swift-lazylist

A Haskell-Like Lazy List in Swift.

Synopsis

Infinite list

let ns = lazylist { $0 } // infinite list of natural numbers
ns.filter{$0 % 2 == 1}.map{$0 * $0}.take(10) // [1, 9, 25, 49, 81, 121, 169, 225, 289, 361]
ns.map{$0 * $0}.filter{$0 % 2 == 1}.take(10) // [1, 9, 25, 49, 81, 121, 169, 225, 289, 361]

Finite list

let p100 = lazylist(Array(0..<100)) // [0, 1, 2 ... 99]
p100.drop(10).take(10)   // [10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
p100.drop(90).take(20)   // [90, 91, 92, 93, 94, 95, 96, 97, 98, 99] -- only 10
p100.drop(100).take(10)  // [] -- no elements left

Indefinite list -- Lazy list with a terminator

let m100 = lazylist { $0 < 100 ? -$0 : nil } // [0, -1, -2 ... -99]
m100.drop(10).take(10)  // [-10, -11, -12, -13, -14, -15, -16, -17, -18, -19]
m100.drop(90).take(20)  // [-90, -91, -92, -93, -94, -95, -96, -97, -98, -99]
m100.drop(100).take(10) // []

Infinite list with seed array

let fibs = lazylist([0,1]){ i, a in a[i-2] + a[i-1] }
fibs.drop(10).take(10) // [55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181] -- F10...F19

let primes = lazylist([2,3]) { i, ps in
    var n = ps[i-1] + 2
    while true {
        for p in ps {
            if n % p == 0 { break }
            if p * p > n  { return n }
        }
        n += 2
    }
}
primes.take(25) // first 25 prime numbers -- primes less than 100

Inifinite list of repeated values

Simple { constant } does not work. Use { i in constant } to make type inference happy.

let theAnswer = lazylist { i in 42 }
theAnswer.drop(40).take(2) // [42, 42]

Caution: Enumerate with care

LazyList conforms to the Sequence protocol so it supports for-in loop like this:

/// you can enumurate it in for-in loop so long as you `break` it anyhow
for (i,v) in enumerate(ns) {
    if i % 7 != 0 { continue }
    if v > 42 { break }
    print("ns[\(i)] = \(v)")
}

However, this is very inefficient unless the list does not depend on the seed array. Such lists:

0. are constructed without seed array
1. AND .filter() is never applied

The natural number list example above meets that condition until you apply .filter(). If you are not sure avoid using enumurator directly. Just take() to get an ordinary array

Prerequisite

Swift 4 or better.