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Values, variables, and pointers

Table of contents

Values, objects, and expressions

Carbon has both abstract values and concrete objects. Carbon values are things like 42, true, and i32 (a type value). Carbon objects have storage where values can be read and written. Storage also allows taking the address of an object in memory in Carbon.

Both objects and values can be nested within each other. For example (true, true) is both a value and also contains two sub-values. When a two-tuple is stored somewhere, it is both a tuple-typed object and contains two subobjects.

These terms are important components in the describing the semantics of Carbon code, but they aren't sufficient. We also need to explicitly and precisely talk about the Carbon expressions that produce or reference values and objects. Categorizing the expressions themselves allows us to be more precise and articulate important differences not captured without looking at the expression itself.

Expression categories

There are three expression categories in Carbon:

  • Value expressions produce abstract, read-only values that cannot be modified or have their address taken.
  • Reference expressions refer to objects with storage where a value may be read or written and the object's address can be taken.
  • Initializing expressions require storage to be provided implicitly when evaluating the expression. The expression then initializes an object in that storage. These are used to model function returns, which can construct the returned value directly in the caller's storage.

Expressions in one category can be converted to any other category when needed. The primitive conversion steps used are:

These conversion steps combine to provide the transitive conversion table:

From: value reference initializing
to value == bind materialize + bind
to reference direct init + materialize == materialize
to initializing direct init copy init ==

Reference expressions formed through temporary materialization are called ephemeral reference expressions and have restrictions on how they are used. In contrast, reference expressions that refer to declared storage are called durable reference expressions. Beyond the restrictions on what is valid, there is no distinction in their behavior or semantics.

Value binding

We call forming a value expression from a reference expression value binding. This forms a value expression that will evaluate to the value of the object in the referenced storage of the reference expression. It may do this by eagerly reading that value into a machine register, lazily reading that value on-demand into a machine register, or in some other way modeling that abstract value.

See the value expressions section for more details on the semantics of value expressions.

Direct initialization

This is the first way we have of initializing storage of an object. There may not be storage for the source of this initialization, as the value expression used for the initialization may be in a machine register or simply be abstractly modeled like a source literal. A canonical example here is zeroing an object.

Copy initialization

This initializes storage for an object based on some other object which already has initialized storage. A classic example here are types which can be copied trivially and where this is implemented as a memcpy of their underlying bytes.

Temporary materialization

We use temporary materialization when we need to initialize an object by way of storage, but weren't provided dedicate storage and can simply bind the result to a value afterward.

Open question: The lifetimes of temporaries is not yet specified.

Binding patterns and local variables with let and var

Binding patterns introduce names that are value expressions by default and are called value bindings. This is the desired default for many pattern contexts, especially function parameters. Values are a good model for "input" function parameters which are the dominant and default style of function parameters:

fn Sum(x: i32, y: i32) -> i32 {
  // `x` and `y` are value expressions here. We can use their value, but not
  // modify them or take their address.
  return x + y;
}

Value bindings require the matched expression to be a value expression, converting it into one as necessary.

A variable pattern can be introduced with the var keyword to create an object with storage when matched. Every binding pattern name introduced within a variable pattern is called a variable binding and forms a durable reference expression to an object within the variable pattern's storage when used. Variable patterns require their matched expression to be an initializing expression and provide their storage to it to be initialized.

fn MutateThing(ptr: i64*);

fn Example() {
  // `1` starts as a value expression, which is what a `let` binding expects.
  let x: i64 = 1;

  // `2` also starts as a value expression, but the variable binding requires it
  // to be converted to an initializing expression by using the value `2` to
  // initialize the provided variable storage that `y` will refer to.
  var y: i64 = 2;

  // Allowed to take the address and mutate `y` as it is a durable reference
  // expression.
  MutateThing(&y);

  // ❌ This would be an error though due to trying to take the address of the
  // value expression `x`.
  MutateThing(&x);
}

Local variables

A local binding pattern can be introduced with either the let or var keyword. The let introducer begins a value pattern which works the same as the default patterns in other contexts. The var introducer immediately begins a variable pattern.

  • let identifier: ( expression | auto ) = value;
  • var identifier: ( expression | auto ) [ = value ];

These are just simple examples of binding patterns used directly in local declarations. Local let and var declarations build on Carbon's general pattern matching design. var declarations implicitly start off within a var pattern. let declarations introduce patterns that bind values by default, the same as function parameters and most other pattern contexts.

The general pattern matching model also allows nesting var sub-patterns within a larger pattern that defaults to matching values. For example, we can combine the two local declarations above into one destructuring declaration with an inner var pattern here:

fn DestructuringExample() {
  // Both `1` and `2` start as value expressions. The `x` binding directly
  // matches `1`. For `2`, the variable binding requires it to be converted to
  // an initializing expression by using the value `2` to initialize the
  // provided variable storage that `y` will refer to.
  let (x: i64, var y: i64) = (1, 2);

  // Just like above, we can take the address and mutate `y`:
  MutateThing(&y);

  // ❌ And this remains an error:
  MutateThing(&x);
}

If auto is used in place of the type for a local binding pattern, type inference is used to automatically determine the variable's type.

These local bindings introduce names scoped to the code block in which they occur, which will typically be marked by an open brace ({) and close brace (}).

Consuming function parameters

Just as part of a let binding can use a var prefix to become a variable pattern and bind names that will form reference expressions to the variable's storage, so can function parameters:

fn Consume(var x: SomeData) {
  // We can mutate and use variable that `x` refers to here.
}

This allows us to model an important special case of function inputs -- those that are consumed by the function, either through local processing or being moved into some persistent storage. Marking these in the pattern and thus signature of the function changes the expression category required for arguments in the caller. These arguments are required to be initializing expressions, potentially being converted into such an expression if necessary, that directly initialize storage dedicated-to and owned-by the function parameter.

This pattern serves the same purpose as C++'s pass-by-value when used with types that have non-trivial resources attached to pass ownership into the function and consume the resource. But rather than that being the seeming default, Carbon makes this a use case that requires a special marking on the declaration.

Reference expressions

Reference expressions refer to objects with storage where a value may be read or written and the object's address can be taken.

Calling a method on a reference expression where the method's self parameter has an addr specifier can always implicitly take the address of the referred-to object. This address is passed as a pointer to the self parameter for such methods.

There are two sub-categories of reference expressions: durable and ephemeral. These refine the lifetime of the underlying storage and provide safety restrictions reflecting that lifetime.

Durable reference expressions

Durable reference expressions are those where the object's storage outlives the full expression and the address could be meaningfully propagated out of it as well.

There are two contexts that require a durable reference expression in Carbon:

  • Assignment statements require the left-hand-side of the = to be a durable reference. This stronger requirement is enforced before the expression is rewritten to dispatch into the Carbon.Assign.Op interface method.
  • Address-of expressions require their operand to be a durable reference and compute the address of the referenced object.

There are several kinds of expressions that produce durable references in Carbon:

  • Names of objects introduced with a variable binding: x
  • Dereferenced pointers: *p
  • Names of subobjects through member access to some other durable reference expression: x.member or p->member
  • Indexing into a type similar to C++'s std::span that implements IndirectIndexWith, or indexing into any type with a durable reference expression such as local_array[i].

Durable reference expressions can only be produced directly by one of these expressions. They are never produced by converting one of the other expression categories into a reference expression.

Ephemeral reference expressions

We call the reference expressions formed through temporary materialization ephemeral reference expressions. They still refer to an object with storage, but it may be storage that will not outlive the full expression. Because the storage is only temporary, we impose restrictions on where these reference expressions can be used: their address can only be taken implicitly as part of a method call whose self parameter is marked with the addr specifier.

Future work: The current design allows directly requiring an ephemeral reference for addr-methods because this replicates the flexibility in C++ -- very few C++ methods are L-value-ref-qualified which would have a similar effect to addr-methods requiring a durable reference expression. This is leveraged frequently in C++ for builder APIs and other patterns. However, Carbon provides more tools in this space than C++ already, and so it may be worth evaluating whether we can switch addr-methods to the same restrictions as assignment and &. Temporaries would never have their address escaped (in a safe way) in that world and there would be fewer different kinds of entities. But this is reserved for future work as we should be very careful about the expressivity hit being tolerable both for native-Carbon API design and for migrated C++ code.

Value expressions

A value cannot be mutated, cannot have its address taken, and may not have storage at all or a stable address of storage. Values are abstract constructs like function input parameters and constants. They can be formed in two ways -- a literal expression like 42, or by reading the value of some stored object.

A core goal of values in Carbon is to provide a single model that can get both the efficiency of passing by value when working with small types such as those that fit into a machine register, but also the efficiency of minimal copies when working with types where a copy would require extra allocations or other costly resources. This directly helps programmers by providing a simpler model to select the mechanism of passing function inputs. But it is also important to enable generic code that needs a single type model that will have consistently good performance.

When forming a value expression from a reference expression, Carbon binds the referenced object to that value expression. This allows immediately reading from the object's storage into a machine register or a copy if desired, but does not require that. The read of the underlying object can also be deferred until the value expression itself is used. Once an object is bound to a value expression in this way, any mutation to the object or its storage ends the lifetime of the value binding, and makes any use of the value expression an error.

Note: this is not intended to ever become "undefined behavior", but instead just "erroneous". We want to be able to detect and report such code as having a bug, but do not want unbounded UB and are not aware of important optimizations that this would inhibit.

Open issue: We need a common definition of erroneous behavior that we can use here (and elsewhere). Once we have that, we should cite it here.

Note: this restriction is also experimental -- we may want to strengthen or weaken it based on experience, especially with C++ interop and a more complete memory safety story.

Even with these restrictions, we expect to make values in Carbon useful in roughly the same places as const &s in C++, but with added efficiency in the case where the values can usefully be kept in machine registers. We also specifically encourage a mental model of a const & with extra efficiency.

The actual representation of a value when bound, especially across function boundaries, is customizable by the type. The defaults are based around preserving the baseline efficiency of C++'s const &, but potentially reading the value when that would be both correct and reliably more efficient, such as into a machine register.

Comparison to C++ parameters

While these are called "values" in Carbon, they are not related to "by-value" parameters as they exist in C++. The semantics of C++'s by-value parameters are defined to create a new local copy of the argument, although it may move into this copy.

Carbon's values are much closer to a const & in C++ with extra restrictions such as allowing copies under "as-if" and preventing taking the address. Combined, these restrictions allow implementation strategies such as in-register parameters.

Polymorphic types

Value expressions and value bindings can be used with polymorphic types, for example:

base class MyBase { ... }

fn UseBase(b: MyBase) { ... }

class Derived {
  extend base: MyBase;
  ...
}

fn PassDerived() {
  var d: Derived = ...;
  // Allowed to pass `d` here:
  UseBase(d);
}

This is still allowed to create a copy or to move, but it must not slice. Even if a copy is created, it must be a Derived object, even though this may limit the available implementation strategies.

Future work: The interaction between a custom value representation and a value expression used with a polymorphic type needs to be fully captured. Either it needs to restrict to a const Self* style representation (to prevent slicing) or it needs to have a model for the semantics when a different value representation is used.

Interop with C++ const & and const methods

While value expressions cannot have their address taken in Carbon, they should be interoperable with C++ const &s and C++ const-qualified methods. This will in-effect "pin" some object (potentially a copy or temporary) into memory and allow C++ to take its address. Without supporting this, values would likely create an untenable interop ergonomic barrier. However, this does create some additional constraints on value expressions and a way that their addresses can escape unexpectedly.

Despite interop requiring an address to implement, C++ allows const & parameters to bind to temporary objects where that address doesn't have much meaning and might not be valid once the called function returns. As a consequence, we don't expect C++ interfaces using a const & to misbehave in practice.

Future work: when a type customizes its value representation, as currently specified this will break the use of const & C++ APIs with such a value. We should extend the rules around value representation customization to require that either the representation type can be converted to (a copy) of the customized type, or implements an interop-specific interface to compute a const pointer to the original object used to form the representation object. This will allow custom representations to either create copies for interop or retain a pointer to the original object and expose that for interop as desired.

Another risk is exposing Carbon's value expressions to const & parameters in this way, as C++ allows casting away const. However, in the absence of mutable members, casting away const does not make it safe to mutate through a const & parameter (or a const-qualified method). C++ allows const & parameters and const member functions to access objects that are declared const. These objects cannot be mutated, even if const is removed, exactly the same as Carbon value expressions. In fact, these kinds of mutations break in real implementations. The result is that Carbon's value expressions will work similarly to const-declared objects in C++, and will interop with C++ code similarly well.

Escape hatches for value addresses in Carbon

Open question: It may be necessary to provide some amount of escape hatch for taking the address of values. The C++ interop above already takes their address functionally. Currently, this is the extent of an escape hatch to the restrictions on values.

If a further escape hatch is needed, this kind of fundamental weakening of the semantic model would be a good case for some syntactic marker like Rust's unsafe, although rather than a region, it would seem better to tie it directly to the operation in question. For example:

class S {
  fn ValueMemberFunction[self: Self]();
  fn AddrMemberFunction[addr self: const Self*]();
}

fn F(s_value: S) {
  // This is fine.
  s_value.ValueMemberFunction();

  // This requires an unsafe marker in the syntax.
  s_value.unsafe AddrMemberFunction();
}

The specific tradeoff here is covered in a proposal alternative.

Initializing expressions

Storage in Carbon is initialized using initializing expressions. Their evaluation produces an initialized object in the storage, although that object may still be unformed.

Future work: More details on initialization and unformed objects should be added to the design from the proposal #257, see #1993. When added, it should be linked from here for the details on the initialization semantics specifically.

The simplest form of initializing expressions are value or durable reference expressions that are converted into an initializing expression. Value expressions are written directly into the storage to form a new object. Reference expressions have the object they refer to copied into a new object in the provided storage.

Future work: The design should be expanded to fully cover how copying is managed and linked to from here.

The first place where an initializing expression is required is to satisfy variable patterns. These require the expression they match to be an initializing expression for the storage they create. The simplest example is the expression after the = in a local var declaration.

The next place where a Carbon expression requires an initializing expression is the expression operand to return statements. We expand more completely on how return statements interact with expressions, values, objects, and storage below.

The last path that requires forming an initializing expression in Carbon is when attempting to convert a non-reference expression into an ephemeral reference expression: the expression is first converted to an initializing expression if necessary, and then temporary storage is materialized to act as its output, and as the referent of the resulting ephemeral reference expression.

Function calls and returns

Function calls in Carbon are modeled directly as initializing expressions -- they require storage as an input and when evaluated cause that storage to be initialized with an object. This means that when a function call is used to initialize some variable pattern as here:

fn CreateMyObject() -> MyType {
  return <return-expression>;
}

var x: MyType = CreateMyObject();

The <return-expression> in the return statement actually initializes the storage provided for x. There is no "copy" or other step.

All return statement expressions are required to be initializing expressions and in fact initialize the storage provided to the function's call expression. This in turn causes the property to hold transitively across an arbitrary number of function calls and returns. The storage is forwarded at each stage and initialized exactly once.

Note that functions without a specified return type work exactly the same as functions with a () return type for the purpose of expression categories.

Deferred initialization from values and references

Carbon also makes the evaluation of function calls and return statements tightly linked in order to enable more efficiency improvements. It allows the actual initialization performed by the return statement with its expression to be deferred from within the body of the function to the caller initializer expression if it can simply propagate a value or reference expression to the caller that is guaranteed to be alive and available to the caller.

Consider the following code:

fn SelectSecond(first: Point, second: Point, third: Point) -> Point {
  return second;
}

fn UsePoint(p: Point);

fn F(p1: Point, p2: Point) {
  UsePoint(SelectSecond(p2, p1, p2));
}

The call to SelectSecond must provide storage for a Point that can be initialized. However, Carbon allows an implementation of the actual SelectSecond function to not initialize this storage when it reaches return second. The expression second is a name bound to the call's argument value expression, and that value expression is necessarily valid in the caller. Carbon in this case allows the implementation to merely communicate that the returned expression is a name bound to a specific value expression argument to the call, and the caller if necessary should initialize the temporary storage. This in turn allows the caller F to recognize that the value expression argument (p1) is already valid to pass as the argument to UsePoint without initializing the temporary storage from it and reading it back out of that storage.

None of this impacts the type system and so an implementation can freely select specific strategies here based on concrete types without harming generic code. Even if generics were to be implemented without monomorphization, for example dynamic dispatch of object-safe interfaces, there is a conservatively correct strategy that will work for any type.

This freedom mirrors that of input values where might be implemented as either a reference or a copy without breaking genericity. Here too, many small types will not need to be lazy and simply eagerly initialize the temporary which is implemented as an actual machine register. But for large types or ones with associated allocated storage, this can reliably avoid extraneous memory allocations and other costs.

Note that this flexibility doesn't avoid the call expression materializing temporary storage and providing it to the function. Whether the function needs this storage is an implementation detail. It simply allows deferring an important case of initializing that storage from a value or reference expression already available in the caller to the caller so that it can identify cases where that initialization is not necessary.

References: This addresses an issue-for-leads about reducing the potential copies incurred by returns.

Declared returned variable

The model of initialization of returns also facilitates the use of returned var declarations. These directly observe the storage provided for initialization of a function's return.

Pointers

Pointers in Carbon are the primary mechanism for indirect access to storage containing some value. Dereferencing a pointer is one of the primary ways to form a durable reference expression.

Carbon pointers are heavily restricted compared to C++ pointers -- they cannot be null and they cannot be indexed or have pointer arithmetic performed on them. In some ways, this makes them more similar to references in C++, but they retain the essential aspect of a pointer that they syntactically distinguish between the pointer and the pointee.

Carbon will still have mechanisms to achieve the equivalent behaviors as C++ pointers. Optional pointers are expected to serve nullable use cases. Slice or view style types are expected to provide access to indexable regions. And even raw pointer arithmetic is expected to be provided at some point, but through specialized constructs given the specialized nature of these operations.

Future work: Add explicit designs for these use cases and link to them here.

Reference types

Unlike C++, Carbon does not currently have reference types. The only form of indirect access are pointers. There are a few aspects to this decision that need to be separated carefully from each other as the motivations and considerations are different.

First, Carbon has only a single fundamental construct for indirection because this gives it a single point that needs extension and configuration if and when we want to add more powerful controls to the indirect type system such as lifetime annotations or other safety or optimization mechanisms. The designs attempts to identify a single, core indirection tool and then layer other related use cases on top. This is motivated by keeping the language scalable as it evolves and reducing the huge explosion of complexity that C++ sees due to having a large space here. For example, when there are N > 1 ways to express indirection equivalently and APIs want to accept any one of them across M different parameters they can end up with N * M combinations.

Second, with pointers, Carbon's indirection mechanism retains the ability to refer distinctly to the pointer and the pointee when needed. This ends up critical for supporting rebinding and so without this property more permutations of indirection would likely emerge.

Third, Carbon doesn't provide a straightforward way to avoid the syntactic distinction between indirect access and direct access.

For a full discussion of the tradeoffs of these design decisions, see the alternatives considered section of P2006:

Pointer syntax

The type of a pointer to a type T is written with a postfix * as in T*. Dereferencing a pointer is a [reference expression] and is written with a prefix * as in *p:

var i: i32 = 42;
var p: i32* = &i;

// Form a reference expression `*p` and assign `13` to the referenced storage.
*p = 13;

This syntax is chosen specifically to remain as similar as possible to C++ pointer types as they are commonly written in code and are expected to be extremely common and a key anchor of syntactic similarity between the languages. The different alternatives and tradeoffs for this syntax issue were discussed extensively in #523 and are summarized in the proposal.

Carbon also supports an infix -> operation, much like C++. However, Carbon directly defines this as an exact rewrite to * and . so that p->member becomes (*p).member for example. This means there is no overloaded or customizable -> operator in Carbon the way there is in C++. Instead, customizing the behavior of *p in turn customizes the behavior of p->.

Future work: As #523 discusses, one of the primary challenges of the C++ syntax is the composition of a prefix dereference operation and other postfix or infix operations, especially when chained together such as a classic C++ frustrations of mixes of dereference and indexing: (*(*p)[42])[13]. Where these compositions are sufficiently common to create ergonomic problems, the plan is to introduce custom syntax analogous to -> that rewrites down to the grouped dereference. However, nothing beyond -> itself is currently provided. Extending this, including the exact design and scope of extension desired, is a future work area.

Dereferencing customization

Carbon should support user-defined pointer-like types such as smart pointers using a similar pattern as operator overloading or other expression syntax. That is, it should rewrite the expression into a member function call on an interface. Types can then implement this interface to expose pointer-like user-defined dereference syntax.

The interface might look like:

interface Pointer {
  let ValueT:! Type;
  fn Dereference[self: Self]() -> ValueT*;
}

Here is an example using a hypothetical TaggedPtr that carries some extra integer tag next to the pointer it emulates:

class TaggedPtr(T:! Type) {
  var tag: Int32;
  var ptr: T*;
}
external impl [T:! Type] TaggedPtr(T) as Pointer {
  let ValueT:! T;
  fn Dereference[self: Self]() -> T* { return self.ptr; }
}

fn Test(arg: TaggedPtr(T), dest: TaggedPtr(TaggedPtr(T))) {
  **dest = *arg;
  *dest = arg;
}

There is one tricky aspect of this. The function in the interface which implements a pointer-like dereference must return a raw pointer which the language then actually dereferences to form a reference expression similar to that formed by var declarations. This interface is implemented for normal pointers as a no-op:

impl [T:! Type] T* as Pointer {
  let ValueT:! Type = T;
  fn Dereference[self: Self]() -> T* { return self; }
}

Dereference expressions such as *x are syntactically rewritten to use this interface to get a raw pointer and then that raw pointer is dereferenced. If we imagine this language level dereference to form a reference expression as a unary deref operator, then (*x) becomes (deref (x.(Pointer.Dereference)())).

Carbon will also use a simple syntactic rewrite for implementing x->Method() as (*x).Method() without separate or different customization.

const-qualified types

Carbon provides the ability to qualify a type T with the keyword const to get a const-qualified type: const T. This is exclusively an API-subsetting feature in Carbon -- for more fundamentally "immutable" use cases, value expressions and bindings should be used instead. Pointers to const-qualified types in Carbon provide access to an object with an API subset that can help model important requirements like ensuring usage is exclusively by way of a thread-safe interface subset of an otherwise thread-compatible type.

Note that const T is a type qualification and is generally orthogonal to expression categories or what form of pattern is used, including for object parameters. Notionally, it can occur both with addr and value object parameters. However, on value patterns, it is redundant as there is no meaningful distinction between a value expression of type T and type const T. For example, given a type and methods:

class X {
  fn Method[self: Self]();
  fn ConstMethod[self: const Self]();
  fn AddrMethod[addr self: Self*]();
  fn AddrConstMethod[addr self: const Self*]();
}

The methods can be called on different kinds of expressions according to the following table:

Expression category: let x: X
(value)		 let x: const X
(const value)		 var x: X
(reference)		 var x: const X
(const reference)
x.Method(); ✅ ✅ ✅ ✅
x.ConstMethod(); ✅ ✅ ✅ ✅
x.AddrMethod(); ❌ ❌ ✅ ❌
x.AddrConstMethod() ❌ ❌ ✅ ✅

The const T type has the same representation as T with the same field names, but all of its field types are also const-qualified. Other than fields, all other members T are also members of const T, and impl lookup ignores the const qualification. There is an implicit conversion from T to const T, but not the reverse. Conversion of reference expressions to value expressions is defined in terms of const T reference expressions to T value expressions.

It is expected that const T will largely occur as part of a pointer, as the express purpose is to form reference expressions. The precedence rules are even designed for this common case, const T* means (const T)*, or a pointer-to-const. Carbon will support conversions between pointers to const-qualified types that follow the same rules as used in C++ to avoid inadvertent loss of const-qualification.

The syntax details of const are also covered in the type operators documentation.

Lifetime overloading

One potential use case that is not obviously or fully addressed by these designs in Carbon is overloading function calls by observing the lifetime of arguments. The use case here would be selecting different implementation strategies for the same function or operation based on whether an argument lifetime happens to be ending and viable to move-from.

Carbon currently intentionally leaves this use case unaddressed. There is a fundamental scaling problem in this style of overloading: it creates a combinatorial explosion of possible overloads similar to other permutations of indirection models. Consider a function with N parameters that would benefit from lifetime overloading. If each parameter benefits independently from the others, as is commonly the case, we would need 2N overloads to express all the possibilities.

Carbon will initially see if code can be designed without this facility. Some of the tools needed to avoid it are suggested above such as the consuming input pattern. But it is possible that more will be needed in practice. It would be good to identify the specific and realistic Carbon code patterns that cannot be expressed with the tools in this proposal in order to motivate a minimal extension. Some candidates based on functionality already proposed here or for classes:

  • Allow overloading between addr me and me in methods. This is among the most appealing as it doesn't have the combinatorial explosion. But it is also very limited as it only applies to the implicit object parameter.
  • Allow overloading between var and non-var parameters.
  • Expand the addr technique from object parameters to all parameters, and allow overloading based on it.

Perhaps more options will emerge as well. Again, the goal isn't to completely preclude pursuing this direction, but instead to try to ensure it is only pursued based on a real and concrete need, and the minimal extension is adopted.

Value representation and customization

The representation of a value expression is especially important because it forms the calling convention used for the vast majority of function parameters -- function inputs. Given this importance, it's important that it is predictable and customizable by the value's type. Similarly, while Carbon code must be correct with either a copy or a reference-based implementation, we want which implementation strategy is used to be a predictable and customizable property of the type of a value.

A type can optionally control its value representation using a custom syntax similar to customizing its destructor. This syntax sets the representation to some type uses a keyword value_rep and can appear where a member declaration would be valid within the type:

class SomeType {
  value_rep = RepresentationType;
}

Open question: The syntax for this is just placeholder, using a placeholder keyword. It isn't final at all and likely will need to change to read well.

The provided representation type must be one of the following:

  • const Self -- this forces the use of a copy of the object.
  • const Self * -- this forces the use of a pointer to the original object.
  • A custom type that is not Self, const Self, or a pointer to either.

If the representation is const Self or const Self *, then the type fields will be accessible as value expressions using the normal member access syntax for value expressions of a type. These will be implemented by either accessing a copy of the object in the non-pointer case or a pointer to the original object in the pointer case. A representation of const Self requires copying to be valid for the type. This provides the builtin functionality but allows explicitly controlling which representation should be used.

If no customization is provided, the implementation will select one based on a set of heuristics. Some examples:

  • Non-copyable types and polymorphic types would use a const Self*.
  • Small objects that are trivially copied in a machine register would use const Self.

When a custom type is provided, it must not be Self, const Self, or a pointer to either. The type provided will be used on function call boundaries and as the implementation representation for let bindings and other value expressions referencing an object of the type. A specifier of value_rep = T; will require that the type containing that specifier satisfies the constraint impls ReferenceImplicitAs where .T = T using the following interface:

interface ReferenceImplicitAs {
  let T:! type;
  fn Convert[addr self: const Self*]() -> T;
}

Converting a reference expression into a value expression for such a type calls this customization point to form a representation object from the original reference expression.

When using a custom representation type in this way, no fields are accessible through a value expression. Instead, only methods can be called using member access, as they simply bind the value expression to the self parameter. However, one important method can be called -- .(ImplicitAs(T).Convert)(). This implicitly converting a value expression for the type into its custom representation type. The customization of the representation above and impls ReferenceImplicitAs where .T = T causes the class to have a builtin impl as ImplicitAs(T) which converts to the representation type as a no-op, exposing the object created by calling ReferenceImplicitAs.Convert on the original reference expression, and preserved as a representation of the value expression.

Here is a more complete example of code using these features:

class StringView {
  private var data_ptr: Char*;
  private var size: i64;

  fn Make(data_ptr: Char*, size: i64) -> StringView {
    return {.data_ptr = data_ptr, .size = size};
  }

  // A typical readonly view of a string API...
  fn ExampleMethod[self: Self]() { ... }
}

class String {
  // Customize the value representation to be `StringView`.
  value_rep = StringView;

  private var data_ptr: Char*;
  private var size: i64;

  private var capacity: i64;

  impl as ReferenceImplicitAs where .T = StringView {
    fn Op[addr self: const Self*]() -> StringView {
      // Because this is called on the String object prior to it becoming
      // a value, we can access an SSO buffer or other interior pointers
      // of `self`.
      return StringView.Make(self->data_ptr, self->size);
    }
  }

  // We can directly declare methods that take `self` as a `StringView` which
  // will cause the caller to implicitly convert value expressions to
  // `StringView` prior to calling.
  fn ExampleMethod[self: StringView]() { self.ExampleMethod(); }

  // Or we can use a value binding for `self` much like normal, but the
  // implementation will be constrained because of the custom value rep.
  fn ExampleMethod2[self: String]() {
    // Error due to custom value rep:
    self.data_ptr;

    // Fine, this uses the builtin `ImplicitAs(StringView)`.
    (self as StringView).ExampleMethod();
  }

  // Note that even though the `Self` type is `const` qualified here, this
  // cannot be called on a `String` value! That would require us to convert to a
  // `StringView` that does not track the extra data member.
  fn Capacity[addr self: const Self*]() -> i64 {
    return self->capacity;
  }
}

It is important to note that the representation type of a value expression is just its representation and does not impact the name lookup or type. Name lookup and impl search occur for the same type regardless of the expression category. But once a particular method or function is selected, an implicit conversion can occur from the original type to the representation type as part of the parameter or receiver type. In fact, this conversion is the only operation that can occur for a value whose type has a customized value representation.

The example above also demonstrates the fundamental tradeoff made by customizing the value representation of a type in this way. While it provides a great deal of control, it may result in some surprising limitations. Above, a method that is classically available on a C++ const std::string& like querying the capacity cannot be implemented with the customized value representation because it loses access to this additional state. Carbon allows type authors to make an explicit choice about whether they want to work with a restricted API and leverage a custom value representation or not.

Open question: Beyond the specific syntax used where we currently have a placeholder value_rep = T;, we need to explore exactly what the best relationship is with the customization point. For example, should this syntax immediately forward declare impl as ReferenceImplicitAs where .T = T, thereby allowing an out-of-line definition of the Convert method and ... where _ to pick up the associated constant from the syntax. Alternatively, the syntactic marker might be integrated into the impl declaration for ReferenceImplicitAs itself.

Alternatives considered

References