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implicit_conversions.md

implicit_conversions.md 12 KB
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Implicit conversions

Table of contents

Overview

When an expression appears in a context in which an expression of a specific type is expected, the expression is implicitly converted to that type if possible.

For built-in types, implicit conversions are permitted when:

  • The conversion is lossless: every possible value for the source expression converts to a distinct value in the target type.
  • The conversion is semantics-preserving: corresponding values in the source and destination type have the same abstract meaning.

These rules aim to ensure that implicit conversions are unsurprising: the value that is provided as the operand of an operation should match how that operation interprets the value, because the identity and abstract meaning of the value are preserved by any implicit conversions that are applied.

It is possible for user-defined types to extend the set of valid implicit conversions. Such extensions are expected to also follow these rules.

Properties of implicit conversions

Lossless

We expect implicit conversion to never lose information: if two values are distinguishable before the conversion, they should generally be distinguishable after the conversion. It should be possible to define a conversion in the opposite direction that restores the original value, but such a conversion is not expected to be provided in general, and might be computationally expensive.

Because an implicit conversion is converting from a narrower type to a wider type, implicit conversions do not necessarily preserve static information about the source value.

Semantics-preserving

We expect implicit conversions to preserve the meaning of converted values. The assessment of this criterion will necessarily be subjective, because the meanings of values generally live in the mind of the programmer rather than in the program text. However, the semantic interpretation is expected to be consistent from one conversion to another, so we can provide a test: if multiple paths of implicit conversions from a type A to a type B exist, and the same value of type A would convert to different values of type B along different paths, then at least one of those conversions must not be semantics-preserving.

A semantics-preserving conversion does not necessarily preserve the meaning of particular syntax when applied to the value. The same syntax may map to different operations in the new type. For example, division may mean different things in integer and floating-point types, and member access may find different members in a derived class pointer versus in a base class pointer.

Examples

Conversion from i32 to Vector(int) by forming a vector of N zeroes is lossless but not semantics-preserving.

Conversion from i32 to f32 by rounding to the nearest representable value is semantics-preserving but not lossless.

Conversion from String to StringView is lossless, because we can compute the String value from the StringView value, and semantics-preserving because the string value denoted is the same. Conversion in the other direction may or may not be semantics-preserving depending on whether we consider the address to be a salient part of a StringView's value.

Built-in types

Data types

The following implicit numeric conversions are available:

  • iN or uN -> iM if M > N
  • uN -> uM if M > N
  • fN -> fM if M > N
  • iN or uN -> fM if every value of type iN or uN can be represeted in fM:
    • i12 or u11 (or smaller) -> f16
    • i25 or u24 (or smaller) -> f32
    • i54 or u53 (or smaller) -> f64
    • i65 or u64 (or smaller) -> f80 (x86 only)
    • i114 or u113 (or smaller) -> f128 (if available)
    • i238 or u237 (or smaller) -> f256 (if available)

In each case, the numerical value is the same before and after the conversion. An integer zero is translated into a floating-point positive zero.

An integer constant can be implicitly converted to any type iM, uM, or fM in which that value can be exactly represented. A floating-point constant can be implicitly converted to any type fM in which that value is between the least representable finite value and the greatest representable finite value (inclusive), and does not fall exactly half-way between two representable values, and converts to the nearest representable finite value.

The above conversions are also precisely those that C++ considers non-narrowing, except:

  • Carbon also permits integer to floating-point conversions in more cases. The most important of these is that Carbon permits i32 to be implicitly converted to f64. Lossy conversions, such as from i32 to f32, are not permitted.

  • What Carbon considers to be an integer constant or floating-point constant may differ from what C++ considers to be a constant expression.

    Note: We have not yet decided what will qualify as a constant in this context, but it will include at least integer and floating-point literals, with optional enclosing parentheses. It is possible that such constants will have singleton types; see issue #508.

Equivalent types

The following conversion is available:

  • T -> U if T is equivalent to U

Two types are equivalent if they can be used interchangeably, implicitly: they have the same set of values with the same meaning and the same representation, with the same set of capabilities and constraints, where the only difference is how the type interprets operations on values of that type.

T is equivalent to U if:

  • T is the same type as U, or
  • T is the facet type U as SomeInterface, or
  • U is the facet type T as SomeInterface, or
  • T is A*, U is B*, and A is equivalent to B, or
  • for some type V, T is equivalent to V and V is equivalent to U.

Note: More type equivalence rules are expected to be added over time.

A prerequisite for types being equivalent is that they are compatible, and in particular that they have the same set of values and the same representation for those values. However, types being compatible does not imply that an implicit conversion, or even an explicit cast, between those types is necessarily valid. This is because the type of a value models not only the representation of the value but also the capabilities that a user of the value has to interact with the value. Two compatible types may expose different capabilities, such as the capability to mutate the object or to access its implementation details, and conversions between such types may require an explicit cast if the conversion is possible at all.

Pointer conversions

The following pointer conversion is available:

  • T* -> U* if T is a subtype of U.

T is a subtype of U if:

  • T is equivalent to U, as described above, or
  • T is equivalent to a class derived from a class equivalent to U.

Note: More type subtyping rules are expected to be added over time.

T* is not necessarily a subtype of U* even if T is a subtype of U. For example, we can convert Derived* to Base*, but cannot convert Derived** to Base** because that would allow storing a Derived2* into a Derived*:

abstract class Base {}
class Derived extends Base {}
class Derived2 extends Base {}
var d2: Derived2 = {};
var p: Derived*;
var q: Derived2* = &d2;
var r: Base** = &p;
// Bad: would store q to p.
*r = q;

Note: If we add const qualification, we could treat const T* as a subtype of const U* if T is a subtype of U, and could treat T as a subtype of const T.

Pointer conversion examples

With these classes:

base class C;
let F: auto = C as Hashable;
class D extends C;

These implicit pointer conversions are permitted:

  • D* -> C*: D is a subtype of C
  • F* -> C*: F is equivalent to C, so F is a subtype of C
  • C* -> F*: C is equivalent to F, so C is a subtype of F
  • F** -> C**: F is equivalent to C, so F* is equivalent to C*, so F* is a subtype of C*
  • D* -> F*: D is derived from C and C is equivalent to D, so D is a subtype of F

These implicit pointer conversions are disallowed:

  • C* -> D*: C is not a subtype of D
  • D** -> C**: D* is not a subtype of C*

Note that "equivalent to" means we can freely convert back and forwards; the difference in the types is just changing which operations are surfaced, not changing anything about the interpretation or switching between different abstractions. In contrast, "subtype of" permits conversion from a more specific type to a more general type, so the reverse conversion is not necessarily valid.

Type-of-types

A type T with type-of-type TT1 can be implicitly converted to the type-of-type TT2 if T satisfies the requirements of TT2.

Semantics

An implicit conversion of an expression E of type T to type U, when permitted, always has the same meaning as the explicit cast expression E as U. Moreover, such an implicit conversion is expected to exactly preserve the value. For example, (E as U) as T, if valid, should be expected to result in the same value as produced by E.

Note: The explicit cast expression syntax has not yet been decided. The use of E as T in this document is provisional.

Extensibility

Implicit conversions can be defined for user-defined types such as classes by implementing the ImplicitAs interface:

interface As(Dest:! Type) {
  fn Convert[me: Self]() -> Dest;
}
interface ImplicitAs(Dest:! Type) extends As(Dest) {}

When attempting to implicitly convert an expression x to type U, the expression is rewritten to x.(ImplicitAs(U).Convert)().

Note: The As interface is intended to be used as the implementation vehicle for explicit casts: x as U would be rewritten as x.(As(U).Convert)(). However, the explicit cast expression syntax has not yet been decided, so this rewrite is provisional.

Note that implicit conversions are not transitive. Even if an impl A as ImplicitAs(B) and an impl B as ImplicitAs(C) are both provided, an expression of type A cannot be implicitly converted to type C. Allowing transitivity would introduce the risk of ambiguity issues as code evolves and would in general require a search of a potentially unbounded set of intermediate types.

Alternatives considered

References