6.2.21. Lexical negation


Detect if the minus sign stands for negation during lexical analysis by checking for the surrounding whitespace.

In Haskell 2010, the minus sign stands for negation when it has no left-hand side. Consider x = - 5 and y = 2 - 5. In x, there’s no expression between the = and -, so the minus stands for negation, whereas in y, there’s 2 to the left of the minus, therefore it stands for subtraction.

This leads to certain syntactic anomalies:

  • (% x) is an operator section for any operator (%) except for (-). (- x) is negated x rather than the right operator section of subtraction. Consequently, it is impossible to write such a section, and users are advised to write (subtract x) instead.
  • Negative numbers must be parenthesized when they appear in function argument position. f (-5) is correct, whereas f -5 is parsed as (-) f 5.

The latter issue is partly mitigated by NegativeLiterals. When it is enabled, -5 is parsed as negative 5 regardless of context, so f -5 works as expected. However, it only applies to literals, so f -x or f -(a*2) are still parsed as subtraction.

With LexicalNegation, both anomalies are resolved:

  • (% x) is an operator section for any operator (%), no exceptions, as long as there’s whitespace between % and x.
  • In f -x, the -x is parsed as the negation of x for any syntactically atomic expression x (variable, literal, or parenthesized expression).
  • The prefix - binds tighter than any infix operator. -a % b is parsed as (-a) % b regardless of the fixity of %.

This means that (- x) is the right operator section of subtraction, whereas (-x) is the negation of x. Note that these expressions will often have different types ((- x) might have type Int -> Int while (-x) will have type Int), and so users mistaking one for the other will likely get a compile error.

Under LexicalNegation, negated literals are desugared without negate. That is, -123 stands for fromInteger (-123) rather than negate (fromInteger 123). This makes LexicalNegation a valid replacement for NegativeLiterals.