Flatmap

flatmap.nim

import future
import options

#[
proc flatMap[X](iter: iterator(): X, mapper: X -> iterator(): X): (iterator(): B) =
  result = proc() =
    yield X


proc iter(): int =
  yield 1
  yield 2
  yield 3

let r =
]#

iterator items[X](o: Option[X]): X =
  if o.isSome:
    yield o.get

proc flatMap[X, Y](enumerable: seq[X], mapper: X -> Y): seq[X] =
  result = newSeq[X]()
  for x in enumerable:
    for y in mapper(x):
      result &= y

let s = @[1, 2, 3]

# Emulating map
echo s.flatMap(x => @[x*x])
echo s.flatMap(x => some(x*x))

# Emulating filter
echo s.flatMap(x => (if x mod 2 == 1: @[x] else: @[]))
echo s.flatMap(x => (if x mod 2 == 1: some(x) else: none(int)))

# flatMap allows map + filter
echo s.flatMap(x => (if x mod 2 == 1: @[x*x] else: @[]))
echo s.flatMap(x => (if x mod 2 == 1: some(x*x) else: none(int)))