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

[Félix Cloutier]

Not a fan of anything that reminds me of Java streams.

Is the case where making an array from the sequence isn't possible significant enough for a new standard API?

Félix

> Le 31 déc. 2015 à 20:50:40, Howard Lovatt via swift-evolution <swift-evolution at swift.org> a écrit :
> 
> +1
> 
> Sent from my iPad
> 
> On 1 Jan 2016, at 11:16 AM, Kevin Ballard via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> 
>> 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() -> BufferedSequence.BufferedGenerator<Base.Generator>
>> 
>>     /// Returns the first element of the underlying sequence, **nondestructively**.
>>     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 generator.
>>     /// - 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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