prefix.swift

// Single-struct version.
// This seems to work just as well as having a custom generator struct.
// I'm not sure if there are other (performance?) implications, however.

struct PrefixSequence<T> : SequenceType
{
    let base: AnySequence<T>
    let maxLength: Int
    
    init<S : SequenceType where S.Generator.Element == T>(_ base: S, _ maxLength: Int) {
        self.base = AnySequence(base)
        self.maxLength = maxLength
    }
    
    func generate() -> AnyGenerator<T>
    {
        let generator = base.generate()
        var remaining = maxLength
        return anyGenerator {
            if remaining-- == 0 { return nil }
            return generator.next()
        }
    }
}

extension SequenceType
{
    // this could also just return PrefixSequence<Generator.Element>,
    // but then callers would have to lazy() it to get map, filter, etc.
    func prefix(maxLength: Int) -> LazySequence<PrefixSequence<Generator.Element>>
    {
        return lazy(PrefixSequence(self, maxLength))
    }
}