

a +b
b- a
c *(4)

The following will not warn:

a + b
b-a
c*(4)

This new warning will encourage users to avoid prefix and suffix uses of binary operators, making future attempts to steal their syntax easier.

Prefix and suffix uses of operators already look strange (to me), and so this is probably a style improvement, anyway.

Note that the stealing of prefix and suffix syntax has been going on for a long time in GHC, such as for Template Haskell and the -XMagicHash extension.

There will be a small maintenance burden.

Some users may find their preferred style is now warned against.

Do nothing. But doing nothing means more debates later when we need to steal more syntax.

Should this warning be extended to operator-like syntax, like ::, <-, and ->? This non-exhaustive list includes syntax that operates something like binary operators, but are not considered binary operators in Haskell's syntax. It is a free choice to whether this proposal should extend to these. I have decided not to do so, in thinking that these key bits of syntax are less likely to be stolen. However, it may be sensible to include them in the warning.