p1885.md

for statement and user types

Pull request

Table of contents

• Abstract
• Problem
• Background
  • Other languages
    • C++
    • Python
    • Rust
    • Typescript
    • Go
• Proposal
• Details
  • for with immutable values
    • Iterate interface
    • let by default
    • Lifetime of rvalues on right-hand side of :
  • Use cases
    • Mutable and immutable elements
    • Random access containers
    • Maps, hash maps, and other containers
    • R-value containers
    • Synthetic data
    • Large containers
    • Filtering, transforming, and reversing
    • Interoperability with C++ types and containers
• Future work
  • for with mutable values
  • Speculative mixin usage
    • Mutable values
    • Alternative views
  • Large elements and Optional
  • C++ interoperability
    • Iterating over C++ types in Carbon
    • Iterating over Carbon types in C++
  • Inversion of control
• Rationale
• Alternatives considered
  • Atomic methods for Iterate
  • Using an iterator instead of a cursor
  • Support getter for both T and T* with Iterate

Abstract

This proposes an interface that can be implemented by user types to support range-based iteration with for.

Problem

The current for proposal does not define a way for types to support range-based for loops.

Examples of use cases for for loops include iterating over:

• Mutable and immutable elements
• Random access containers
• Maps, hash maps, and complex containers
• R-value containers
• Synthetic data
• Large containers, and large elements
• Filtering, transforming, and reversing
• Interoperability with C++ types and containers

The goals for the solution includes:

• Easy to support for user types, with minimal requirements
• Simple and clear usage
• Minimal performance overhead

Background

The original proposal of for loops were discussed in p0353, and the basic design documented in control_flow/loops#for.

The resulting syntax was:

for ( var declaration in expression ) { statements }

We now need a way to allow supporting this syntax for user and library container types.

Other languages

This section highlights some examples of how iterators and range-based for user-defined types is addressed in different languages.

C++

C++ requires either .begin()/.end() methods, or begin(<container>)/end(<container>) to be supported for range-based for.

Python

Python requires 2 dunder methods, __iter__ on the container and __next__ on the iterator, to be usable with a for <element> in <container>: construct.

Generator functions act like iterators. They are executed until the next yield statement, whose argument is returned as the next value to the for loop. Iteration ends when the function returns.

def generate_fibonacci():
  yield 0;
  n1, n2 = 0, 1;
  while True:
    n1, n2 = n2, n1+n2;
    yield n1;

for n in generate():
  print(n);

Rust

Rust supports iterating over a container with for using

• ranges for n in 1..100
• iterators for element in container.iter() or .iter_mut()

The Iterator type can be used with custom types using the following syntax (though specific):

struct Fibonacci {
    curr: u32,
    next: u32,
}

impl Iterator for Fibonacci {
    type Item = u32;
    fn next(&mut self) -> Option<Self::Item> {
        // calculate and yield value
        [...]
    }
}

for i in fibonacci().take(4) {
    ...
}

Typescript

Typescript uses the .iterator property, along with two forms of range-based for loops: for..in and for..of.

let pets = new Set(['Cat', 'Dog', 'Hamster']);
pets['species'] = 'mammals';
for (let pet in pets) {
    console.log(pet); // "species"
}
for (let pet of pets) {
    console.log(pet); // "Cat", "Dog", "Hamster"
}

Using the .iterator property, Typescript allow to lazily generate data easily using functions, classes, or objects:

const reverse = (arr) => ({
    [Symbol.iterator]() {
        let i = arr.length;
        return {
            next: () => ({
                value: arr[--i],
                done: i < 0,
            }),
        };
    },
});

Go

Go does not officially support customizing for for user types.

Proposal

Provide interfaces that can be implemented by user types to enable support for ranged-for loops.

We propose:

• A new Iterate interface to support for loops
• Using a cursor-based approach
• Making the for loop value let by default, that is, non-reassignable

Below is a high-level example of this concept:

// Add support for `for`
class MyRange {
  impl as Iterate where .ElementType = i32 and .CursorType = i32 {
    // Implement `Iterate`
  }
}
// Usage
let my_range: MyRange = {...};
for (item: auto in my_range) {
  // Do something with `item`
}

Details

for with immutable values

Iterate interface

This proposal exposes a new Iterate. This interface:

• Relies on a cursor-based approach: minimize implementation effort for developers, could support R-values.
• Expects an ElementType and CursorType.
• Combines "advance", "get", and "bounds check" into a single Next(cursor) method that returns an optional value.

The Iterate interface is:

interface Iterate {
  let ElementType:! type;
  let CursorType:! type;
  fn NewCursor[self: Self]() -> CursorType;
  fn Next[self: Self](cursor: CursorType*) -> Optional(ElementType);
}

A naive Carbon implementation of the for loop could be:

var cursor: range.(Iterate.CursorType) = range.(Iterate.NewCursor)();
var iter: Optional(range.(Iterate.ElementType)) = range.(Iterate.Next)(&cursor);

// A. Possible implementation
while (iter.HasValue()) {
  ExecuteForBlock(iter.Get());
  iter = container.(Iterate.Next)(&cursor);
}

// B. Possible alternative depending on the API for Optional(T):
while (true) {
  match (iter) {
    case .None => { break; }
    case .Some(let value: range.(Iterate.ElementType)) => {
      ExecuteForBlock(value);
      iter = container.(Iterate.Next)(&cursor);
    }
  }
}

The cursor tracks progression, and the Next method advances the cursor and returns an Optional value. An empty Optional indicates that we have reached the end.

Below is a sample implementation of that interface for a user type.

class MyIntContainer {
  var data: ...
  fn Size[self: Self]() -> i32 {
      return 4;
  }
  impl as Iterate where .ElementType = i32 and .CursorType = i32 {
    fn NewCursor[self: Self]() -> i32 { return 0; }
    fn Next[self: Self](cursor: i32*) -> Optional(i32) {
      // Advance and return value, or return empty
      if (*cursor < self.Size()) {
        let index: i32 = *cursor;
        *cursor = index + 1;
        return Optional(i32).Create(self.data[index]);
      } else {
        return Optional(i32).CreateEmpty();
      }
    }
  }
}

Which can be used as follow:

fn Main() -> i32 {
  var container: MyIntContainer = {.data = (1, 2, 3, 4)};
  // Implicit `let` value declaration
  for (value: i32 in container) {
    Print("{0}", value);
  }
  return 0;
}

let by default

To keep usage simple and non-surprising, we propose defaulting variable declarations to let declarations. This is consistent with function parameters and scrutinees for pattern matching, which are immutable by default.

// Implicitly `let item: T`
for (item: T in my_range) {
  // `item` cannot be reassigned
}

This default can be overridden by adding the var keyword:

for (var item: T in my_range) {
  // `item` can be modified, but this will not modify `my_range`
  // because it is a copy of the element with its own storage
}

Lifetime of rvalues on right-hand side of :

It will be important to ensure that temporary entities on the right-hand side of : are alive during the execution of the for loop to prevent invalid memory access (Note: this was only recently addressed for C++ by P2644). This applies to Carbon and C++ interoperability.

// Example using a temporary range, with `GetElements()` returning an object that implements `Iterate`
for (item: T in bucket.GetElements()) {
  // We need to make sure the object is alive for the duration of the `for` loop
}

Use cases

This section covers the use cases highlighted in the introduction, and usage with the proposed design.

Mutable and immutable elements

This proposal covers strictly immutable elements, and proposes defaulting element declaration to let, to minimize surprises and clarify the intent. Mutable elements are mentioned in the Future work section.

Random access containers

For random access containers, the cursor would represent a numerical index, incremented within Next().

Maps, hash maps, and other containers

Because the cursor type is defined by the developer, and opaque to the user, it could be a hashmap index, a string, or pointer to a node, without changing the usage for users.

The ElementType can be a tuple, such as a (key, value) for maps, or a single value. See [Future work][#future-work] for other examples.

R-value containers

R-values are likely to be limited in terms of addressing and mutation. The impact is undefined until the corresponding design is finalized.

The cursor approach used for Iterate has the slight advantage of not requiring pointers (and addressing) for some container types. This may be beneficial as a first iteration, until the dependent design is updated, at which point we will have to define how to handle this special case.

Synthetic data

Next() can be trivially implemented to return a single computed value on-demand based on the cursor, without the need for storage. If the sequence is infinite, Next() will never return an empty Optional, and the for loop will continue until interrupted by the user.

class SquaredSequence {
  impl as Iterate where .ElementType = i32 and .CursorType = i32 {
    fn NewCursor[self: Self]() -> i32 { return -1; }
    fn Next[self: Self](cursor: i32*) -> Optional(i32) {
      // Advance and return computed value
      *cursor = *cursor + 1;
      return Optional(i32).Create((*cursor) * (*cursor));
    }
  }
}

Large containers

The proposal does not present any limitation regarding large collections. The cursor can be adapted to the collection size, and no copy of the container should occur both for immutable collections, and future mutable elements.

Filtering, transforming, and reversing

Provided the traversal order is the same, generic filters and transformers are supported by implementing a proxy Iterate interface, internally or externally.

This example illustrates a simple proxy interface.

class BasicFilter(T:! Iterate) {
  var seq: T*;
  var value: T.ElementType;
  impl as Iterate where .ElementType = T.ElementType and .CursorType = T.CursorType {
    fn NewCursor[self: Self]() -> T.ElementType { return self.seq->NewCursor(); }
    fn Next[self: Self](cursor: T.CursorType*) -> Optional(T.ElementType) {
      var res: auto = self.seq->Next(cursor);
      while (res.has_value() && res.get() == value) {
        res = self.seq->Next(cursor);
      }
      return res;
    }
  }
}

fn Main() -> i32 {
  let container: MyIntContainer = MyIntContainer.Create(0,1,0,2,3,0);
  let no_zeros: BasicFilter(MyIntContainer) =
      { .seq = &container, .value = 0 };
  // Prints "123"
  for (v: i32 in no_zeros) {
    Print("{0}", v);
  }
  return 0;
}

On the other hand, changing how the collection is traversed (for example reverse), requires internal knowledge about the data layout, and a custom implementation. One approach for containers that support reverse iteration would be to implement a different interface with a Prev() method. A proxy could be exposed to make it usable with for directly.

Below is an example of what such an interface could look like:

interface IterateBidirectional;
class ReverseProxy(T:! IterateBidirectional);
interface IterateBidirectional {
  extends Iterate;
  fn NewEndCursor[self: Self]() -> CursorType;
  fn Prev[self: Self](cursor: CursorType*) -> Optional(ElementType);
  final fn Reversed[self: Self]() -> ReverseProxy(Self);
}
class ReverseProxy(T:! IterateBidirectional) {
  var container: T;
  impl as Iterate where .ElementType = T.ElementType and .CursorType = T.CursorType {
    fn NewCursor[self: Self]() -> CursorType {
      return self.container.NewEndCursor();
    }
    fn Next[self: Self](cursor: CursorType*) -> Optional(ElementType) {
      return self.container.Prev(cursor);
    }
  }
}

fn IterateBidirectional.Reversed[self: Self]() -> ReverseProxy(Self) {
  return {.container = self};
}

/// Usage
class MyContainer {
  fn Create(...) { ... }
  impl as Iterate ... { ... }
  impl as IterateBidirectional ... { ... }
}
var container: MyContainer = MyContainer.Create(...);
for (v: container.(Iterate.ElementType) in container.Reversed()) {...}

Interoperability with C++ types and containers

The cursor approach deviates from the C++ iterator approach, requiring some adaptation.

Given the current state of the design, C++ interoperability is yet to be defined, and suggestions have been highlighted in the Future work section.

Future work

for with mutable values

Supporting mutable values could be achieved using a second interface that provides a proxy or view for the data, exposing an Iterate interface with ElementType*

// Object implementing `MutableIterate` returned by a function
for (p: T* in my_range.AddrRange()) {}

The example MutableIterate interface below acts as a proxy for the collection, and implements our Iterate interface.

interface MutableIterate {
  impl as Iterate;
  alias ElementType = Iterate.ElementType;
  let ProxyType:! Iterate where .ElementType = ElementType*
      and .CursorType is ImplicitAs((Self as Iterate).CursorType);
  fn AddrRange[addr self: Self*]() -> ProxyType;
}

class MyIntContainer {
  //...
  impl as Iterate ...
  external impl as MutateIterate where .ProxyType = Self {
    ...
  }
}

Speculative mixin usage

Mixins are currently in early design stages. This section highlights possible uses speculating on the final design. Some may include:

• Improved semantics for views compared to getter methods: no direct side effect from using the view
• Facilitate code reuse, compared to reimplementing an interface
• Direct access to self, limiting needs for pointers and address resolution

Mutable values

Named mixins, used as programmable members, could expose a view with mutable values:

// Using a named mixin to expose mutable values
for (p: my_range.(Iterate.ElementType)* in my_range.mutable) {}

Used in this way, this clarifies that this a view of the data, like an attribute, with no direct side effects, when compared to a method.

Alternative views

Similarly, mixins could allow supporting other views into the container.

For example, maps could expose a view with "key" and "value" only, in addition to the default (key, value) tuple.

// Using a named mixin to expose mutable values
for (k: my_dict.(Iterate.CursorType) in my_dict.keys) {}

And "reversed" view could also be used for reverse iteration:

// Reversing
for (v: my_dict.(Iterate.ElementType) in my_dict.reversed) {}

Large elements and Optional

Large elements would have a negative impact if the Optional cannot leverage unused bit patterns within the type, nor present a view instead of a copy.

If this proves difficult, we can let implementers of the container choose whether to expose values or pointers to values. When electing to "pointers to values", an adapter can be provided to support transforming the range of pointers into a range a values. This choice allows supporting either iteration over container r-values (ElementType = T), or efficient iteration (ElementType = T*), but not both. While acceptable as an intermediate solution, a better alternative will be needed in the future.

adapter IterateByValue(T:! Iterate where .CursorType impls Deref) for T {
  impl as Iterate
      where .CursorType = T.CursorType
        and .ElementType = T.CursorType.(Deref.Result) {
        ...
  }
}

// Used like:
var frames: NetworkFramesContainerByPtr = ...
for (i: NetworkFrame in frames as IterateByValue(NetworkFramesContainerByPtr))
{
  ...
}

Alternatively, we can allow the binding in the for loop to be an addr pattern, implemented by calling into a separate interface on the container. This puts the choice of using values or pointers to values in the hands of the for loop author. While this deviates from the ergonomics of let (where the compiler can choose to copy or alias the value depending on what is more efficient), this option would provide parity with C++ for loops (where the author chooses between value and reference).

C++ interoperability

Iterating over C++ types in Carbon

For reference, range-based for loops in C++ requires:

• begin() and end() methods or free functions, and
• the type returned supports pre-increment ++, indirection *, and inequality != operations

See range-based for statement for more details.

A limitation of the approach proposed in this document is the need for adaptation to interoperate with the C++ iterator model. The adapter would be a Carbon type that satisfies the Cursor interface, implemented in terms of a C++ iterator.

• NewCursor providing the "start iterator" returned by begin()
• Next doing the "increment, bounds check, and returning value"

Two different approaches are identified:

• One is to have a templated impl of Iterate, for anything with the right method signatures (begin, end, and so on)
• Another is a templated adapter that implements Iterate when requested

// Template impl approach
template constraint CppIterator { ... }
template constraint CppContainer {
  // Speculative
  for_unique IteratorType: CppIterator;
  fn begin() -> IteratorType;
  fn end() -> IteratorType;
}
external impl forall [template T:! CppContainer] T as Iterate
    where .CursorType = T.(CppContainer.IteratorType)
      and .ElementType = .CursorType.ElementType {
  fn NewCursor[self: Self]() -> CursorType {
    return self.(CppContainer.begin)();
  }
  fn Next[self: Self](cursor: CursorType*) -> Optional(ElementType) {
    if (*cursor == self.(CppContainer.end)()) {
      return Optional(ElementType).MakeEmpty();
    }
    let value: ElementType = **cursor;
    ++*cursor;
    return Optional(ElementType).MakeValue(value);
  }
}
// Used like:
var v: Cpp.std.vector(i32) = ...
for (i: i32 in v) { ... }

// Adapter approach
adapter CppIterate(template T:! type) for T {
  impl as Iterate
      // Speculative syntax. Alternatively, a cursor type parameter
      // would avoid the need for deduction.
      where .CursorType = typeof(T.begin())
        and .ElementType = .CursorType.(Deref.Result) {
    fn NewCursor[self: Self]() -> CursorType {
      return self.begin();
    }
    fn Next[self: Self](cursor: CursorType*) -> Optional(ElementType) {
      if (*cursor == self.end()) {
        return Optional(ElementType).MakeEmpty();
      }
      let value: ElementType = **cursor;
      ++*cursor;
      return Optional(ElementType).MakeValue(value);
    }
  }
}
// Used like:
var v: Cpp.std.vector(i32) = ...
for (i: i32 in v as CppIterate(Cpp.std.vector(i32))) { ... }

These two options could allow interoperability with a compatible C++ type, either implicitly (template approach) or explicitly (adapter approach).

Iterating over Carbon types in C++

There need to be begin() and end() methods or free functions accessible to C++ for any Carbon type that implements the Iterate interface, in addition to overloads of the *, ++, and != operators for the types returned.

The Iterate interface does not expose a way to have a cursor nor an iterator to the past-last element. An option would be to have end() return a predefined invalid value, to satisfy iterator comparison when Next() returns an empty Optional.

Below is sample implementation to iterate on a Carbon Iterate. Note that we need to copy and cache the last value, to allow for *it to be called. This puts a requirement on ElementType to be copyable for this interoperability to be usable.

namespace Carbon {
// A C++ input iterator for any type that implements the Carbon `Iterate` interface.
template <typename Iterate>
class IterateIterator {
 public:
  // Returns an "Invalid" iterator representing `end()`
  static auto Invalid(const Iterate& container) -> IterateIterator<Iterate> {
    return IterateIterator<Iterate>(container, std::nullopt);
  }

  IterateIterator(const Iterate& container,
                  std::optional<typename Iterate::CursorType> cursor)
      : container_(&container), cursor_(cursor) {
    if (cursor_) {
      next();
    }
  }
  IterateIterator(const IterateIterator& other)
      : container_(other.container_), cursor_(other.cursor_),
      value_(other) {}

  auto operator*() const -> const typename Iterate::ElementType& {
    assert(cursor_);
    return value_;
  }
  auto operator++() -> IterateIterator<Iterate>& {
    next();
    return *this;
  }
  auto operator==(const IterateIterator<Iterate>& rhs) -> bool {
    return container_ == rhs.container_ && cursor_ == rhs.cursor_;
  }
  auto operator!=(const IterateIterator<Iterate>& rhs) -> bool {
    return !(*this == rhs);
  }

 private:
  void next() {
    assert(cursor_);
    if (const auto v = container_->Next(*cursor_); v) {
      value_ = *std::move(v);
    } else {
      cursor_ = std::nullopt;
    }
  }

  const Iterate* container_;
  std::optional<typename Iterate::CursorType> cursor_;
  typename Iterate::ElementType value_;
};

// The C++ side of a custom Carbon type implementing
// `Iterate` with CursorType `C`, and ElementType `T`
// could have `begin()` and `end()` methods defined as:
class MyIterateType {
  // [...]
  auto begin() -> IterateIterator<Iterate<C, T>> {
    return IterateIterator<Iterate<C, T>>(*this, NewCursor());
  }
  auto end() -> IterateIterator<Iterate<C, T>> {
    return IterateIterator<Iterate<C, T>>::Invalid(*this);
  }
};
}  // namespace Carbon

// Usage:
Carbon::MyIterateType container;
for (auto s : container) { /*...*/ }

Inversion of control

Using an Optional has downsides, discussed in the "alternatives considered" section. A possible approach would be to invert the control of the for loop, by having the container call in the for block for each value, passing the value as argument. This would work similarly to Python generator functions.

This has the following advantages:

• Removes the need for an Optional, and the associated overheads, copies, and unwrapping.
• No boundary checks needed at the for level
• Compatible with R-value containers

Limitations:

• The context needed by the for body will needs to be available or provided
• No optimizer known to the team can reliably produce efficient code for this construct.

Rationale

This proposal offers a first step to support a needed way to iterate using a for loop. More specifically:

• Requires a single interface to be implemented with a combined Next method, uses cursors instead of their more complex iterator counterpart, and defines loop values as let by default to be less error prone (Code that is easy to read, understand, and write).
• Makes use of Library APIs only.

Alternatives considered

Atomic methods for Iterate

An option was to define methods such as Get, Advance, IsAtEnd on Iterate, instead of a unique Next methods. While being more atomic, the concern is that these individual methods will all need to be called separately while iterating, while a single call to Next performs these three operations together, providing more opportunities for optimization.

Using a single operation to return an optional is also consistent with Rust’s approach.

The combined Next method forces us to return an Optional, which has downsides:

• potential performance overhead to wrapping and unwrapping,
• storage overhead for Optional itself, and
• may require additional copying of the value wrapped.

Those may be mitigated by leveraging unused bit patterns, passing a view of the value, or inverting control.

Using an iterator instead of a cursor

The iterator approach was considered but proved to have key limitations that a cursor approach does not have:

• It requires implementing 2 interfaces instead of 1, and is more complex due to having the iteration logic separated from the container itself
• More difficult to harden or troubleshoot, compared to a cursor that allows bounds checking in debug & hardened build modes
• Can pose problems with ranges that consumes their input (See Barry Revzin’s "take(5)" presentation from C++Now 2023), when compared to a combined Next() approach.
• Higher overhead
• Proves to be difficult to support for R-values

On the other hand, the cursor approach deviates more from C++ than iterator, and may require some more effort for C++ interoperability.

Support getter for both T and T* with Iterate

Because some collections will be read-only, we want to separate mutable and immutable interfaces. A single interface would force developers to implement or stub the mutable portion of the interface, even if it is not applicable.