Nim Iterators and Sequences — Lazy and Eager Collection Processing

# Custom iterator
iterator countdown(n: int): int =
  var i = n
  while i >= 0:
    yield i
    dec i

for i in countdown(5):
  echo i  # 5, 4, 3, 2, 1, 0

# Built-in iterators
for i in 0..5: echo i        # Range
for i in items(@[1,2,3]):    # Items in seq
for i in pairs(@[1,2,3]):    # Index-value pairs

var s = @[1, 2, 3]      # Create
s.add(4)                # Append
s.insert(0, 0)          # Insert at index
s.delete(2)             # Delete by index

# Map and filter
import sequtils
let doubled = s.mapIt(it * 2)
let evens = s.filterIt(it mod 2 == 0)
let sum = s.foldl(a + b)