Generics vs Typedesc : The Movie

gendesc.nim

# EXAMPLE A
iterator count* [T](start: T, step = 1): T {.closure.} =
  ## An iterator function that returns evenly spaced values of ordinal T,
  ## starting with `start`
  ## Note, this doesn't have any protections against overflow errors!
  ##
  ## Parameters:
  ##   start: A starting ordinal
  ##   step: The space between returned values.
  var i = start
  if step < 0:
    while True:
      dec(i, step)
      yield i
  if step > 0:
    while True:
      inc(i, step)
      yield i

when defined(isMainModule):
  block countBlock:
    var intSeq = newSeq[int]()
    var countIterator = count[int]
    for i in 1..5:
      intSeq.add(countIterator(0,5))
    assert(intSeq == @[5,10,15,20,25])
    echo(repr(intSeq))

# EXAMPLE B
iterator count* (T:typedesc, start: T, step = 1): T {.closure.} =
  ## An iterator function that returns evenly spaced values of ordinal T,
  ## starting with `start`
  ## Note, this doesn't have any protections against overflow errors!
  ##
  ## Parameters:
  ##   start: A starting ordinal
  ##   step: The space between returned values.
  var i = start
  if step < 0:
    while True:
      dec(i, step)
      yield i
  if step > 0:
    while True:
      inc(i, step)
      yield i

when defined(isMainModule):
  block countBlock:
    var intSeq = newSeq[int]()
    var countIterator = count(int)
    for i in 1..5:
      intSeq.add(countIterator(0,5))
    assert(intSeq == @[5,10,15,20,25])
    echo(repr(intSeq))

The correct way to define an iterator like count is this:

# EXAMPLE A
proc count*[T](start: T, step = 1): auto =
  iterator counter: T {.closure.} =
    ## An iterator function that returns evenly spaced values of ordinal T,
    ## starting with `start`
    ## Note, this doesn't have any protections against overflow errors!
    ##
    ## Parameters:
    ##   start: A starting ordinal
    ##   step: The space between returned values.
    var i = start
    if step < 0:
      while True:
        dec(i, step)
        yield i
    if step > 0:
      while True:
        inc(i, step)
        yield i

  return counter