Swift Sequences and Lazy Evaluation | Hacker News Reader

Swift Sequences and Lazy Evaluation

scottlogic.com

54 pointsianstallings9 comments

http://robots.thoughtbot.com/swift-sequences has a somewhat more advanced and in-depth overview of Sequence and Generator, although it doesn't use the laziness possibilities. It also points to the "Advanced Swift" WWDC session which covers the subject.

There's a different syntax available for sequences utilizing trailing closure syntax, in my opinion it's much more succinct, 13 lines vs. 22

  let Fibbonacci = SequenceOf { 

    var current = -1, last = 0

    return GeneratorOf {
      if current == -1 {
        current = 0
      } else if current == 0 {
        current = 1
      } else {
        var next = last + current
        last = current
        current = next
      }
      return current
    }
  }

And they'd both be five lines shorter if the fibonacci implementation weren't overcomplicated, making your gain even more significant (8 lines vs 17).

  let Fibbonacci = SequenceOf {
 
    var current = 0, next = 1
    
    return GeneratorOf {
      var ret = current
      current = next
      next = next + ret
      return ret
    }
  }

It's interesting how that sort of thing actually hurts the article. When I skimmed through that code I got stuck on "Why is this Fibonacci implementation so verbose?" rather than the actual topic of the paragraph in question.

And branch-less to boot

Good point, I was thinking that myself but wanted to keep the core algorithm to show specifically the boiler plate involved in solving in an OO style.

this stuff is always interesting. i don't understand how it ends up in the language in the form of 'for-in' though.

the syntax is similar to the old school BASIC for loop, but the implementation is heavier in the vast majority of cases where you write simple code for the loop body (i.e. avoiding heavyweight language constructs). Of course unless the compiler optimised out the lazy evaluations...

i would like laziness in this situation to be described by a keyword please, so that i don't have a whole language feature being slower by premature optimisation I don't get a say in... :)

I think you misunderstand, the Swift for-in relies on a sequence, i.e. an object that 'feeds' it a certain number of items. However, the sequence does not have to be lazy in its evaluation. And there is nothing that couples for-in to the concept of lazy evaluation.

When you use for-in to iterate over arrays, strings, dictionaries etc ... it is an iteration of the type you are no doubt familiar with, where you are iterating over some fixed pre-computed structure.

However, sequences can also generate their 'next' value lazily.

It is something that I consider to be an interesting topic, but would certainly not advocate that everything should become a lazy operation.

> i would like laziness in this situation to be described by a keyword please, so that i don't have a whole language feature being slower by premature optimisation I don't get a say in... :)

Rust uses the exact same external iterator pattern[0] yet is competitive with[1] a C-style for loop[2]

[0] http://doc.rust-lang.org/std/iter/

[1] up to and including inlining and full unrolling

[2] http://www.reddit.com/r/rust/comments/2957fg/can_i_request_a...

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