return-nil-from-proc.nim

proc makeJumpPlayerCommand(player: var Player,
                           platform: Platform): Command =
  var
    moveActorCommand = makeMoveActorCommand(player, platform.x,
                                            platform.y)

  if not isStrongEnough(player.jumpStrength, platform.altitude):
    echo("$1 is not strong enough to jump to the platform" % [player.name])
    return nil # <== Error: type mismatch: got (nil) but expected 'Command'

  result.execute = proc =
    moveActorCommand.execute()

  result.undo = proc =
    moveActorCommand.undo()


var
    player = getPlayer()
    player = getPlatform()
    command = makeJumpPlayerCommand(player, platform)

if command:
  command.execute()