Swift: Lazily encapsulating chains of map, filter, flatMap

Question

I have a list of animals:

let animals = ["bear", "dog", "cat"]

And some ways to transform that list:

typealias Transform = (String) -> [String]

let containsA: Transform = { $0.contains("a") ? [$0] : [] }
let plural:    Transform = { [$0 + "s"] }
let double:    Transform = { [$0, $0] }

As a slight aside, these are analogous to filter (outputs 0 or 1 element), map (exactly 1 element) and flatmap (more than 1 element) respectively but defined in a uniform way so that they can be handled consistently.

I want to create a lazy iterator which applies an array of these transforms to the list of animals:

extension Array where Element == String {
  func transform(_ transforms: [Transform]) -> AnySequence<String> {

    return AnySequence<String> { () -> AnyIterator<String> in
      var iterator = self
        .lazy
        .flatMap(transforms[0])
        .flatMap(transforms[1])
        .flatMap(transforms[2])
        .makeIterator()

      return AnyIterator {
        return iterator.next()
      }
    }
  }
}

which means I can lazily do:

let transformed = animals.transform([containsA, plural, double])

and to check the result:

print(Array(transformed))

I'm pleased with how succinct this is but clearly:

        .flatMap(transforms[0])
        .flatMap(transforms[1])
        .flatMap(transforms[2])

is an issue as it means the transform function will only work with an array of 3 transforms.

Edit: I tried:

  var lazyCollection = self.lazy
  for transform in transforms {
    lazyCollection = lazyCollection.flatMap(transform) //Error
  }
  var iterator = lazyCollection.makeIterator()

but on the marked row I get error:

Cannot assign value of type 'LazyCollection< FlattenCollection< LazyMapCollection< Array< String>, [String]>>>' to type 'LazyCollection< Array< String>>'

which I understand because each time around the loop another flatmap is being added, so the type is changing.

How can I make the transform function work with an array of any number of transforms?

One WET solution for a limited number of transforms would be (but YUK!)

  switch transforms.count {
  case 1:
    var iterator = self
      .lazy
      .flatMap(transforms[0])
      .makeIterator()
    return AnyIterator {
      return iterator.next()
    }
  case 2:
    var iterator = self
      .lazy
      .flatMap(transforms[0])
      .flatMap(transforms[1])
      .makeIterator()
    return AnyIterator {
      return iterator.next()
    }
  case 3:
    var iterator = self
      .lazy
      .flatMap(transforms[0])
      .flatMap(transforms[1])
      .flatMap(transforms[2])
      .makeIterator()
    return AnyIterator {
      return iterator.next()
    }
  default:
    fatalError(" Too many transforms!")
  }

Whole code:

let animals = ["bear", "dog", "cat"]

typealias Transform = (String) -> [String]

let containsA: Transform = { $0.contains("a") ? [$0] : [] }
let plural:    Transform = { [$0 + "s"] }
let double:    Transform = { [$0, $0] }

extension Array where Element == String {
  func transform(_ transforms: [Transform]) -> AnySequence<String> {

    return AnySequence<String> { () -> AnyIterator<String> in
      var iterator = self
        .lazy
        .flatMap(transforms[0])
        .flatMap(transforms[1])
        .flatMap(transforms[2])
        .makeIterator()

      return AnyIterator {
        return iterator.next()
      }
    }
  }
}

let transformed = animals.transform([containsA, plural, double])

print(Array(transformed))

Answer 1

Another approach to achieve what you want:

Edit: I tried:

var lazyCollection = self.lazy
for transform in transforms {
    lazyCollection = lazyCollection.flatMap(transform) //Error
}
var iterator = lazyCollection.makeIterator()

You were very near to your goal, if the both types in the Error line was assignable, your code would have worked.

A little modification:

var lazySequence = AnySequence(self.lazy)
for transform in transforms {
    lazySequence = AnySequence(lazySequence.flatMap(transform))
}
var iterator = lazySequence.makeIterator()

Or you can use reduce here:

var transformedSequence = transforms.reduce(AnySequence(self.lazy)) {sequence, transform in
    AnySequence(sequence.flatMap(transform))
}
var iterator = transformedSequence.makeIterator()

Whole code would be:

(EDIT Modified to include the suggestions from Martin R.)

let animals = ["bear", "dog", "cat"]

typealias Transform<Element> = (Element) -> [Element]

let containsA: Transform<String> = { $0.contains("a") ? [$0] : [] }
let plural:    Transform<String> = { [$0 + "s"] }
let double:    Transform<String> = { [$0, $0] }

extension Sequence {
    func transform(_ transforms: [Transform<Element>]) -> AnySequence<Element> {
        return transforms.reduce(AnySequence(self)) {sequence, transform in
            AnySequence(sequence.lazy.flatMap(transform))
        }
    }
}

let transformed = animals.transform([containsA, plural, double])

print(Array(transformed))

Answer 2

You can apply the transformations recursively if you define the method on the Sequence protocol (instead of Array). Also the constraint where Element == String is not needed if the transformations parameter is defined as an array of (Element) -> [Element].

extension Sequence {
    func transform(_ transforms: [(Element) -> [Element]]) -> AnySequence<Element> {
        if transforms.isEmpty {
            return AnySequence(self)
        } else {
            return lazy.flatMap(transforms[0]).transform(Array(transforms[1...]))
        }
    }
}

Answer 3

How about fully taking this into the functional world? For example using (dynamic) chains of function calls, like filter(containsA) | map(plural) | flatMap(double).

With a little bit of reusable generic code we can achieve some nice stuff.

Let's start with promoting some sequence and lazy sequence operations to free functions:

func lazy<S: Sequence>(_ arr: S) -> LazySequence<S> {
    return arr.lazy
}

func filter<S: Sequence>(_ isIncluded: @escaping (S.Element) throws -> Bool) -> (S) throws -> [S.Element] {
    return { try $0.filter(isIncluded) }
}

func filter<L: LazySequenceProtocol>(_ isIncluded: @escaping (L.Elements.Element) -> Bool) -> (L) -> LazyFilterSequence<L.Elements> {
    return { $0.filter(isIncluded) }
}

func map<S: Sequence, T>(_ transform: @escaping (S.Element) throws -> T) -> (S) throws -> [T] {
    return { try $0.map(transform) }
}

func map<L: LazySequenceProtocol, T>(_ transform: @escaping (L.Elements.Element) -> T) -> (L) -> LazyMapSequence<L.Elements, T> {
    return { $0.map(transform) }
}

func flatMap<S: Sequence, T: Sequence>(_ transform: @escaping (S.Element) throws -> T) -> (S) throws -> [T.Element] {
    return { try $0.flatMap(transform) }
}

func flatMap<L: LazySequenceProtocol, S: Sequence>(_ transform: @escaping (L.Elements.Element) -> S) -> (L) -> LazySequence<FlattenSequence<LazyMapSequence<L.Elements, S>>> {
    return { $0.flatMap(transform) }
}

Note that the lazy sequences counterparts are more verbose that the regular Sequence ones, but this is due to the verbosity of LazySequenceProtocol methods.

With the above we can create generic functions that receive arrays and return arrays, and this type of functions are extremely fitted for pipelining, so let's define a pipeline operator:

func |<T, U>(_ arg: T, _ f: (T) -> U) -> U {
    return f(arg)
}

Now all we need is to feed something to these functions, but to achieve this we'll need a little bit of tweaking over the Transform type:

typealias Transform<T, U> = (T) -> U

let containsA: Transform<String, Bool> = { $0.contains("a") }
let plural:    Transform<String, String> = { $0 + "s" }
let double:    Transform<String, [String]> = { [$0, $0] }

With all the above in place, things get easy and clear:

let animals = ["bear", "dog", "cat"]
let newAnimals = lazy(animals) | filter(containsA) | map(plural) | flatMap(double)
print(Array(newAnimals)) // ["bears", "bears", "cats", "cats"]