MyPrefixWhile.swift

public struct MyPrefixWhile<Base: Sequence> : Sequence {
  var predicate: (Base.Element)->Bool
  var base: Base
  
  public struct Iterator : IteratorProtocol {
    var predicate: (Base.Element)->Bool
    var base: Base.Iterator?
    public mutating func next() -> Base.Element? {
      guard let r = base?.next() else { return nil }
      if predicate(r) { return r }
      base = nil
      return nil
    }
  }

  public func makeIterator() -> Iterator {
    return Iterator(predicate: predicate, base: base.makeIterator())
  }
}

extension Sequence {
  public func myPrefix(
    while predicate: @escaping (Element) -> Bool
  ) -> MyPrefixWhile<Self> {
    // Wrap it in AnySequence if you must
    return MyPrefixWhile(predicate: predicate, base: self)
  }
}

let positiveInts = sequence(first: 0) { $0 + 1 }

print(Array(positiveInts.myPrefix { $0 < 0 }))
print(Array(positiveInts.myPrefix { $0 < 10 }))