LazySeq & GeneratedSeq

Everybody heard about lazy sequences, that's not new. But what the hell is Generated Sequence?

GeneratedSeq is basically a nice wrapper around closures countFn and generateFn. It doesn't store any values it gets from this functions, just gives more human-like interface (you can treat it as Sequence or Collection). It will use generateFn on every call to get the items with those indexes.

LazySeq is subclass of GeneratedSeq that actually saves values to storage index->value dictionary (is available for lookup) once they are calculated. Next time lookup occurs, saved value is taken without re-evaluation. To force re-evaluation, you can use resetStorage method.

Version 0.6.1 introduces single-value transform, with both no-storage GeneratedTransform and stored LazyTransform options.

Example

Lets have a try!

let seq = GeneratedSeq(count: { () -> Int in
                    return 5
                }, generate: { (idx, _) -> String? in
                    guard (idx < 5) else {
                        return nil
                    }
                    return "item\(idx)"
                })

seq // GeneratedSeq<String>
seq[2] // "item2"
seq[2..<5] // ["item2", "item3", "item4"]
seq[5] // crash, index out of range
seq.get(0) // Optional("item0")
seq.get(5) // nil

Q: Why generate returns optional, while non-optional seq is created?

A: Sometimes, you cannot create items beyond some index, but it doesn't mean sequence will be broken, because count function will limit us to non-nil results.

.map function, on the other side, doesn't return optional values on transformation, because you are guaranteed to have the item if the first place, and will never run off-bounds.

Q: What is going on in second parameter of generate closure?

A: When we get our value with .get(idx: context:) function, we can pass anything to the generate function.

let seq = GeneratedSeq(count: { () -> Int in
                    return 5
                }, generate: { (idx, context) -> String in
                    return "item\(idx) with context \(context)"
                })
seq[2] // "item2 with context nil"
seq.get(2, [3, 4]) // "item2 with context [3, 4]"

You can pass closures to the context too :)

LazyTransform example:

var a: [Int] = [1, 2, 3]
let transform = LazyTransform({ () -> Int in
    return a.count
})
transform.value() // 3
a = [1, 2, 3, 4]
transform.value() // 3 (because it's stored, GeneratedTransform would yield 4)
transform.reset()
transform.value() // 4

Special olympics

Fibonacci!

var seq: LazySeq<Int>! // so we can reference it inside
seq = LazySeq(count: nil, generate: { (idx, _) -> Int in
    if idx <= 1 {
        return 1
    }
    return seq[idx-1]+seq[idx-2]
})

seq.prefix(10) // [1, 1, 2, 3, 5, 8, 13, 21, 34, 55]
seq[10..<15] // [89, 144, 233, 377, 610]

Installation

LazySeq is available through CocoaPods. To install it, simply add the following line to your Podfile:

pod 'LazySeq'

Author

Oleksii Horishnii, [email protected]

License

LazySeq is available under the MIT license. See the LICENSE file for more info.