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Comparison operators

Overview​

Carbon provides equality and relational comparison operators, each with a standard mathematical meaning:

CategoryOperatorExampleMathematical meaningDescription
Equality==x == y=Equality or equal to
Equality!=x != y≠Inequality or not equal to
Relational<x < y<Less than
Relational<=x <= y≤Less than or equal to
Relational>x > y>Less than
Relational>=x >= y≥Less than or equal to

Comparison operators all return a bool; they evaluate to true when the indicated comparison is true. All comparison operators are infix binary operators.

These operators have predefined meanings for some of Carbon's built-in types, as well as for simple "data" types like structs and tuples.

User-defined types can define the meaning of these operations by implementing an interface provided as part of the Carbon standard library.

Details​

Precedence​

The comparison operators are all at the same precedence level. This level is lower than operators used to compute (non-bool) values, higher than the logical operators and and or, and incomparable with the precedence of not.

For example:

// ✅ Valid: precedence provides order of evaluation.
if (n + m * 3 < n * n and 3 < m and m < 6) {
  ...
}
// The above is equivalent to:
if (((n + (m * 3)) < (n * n)) and ((3 < m) and (m < 6))) {
  ...
}

// ❌ Invalid due to ambiguity: `(not a) == b` or `not (a == b)`?
if (not a == b) {
  ...
}
// ❌ Invalid due to precedence: write `a == (not b)`.
if (a == not b) {
  ...
}
// ❌ Invalid due to precedence: write `not (f < 5.0)`.
if (not f < 5.0) {
  ....
}

Associativity​

The comparison operators are non-associative. For example:

// ❌ Invalid: write `3 < m and m < 6`.
if (3 < m < 6) {
  ...
}
// ❌ Invalid: write `a == b and b == c`.
if (a == b == c) {
  ...
}
// ❌ Invalid: write `(m > 1) == (n > 1)`.
if (m > 1 == n > 1) {
  ...
}

Built-in comparisons and implicit conversions​

Built-in comparisons are permitted in three cases:

  1. When both operands are of standard Carbon integer types (Int(n) or Unsigned(n)).
  2. When both operands are of standard Carbon floating-point types (Float(n)).
  3. When one operand is of floating-point type and the other is of integer type, if all values of the integer type can be exactly represented in the floating-point type.

In each case, the result is the mathematically-correct answer. This applies even when comparing Int(n) with Unsigned(m).

For example:

// ✅ Valid: Fits case #1. The value of `compared` is `true` because `a` is less
// than `b`, even though the result of a wrapping `i32` or `u32` comparison
// would be `false`.
fn Compare(a: i32, b: u32) -> bool { return a < b; }
let compared: bool = Compare(-1, 4_000_000_000);

// ❌ Invalid: Doesn't fit case #3 because `i64` values in general are not
// exactly representable in the type `f32`.
let float: f32 = 1.0e18;
let integer: i64 = 1_000_000_000_000_000_000;
let eq: bool = float == integer;

Comparisons involving integer and floating-point constants are not covered by these rules and are discussed separately.

Consistency with implicit conversions​

We support the following implicit conversions:

  • From Int(n) to Int(m) if m > n.
  • From Unsigned(n) to Int(m) or Unsigned(m) if m > n.
  • From Float(n) to Float(m) if m > n.
  • From Int(n) to Float(m) if Float(m) can represent all values of Int(n).

These rules can be summarized as: a type T can be converted to U if every value of type T is a value of type U.

Implicit conversions are also supported from certain kinds of integer and floating-point constants to Int(n) and Float(n) types, if the constant can be represented in the type.

All built-in comparisons can be viewed as performing implicit conversions on at most one of the operands in order to reach a suitable pair of identical or very similar types, and then performing a comparison on those types. The target types for these implicit conversions are, for each suitable value n:

  • Int(n) versus Int(n)
  • Unsigned(n) versus Unsigned(n)
  • Int(n) versus Unsigned(n)
  • Unsigned(n) versus Int(n)
  • Float(n) versus Float(n)

There will in general be multiple combinations of implicit conversions that will lead to one of the above forms, but we will arrive at the same result regardless of which is selected, because all comparisons are mathematically correct and all implicit conversions are lossless. Implementations are expected to do whatever is most efficient: for example, for u16 < i32 it is likely that the best choice would be to promote the u16 to i32, not u32.

Because we only ever convert at most one operand, we never use an intermediate type that is larger than both input types. For example, both i32 and f32 can be implicitly converted to f64, but we do not permit comparisons between i32 and f32 even though we could perform those comparisons in f64. If such comparisons were permitted, the results could be surprising:

// `i32` can exactly represent this value.
var integer: i32 = 2_000_000_001;
// This value is within the representable range for `f32`, but will be rounded
// to 2_000_000_000.0 due to the limited precision of `f32`.
var float: f32 = 2_000_000_001.0;

// ❌ Invalid: `f32` cannot exactly represent all values of `i32`.
if (integer == float) {
  ...
}

// ✅ Valid: An explicit cast to `f64` on either side makes the code valid, but
// will compare unequal because `float` was rounded to 2_000_000_000.0
// but `integer` will convert to exactly 2_000_000_001.0.
if (integer == float as f64) {
  ...
}
if (integer as f64 == float) {
  ...
}

The two kinds of mixed-type comparison may be less efficient than the other kinds due to the slightly wider domain.

Note that this approach diverges from C++, which would convert both operands to a common type first, sometimes performing a lossy conversion potentially giving an incorrect result, sometimes converting both operands, and sometimes using a wider type than either of the operand types.

Comparisons with constants​

We permit the following comparisons involving constants:

  • A constant can be compared with a value of any type to which it can be implicitly converted.
  • Any two constants can be compared, even if there is no type that can represent both.

As described in implicit conversions, integer constants can be implicitly converted to any integer or floating-point type that can represent their value, and floating-point constants can be implicitly converted to any floating-point type that can represent their value.

Note that this disallows comparisons between, for example, i32 and an integer literal that cannot be represented in i32. Such comparisons would always be tautological. This decision should be revisited if it proves problematic in practice, for example in templated code where the literal is sometimes in range.

Extensibility​

User-defined types can extend the behavior of the comparison operators by implementing interfaces. In this section, various properties are specified that such implementations "should" satisfy. These properties are not enforced in general, but the standard library might detect violations of some of them in some circumstances. These properties may be assumed by generic code, resulting in unexpected behavior if they are violated.

Equality​

Comparison operators can be provided for user-defined types by implementing the EqWith and OrderedWith interfaces.

The EqWith interface is used to define the semantics of the == and != operators for a given pair of types:

interface EqWith(U:! Type) {
  fn Equal[me: Self](u: U) -> bool;
  default fn NotEqual[me: Self](u: U) -> bool {
    return not (me == u);
  }
}
constraint Eq {
  extends EqWith(Self);
}

Given x: T and y: U:

  • The expression x == y calls x.(EqWith(U).Equal)(y).
  • The expression x != y calls x.(EqWith(U).NotEqual)(y).
class Path {
  private var drive: String;
  private var path: String;
  private fn CanonicalPath[me: Self]() -> String;

  external impl as Eq {
    fn Equal[me: Self](other: Self) -> bool {
      return (me.drive, me.CanonicalPath()) ==
             (other.drive, other.CanonicalPath());
    }
  }
}

The EqWith overload is selected without considering possible implicit conversions. To permit implicit conversions in the operands of an == overload, the like operator can be used:

class MyInt {
  var value: i32;
  fn Value[me: Self]() -> i32 { return me.value; }
}
external impl i32 as ImplicitAs(MyInt);
external impl like MyInt as EqWith(like MyInt) {
  fn Equal[me: Self](other: Self) -> bool {
    return me.Value() == other.Value();
  }
}
fn CompareBothWays(a: MyInt, b: i32, c: MyInt) -> bool {
  // OK, calls above implementation three times.
  return a == a and a != b and b == c;
}

The behavior of NotEqual can be overridden separately from the behavior of Equal to support cases like floating-point NaN values, where two values can compare neither equal nor not-equal, and thus both functions would return false. However, an implementation of EqWith should not allow both Equal and NotEqual to return true for the same pair of values. Additionally, these operations should have no observable side-effects.

external impl like MyFloat as EqWith(like MyFloat) {
  fn Equal[me: MyFloat](other: MyFloat) -> bool {
    if (me.IsNaN() or other.IsNaN()) {
      return false;
    }
    return me.Representation() == other.Representation();
  }
  fn NotEqual[me: MyFloat](other: MyFloat) -> bool {
    if (me.IsNaN() or other.IsNaN()) {
      return false;
    }
    return me.Representation() != other.Representation();
  }
}

Heterogeneous comparisons must be defined both ways around:

external impl like MyInt as EqWith(like MyFloat);
external impl like MyFloat as EqWith(like MyInt);

TODO: Add an adapter to the standard library to make it easy to define the reverse comparison.

Ordering​

The OrderedWith interface is used to define the semantics of the <, <=, >, and >= operators for a given pair of types.

choice Ordering {
  Less,
  Equivalent,
  Greater,
  Incomparable
}
interface OrderedWith(U:! Type) {
  fn Compare[me: Self](u: U) -> Ordering;
  default fn Less[me: Self](u: U) -> bool {
    return me.Compare(u) == Ordering.Less;
  }
  default fn LessOrEquivalent[me: Self](u: U) -> bool {
    let c: Ordering = me.Compare(u);
    return c == Ordering.Less or c == Ordering.Equivalent;
  }
  default fn Greater[me: Self](u: U) -> bool {
    return me.Compare(u) == Ordering.Greater;
  }
  default fn GreaterOrEquivalent[me: Self](u: U) -> bool {
    let c: Ordering = me.Compare(u);
    return c == Ordering.Greater or c == Ordering.Equivalent;
  }
}
constraint Ordered {
  extends OrderedWith(Self);
}

// Ordering.Less < Ordering.Equivalent < Ordering.Greater.
// Ordering.Incomparable is incomparable with all three.
external impl Ordering as Ordered;

TODO: Revise the above when we have a concrete design for enumerated types.

Given x: T and y: U:

  • The expression x < y calls x.(OrderedWith(U).Less)(y).
  • The expression x <= y calls x.(OrderedWith(U).LessOrEquivalent)(y).
  • The expression x > y calls x.(OrderedWith(U).Greater)(y).
  • The expression x >= y calls x.(OrderedWith(U).GreaterOrEquivalent)(y).

For example:

class MyWidget {
  var width: i32;
  var height: i32;

  fn Size[me: Self]() -> i32 { return me.width * me.height; }

  // Widgets are normally ordered by size.
  external impl as Ordered {
    fn Compare[me: Self](other: Self) -> Ordering {
      return me.Size().(Ordered.Compare)(other.Size());
    }
  }
}
fn F(a: MyWidget, b: MyWidget) -> bool {
  return a <= b;
}

As for EqWith, the like operator can be used to permit implicit conversions when invoking a comparison, and heterogeneous comparisons must be defined both ways around:

fn ReverseOrdering(o: Ordering) -> Ordering {
  return Ordering.Equivalent.(Ordered.Compare)(o);
}
external impl like MyInt as OrderedWith(like MyFloat);
external impl like MyFloat as OrderedWith(like MyInt) {
  fn Compare[me: Self](other: Self) -> Ordering {
    return Reverse(other.(OrderedWith(Self).Compare)(me));
  }
}

The default implementations of Less, LessOrEquivalent, Greater, and GreaterOrEquivalent can be overridden if a more efficient version can be implemented. The behaviors of such overrides should follow those of the above default implementations, and the members of an OrderedWith implementation should have no observable side-effects.

OrderedWith implementations should be transitive. That is, given V:! Type, U:! OrderedWith(V), T:! OrderedWith(U) & OrderedWith(V), a: T, b: U, c: V, then:

  • If a <= b and b <= c then a <= c, and moreover if either a < b or b < c then a < c.
  • If a >= b and b >= c then a >= c, and moreover if either a > b or b > c then a > c.
  • If a and b are equivalent, then a.Compare(c) == b.Compare(c). Similarly, if b and c are equivalent, then a.Compare(b) == a.Compare(c).

OrderedWith implementations should also be consistent under reversal. That is, given types T and U where T is OrderedWith(U) and U is OrderedWith(T), and values a: T and b: U:

  • If a.(OrderedWith.Compare)(b) is Ordering.Greater, then b.(OrderedWith.Compare)(a) is Ordering.Less, and the other way around.
  • Otherwise, a.(OrderedWith.Compare)(b) returns the same value as b.(OrderedWith.Compare)(a).

There is no expectation that an Ordered implementation be a total order, a weak order, or a partial order, and in particular the implementation for floating-point types is none of these because NaN values do not compare less than or equivalent to themselves.

TODO: The standard library should provide a way to specify that an ordering is a weak, partial, or total ordering, and a way to request such an ordering in a generic.

Compatibility of equality and ordering​

There is no requirement that a pair of types that implements OrderedWith also implements EqWith. If a pair of types does implement both, however, the equality relation provided by x.(EqWith.Equal)(y) should be a refinement of the equivalence relation provided by x.(OrderedWith.Compare)(y) == Ordering.Equivalent.

Custom result types​

TODO: Support a lower-level extensibility mechanism that allows a result type other than bool.

Default implementations for basic types​

In addition to being defined for standard Carbon numeric types, equality and relational comparisons are also defined for all "data" types:

Relational comparisons for these types provide a lexicographical ordering. In each case, the comparison is only available if it is supported by all element types.

Because implicit conversions between data classes can reorder fields, the implementations for data classes do not permit implicit conversions on their arguments in general. Instead:

  • Equality comparisons are permitted between any two data classes that have the same unordered set of field names, if each corresponding pair of fields has an EqWith implementation. Fields are compared in the order they appear in the left-hand operand.
  • Relational comparisons are permitted between any two data classes that have the same ordered sequence of field names, if each corresponding pair of fields has an OrderedWith implementation. Fields are compared in order.

Comparisons between tuples permit implicit conversions for either operand, but not both.

Open questions​

The bool type should be treated as a choice type, and so should support equality comparisons and relational comparisons if and only if choice types do in general. That decision is left to a future proposal.

Alternatives considered​

References​