Expressions And Operators: Comparisons

Hack provides comparison operators. They operate on two operands and return bool.

• <, which represents less-than
• >, which represents greater-than
• <=, which represents less-than-or-equal-to
• >=, which represents greater-than-or-equal-to

The type of the result is bool.

Comparison operators are typically used with numbers:

1 < 2;     // true
1.0 <= 1.5; // true

However, comparisons also work on strings. This uses a lexicographic ordering unless both strings are numeric, in which case the numeric values are used:

"a" < "b";   // true
"ab" < "b";  // true

"01" < "1";  // false (1 == 1)

The Spaceship Operator

Often referred to as the spaceship operator, the binary operator <=> compares the values of its operands and returns an int result. If the left-hand value is less than the right-hand value, the result is some unspecified negative value; else, if the left-hand value is greater than the right-hand value, the result is some unspecified positive value; otherwise, the values are equal and the result is zero. For example:

1 <=> 1;         // 0; equal
1 <=> 2;         // negative; 1 < 2
2 <=> 1;         // positive; 2 > 1

"a" <=> "a";     // 0; same length and content
"a" <=> "b";     // negative; a is lower than b in the collating sequence
"b" <=> "a";     // positive; b is higher than a in the collating sequence
"a" <=> "A";     // positive; lowercase a is higher than uppercase A

"a" <=> "aa";    // negative; same leading part, but a is shorter than aa
"aa" <=> "a";    // positive; same leading part, but aa is longer than a
"aa" <=> "aa";   // 0; same length and content