F# Cond Zip

gistfile1.fs

open System.Linq

let a = [0; 5; 10]
let b = [0 .. 10]

// enumerates left and yields element pairs combined by consuming right until 'consume' is false or either sequence is exhausted
let condZip (left:seq<'a>) (right:seq<'b>) consume project = 
  seq {
    use en = right.GetEnumerator()
    let hadNext = ref(en.MoveNext())
    for l in left do
      while (!hadNext && consume l en.Current) do // had next is used to ensure we do not drop elements when until returns false
        yield project l en.Current
        hadNext := en.MoveNext()
  }


let result = condZip a b (fun l r -> r <= l) (fun l r -> (r, l))
let expected = [(0, 0);(1, 5);(2, 5);(3, 5);(4, 5);(5, 5);(6, 10);(7, 10);(8, 10);(9, 10);(10, 10)]

Enumerable.SequenceEqual(result, expected)