[swift-evolution] Proposal: Add types BufferedSequence, BufferedGenerator

Kevin Ballard kevin at sb.org
Thu Dec 31 18:16:44 CST 2015

BufferedSequence is a sequence adaptor that wraps any underlying sequence and provides a stable `first` property. BufferedGenerator is a generator adaptor that wraps any underlying generator and provides arbitrary lookahead with a `peek(n: Int)` method.

The method name "peek()" has precedent in various languages (e.g. Rust
with std::iter::Peekable, C++ with std::basic_istream::peek, Ruby with
Enumerator#peek, etc). I considered the name "lookahead()" but I decided
it made more sense to use that name for the property that describes how
much lookahead there is.

The proposed API looks like this:

/// A sequence adaptor that adds a nondestructive `first`
property to any

/// underlying sequence.

/// - Note: If the underlying sequence is not destructively
"consumed" by

///   iteration, then neither is `BufferedSequence`.

public class BufferedSequence<Base : SequenceType> : SequenceType {

public init(_ base: Base)

/// Returns ` BufferedGenerator` with a lookahead size of `1`.

public func generate() ->

/// Returns the first element of the underlying sequence,

public var first: Base.Generator.Element? { get }


/// A generator adaptor that adds a nondestructive `peek()`
method to any

/// underlying generator.

public struct BufferedGenerator<Base : GeneratorType> : GeneratorType {

/// Construct an instance that buffers access to an underlying

/// - Parameter base: The underlying generator.

/// - Parameter lookahead: The amount of lookahead to allow.
Default is `1`.

///   Values less than `1` will be treated the same as `1`.

public init(_ base: Base, lookahead: Int = default)

/// The amount of lookahead that this generator offers.

/// - Invariant: `lookahead >= 1`.

public let lookahead: Int

/// Advance to the next element and return it, or `nil` if no next
element exists.


/// - Requires: Neither `next()` nor `peek()` have been applied
to a copy of

///   `self` since the copy was made, and no preceding call to
`self.next()` has

///   returned `nil`.

public mutating func next() -> Base.Element?

/// Returns the value that will be returned from subsequent calls
to `next()`.

/// - Parameter n: The number of elements to look ahead. Default is
`0`. A value

///   of `0` means to look at the next element.

/// - Precondition: `n >= 0 && n < lookahead`.

/// - Requires: Neither `next()` nor `peek()` have been applied
to a copy of

///   `self` since the copy was made, and no preceding call to
`self.next()` has

///   returned `nil`.

/// - Note: It is safe to peek at values past the end of the
underlying generator

///  (`peek()` will return `nil` in such cases). It is also safe to
call `peek()`

///  repeatedly, even after it's returned `nil`, and similarly it is
safe to call

///  `next()` after `peek()` has returned `nil`.

public mutating func peek(n: Int = default) -> Base.Element?


BufferedSequence is a class because the generate() function needs to
mutate it. After the `first` property has been accessed, it needs to
cache the generator it used for that so it can return it from the next
call to generate(), but it also needs to nil out that cache at that time
so it doesn't try and return the same generator instance a second time
on a subsequent call to generate() (this way BufferedSequence can be
written to support non-destructive iteration if the underlying sequence
is non-destructive).

I've already started sketching out an implementation as well. I believe
it should be possible to optimize BufferedGenerator for a lookahead of 1
to avoid the heap allocation of an array.

-Kevin Ballard
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