Associativity

The Associativity of an infix operator can be either none, left or right. It describes how the parser should handle expressions in which the same operator is applied multiple times.
infix operator +- { associativity none, precedence 120 }
// ^^^^
10 +- 20 +- 30
Depending on the associativity, the expression can be evaluated in different ways:
Associativity
Desugared Expression
none
error
left
(10 +- 20) +- 30
right
10 +- (20 +- 30)
If the associativity of the +- operator is set to none, an expression like the above is not allowed, and parenthesis have to be placed explicitly. Otherwise, the compiler will produce a Syntax Error diagnostic message.
It is not allowed to assign an associativity to prefix or postfix operators.