Negative indices workaround

workaround.nim

proc `..<`(a, b: int): auto =
  if a < 0:
    raise newException(IndexError, "Left index == " & $a & ", out of range")
  if b <= 0:
    return 1 .. 0
  if a >= b:
    return a .. <a
  return a .. <b

proc `.!`(a, b: int): auto =
  if a < 0:
    raise newException(IndexError, "Left index == " & $a & ", out of range")
  if b < 0:
    return 1 .. 0
  if a > b:
    return a .. <a
  return a .. b
    
template asserr(x: expr): stmt {.immediate.}=
  ## assert that this expression produces an IndexError
  try:
    discard x
    assert false
  except IndexError:
    discard

var s0: seq[int] = @[]
var s1 = @[0]
var s4 = @[0, 1, 2, 3]

assert s0[0 ..< 0] == @[]
assert s1[0 ..< 1] == @[0]
assert s4[0 ..< 0] == @[]
assert s1[1 ..< 1] == @[]
assert s4[1 ..< 2] == @[1]
assert s4[0 ..< 3] == @[0, 1, 2]
assert s4[0 ..< 4] == @[0, 1, 2, 3]
asserr s0[0 ..< 1]
asserr s1[-1 ..< 0]
asserr s4[0 ..< 100]

assert s0[0 .! -1] == @[]
assert s1[0 .! 0] == @[0]
assert s4[0 .! -1] == @[]
assert s1[1 .! 0] == @[]
assert s4[1 .! 1] == @[1]
assert s4[0 .! 2] == @[0, 1, 2]
assert s4[0 .! 3] == @[0, 1, 2, 3]
asserr s0[0 .! 0]
asserr s1[-1 .! -1]
asserr s4[0 .! 99]

assert s0[0 .. < 0] == @[]
assert s1[0 .. < 1] == @[0]
assert s4[0 .. < 0] == @[0, 1, 2, 3]
assert s1[1 .. < 1] == @[]
assert s4[1 .. < 2] == @[1]
assert s4[0 .. < 3] == @[0, 1, 2]
assert s4[0 .. < 4] == @[0, 1, 2, 3]
asserr s0[0 .. < 1]
assert s1[-1 .. < 0] == @[0]
asserr s4[0 .. < 100]