diff --git a/nonebot/command.py b/nonebot/command/__init__.py
similarity index 82%
rename from nonebot/command.py
rename to nonebot/command/__init__.py
index c949778c..3fda70af 100644
--- a/nonebot/command.py
+++ b/nonebot/command/__init__.py
@@ -3,16 +3,18 @@ import re
 import shlex
 from datetime import datetime
 from typing import (
-    Tuple, Union, Callable, Iterable, Any, Optional, List, Dict
+    Tuple, Union, Callable, Iterable, Any, Optional, List, Dict,
+    Awaitable
 )
 
-from . import NoneBot, permission as perm
-from .helpers import context_id, send, render_expression
-from .log import logger
-from .message import Message
-from .session import BaseSession
-from .typing import (
-    Context_T, CommandName_T, CommandArgs_T, Message_T
+from nonebot import NoneBot, permission as perm
+from nonebot.command.argfilter import ArgFilter_T, ValidateError
+from nonebot.helpers import context_id, send, render_expression
+from nonebot.log import logger
+from nonebot.message import Message
+from nonebot.session import BaseSession
+from nonebot.typing import (
+    Context_T, CommandName_T, CommandArgs_T, Message_T, State_T
 )
 
 # key: one segment of command name
@@ -27,19 +29,25 @@ _aliases = {}  # type: Dict[str, CommandName_T]
 # value: CommandSession object
 _sessions = {}  # type: Dict[str, CommandSession]
 
+CommandHandler_T = Callable[['CommandSession'], Any]
+
 
 class Command:
     __slots__ = ('name', 'func', 'permission',
                  'only_to_me', 'privileged', 'args_parser_func')
 
-    def __init__(self, *, name: CommandName_T, func: Callable,
-                 permission: int, only_to_me: bool, privileged: bool):
+    def __init__(self, *,
+                 name: CommandName_T,
+                 func: CommandHandler_T,
+                 permission: int,
+                 only_to_me: bool,
+                 privileged: bool):
         self.name = name
         self.func = func
         self.permission = permission
         self.only_to_me = only_to_me
         self.privileged = privileged
-        self.args_parser_func = None
+        self.args_parser_func: Optional[CommandHandler_T] = None
 
     async def run(self, session, *,
                   check_perm: bool = True,
@@ -56,8 +64,28 @@ class Command:
         if self.func and has_perm:
             if dry:
                 return True
-            if self.args_parser_func:
-                await self.args_parser_func(session)
+
+            if session.current_arg_filters is not None and \
+                    session.current_key is not None:
+                # argument-level filters are given, use them
+                arg = session.current_arg
+                for f in session.current_arg_filters:
+                    try:
+                        res = f(arg)
+                        if isinstance(res, Awaitable):
+                            res = await res
+                        arg = res
+                    except ValidateError as e:
+                        # validation failed
+                        session.pause(e.message)
+
+                # passed all filters
+                session.state[session.current_key] = arg
+            else:
+                # fallback to command-level args_parser_func
+                if self.args_parser_func:
+                    await self.args_parser_func(session)
+
             await self.func(session)
             return True
         return False
@@ -77,14 +105,14 @@ class Command:
 class CommandFunc:
     __slots__ = ('cmd', 'func')
 
-    def __init__(self, cmd: Command, func: Callable):
+    def __init__(self, cmd: Command, func: CommandHandler_T):
         self.cmd = cmd
         self.func = func
 
-    def __call__(self, *args, **kwargs):
-        return self.func(*args, **kwargs)
+    def __call__(self, session: 'CommandSession') -> Any:
+        return self.func(session)
 
-    def args_parser(self, parser_func: Callable):
+    def args_parser(self, parser_func: CommandHandler_T) -> CommandHandler_T:
         """
         Decorator to register a function as the arguments parser of
         the corresponding command.
@@ -110,7 +138,7 @@ def on_command(name: Union[str, CommandName_T], *,
     :param shell_like: use shell-like syntax to split arguments
     """
 
-    def deco(func: Callable) -> Callable:
+    def deco(func: CommandHandler_T) -> CommandHandler_T:
         if not isinstance(name, (str, tuple)):
             raise TypeError('the name of a command must be a str or tuple')
         if not name:
@@ -153,7 +181,7 @@ class CommandGroup:
                  privileged: Optional[bool] = None,
                  shell_like: Optional[bool] = None):
         self.basename = (name,) if isinstance(name, str) else name
-        self.permission = permission
+        self.permission = permission  # TODO: use .pyi
         self.only_to_me = only_to_me
         self.privileged = privileged
         self.shell_like = shell_like
@@ -204,10 +232,10 @@ def _find_command(name: Union[str, CommandName_T]) -> Optional[Command]:
     return cmd if isinstance(cmd, Command) else None
 
 
-class _FurtherInteractionNeeded(Exception):
+class _PauseException(Exception):
     """
-    Raised by session.pause() indicating that the command should
-    enter interactive mode to ask the user for some arguments.
+    Raised by session.pause() indicating that the command session
+    should be paused to ask the user for some arguments.
     """
     pass
 
@@ -244,23 +272,48 @@ class SwitchException(Exception):
 
 
 class CommandSession(BaseSession):
-    __slots__ = ('cmd', 'current_key', 'current_arg',
+    __slots__ = ('cmd', 'current_key', 'current_arg', 'current_arg_filters',
                  'current_arg_text', 'current_arg_images',
-                 'args', '_last_interaction', '_running')
+                 '_state', '_last_interaction', '_running')
 
     def __init__(self, bot: NoneBot, ctx: Context_T, cmd: Command, *,
                  current_arg: str = '', args: Optional[CommandArgs_T] = None):
         super().__init__(bot, ctx)
         self.cmd = cmd  # Command object
-        self.current_key = None  # current key that the command handler needs
-        self.current_arg = None  # current argument (with potential CQ codes)
-        self.current_arg_text = None  # current argument without any CQ codes
-        self.current_arg_images = None  # image urls in current argument
-        self.refresh(ctx, current_arg=current_arg)
-        self.args = args or {}
+
+        # unique key of the argument that is currently requesting (asking)
+        self.current_key: Optional[str] = None
+
+        # initialize current argument
+        self.current_arg: str = ''  # with potential CQ codes
+        self.current_arg_text: str = ''  # without any CQ codes TODO: property
+        self.current_arg_images: List[str] = []  # image urls
+        self.refresh(ctx, current_arg=current_arg)  # fill the above
+
+        # initialize current argument filters
+        self.current_arg_filters: Optional[List[ArgFilter_T]] = None
+
+        self._state: State_T = {}
+        if args:
+            self._state.update(args)
         self._last_interaction = None  # last interaction time of this session
         self._running = False
 
+    @property
+    def state(self) -> State_T:
+        """
+        State of the session.
+
+        This contains all named arguments and
+        other session scope temporary values.
+        """
+        return self._state
+
+    @property
+    def args(self) -> CommandArgs_T:
+        """Deprecated. Use `session.state` instead."""
+        return self.state
+
     @property
     def running(self) -> bool:
         return self._running
@@ -292,7 +345,7 @@ class CommandSession(BaseSession):
         Shell-like argument list, similar to sys.argv.
         Only available while shell_like is True in on_command decorator.
         """
-        return self.get_optional('argv', [])
+        return self.state.get('argv', [])
 
     def refresh(self, ctx: Context_T, *, current_arg: str = '') -> None:
         """
@@ -308,38 +361,46 @@ class CommandSession(BaseSession):
         self.current_arg_images = [s.data['url'] for s in current_arg_as_msg
                                    if s.type == 'image' and 'url' in s.data]
 
-    def get(self, key: Any, *,
-            prompt: Optional[Message_T] = None, **kwargs) -> Any:
+    def get(self, key: str, *,
+            prompt: Optional[Message_T] = None,
+            arg_filters: Optional[List[ArgFilter_T]] = None,
+            **kwargs) -> Any:
         """
         Get an argument with a given key.
 
         If the argument does not exist in the current session,
-        a FurtherInteractionNeeded exception will be raised,
-        and the caller of the command will know it should keep
-        the session for further interaction with the user.
+        a pause exception will be raised, and the caller of
+        the command will know it should keep the session for
+        further interaction with the user.
 
         :param key: argument key
         :param prompt: prompt to ask the user
+        :param arg_filters: argument filters for next user input
         :return: the argument value
         """
-        value = self.get_optional(key)
-        if value is not None:
-            return value
+        if key in self.state:
+            return self.state[key]
 
         self.current_key = key
+        self.current_arg_filters = arg_filters
+        # TODO: self.current_send_kwargs
         # ask the user for more information
         self.pause(prompt, **kwargs)
 
-    def get_optional(self, key: Any,
+    def get_optional(self, key: str,
                      default: Optional[Any] = None) -> Optional[Any]:
-        """Simply get a argument with given key."""
-        return self.args.get(key, default)
+        """
+        Simply get a argument with given key.
+
+        Deprecated. Use `session.state.get()` instead.
+        """
+        return self.state.get(key, default)
 
     def pause(self, message: Optional[Message_T] = None, **kwargs) -> None:
         """Pause the session for further interaction."""
         if message:
             asyncio.ensure_future(self.send(message, **kwargs))
-        raise _FurtherInteractionNeeded
+        raise _PauseException
 
     def finish(self, message: Optional[Message_T] = None, **kwargs) -> None:
         """Finish the session."""
@@ -564,9 +625,7 @@ async def _real_run_command(session: CommandSession,
             handled = future.result()
         except asyncio.TimeoutError:
             handled = True
-        except (_FurtherInteractionNeeded,
-                _FinishException,
-                SwitchException) as e:
+        except (_PauseException, _FinishException, SwitchException) as e:
             raise e
         except Exception as e:
             logger.error(f'An exception occurred while '
@@ -574,7 +633,7 @@ async def _real_run_command(session: CommandSession,
             logger.exception(e)
             handled = True
         raise _FinishException(handled)
-    except _FurtherInteractionNeeded:
+    except _PauseException:
         session.running = False
         if disable_interaction:
             # if the command needs further interaction, we view it as failed
diff --git a/nonebot/command/argfilter/__init__.py b/nonebot/command/argfilter/__init__.py
new file mode 100644
index 00000000..507f89ac
--- /dev/null
+++ b/nonebot/command/argfilter/__init__.py
@@ -0,0 +1,8 @@
+from typing import Callable, Any, Awaitable, Union
+
+ArgFilter_T = Callable[[Any], Union[Any, Awaitable[Any]]]
+
+
+class ValidateError(ValueError):
+    def __init__(self, message=None):
+        self.message = message
diff --git a/nonebot/command/argfilter/converters.py b/nonebot/command/argfilter/converters.py
new file mode 100644
index 00000000..2dff731f
--- /dev/null
+++ b/nonebot/command/argfilter/converters.py
@@ -0,0 +1,40 @@
+from typing import Optional, List
+
+
+def _simple_chinese_to_bool(text: str) -> Optional[bool]:
+    """
+    Convert a chinese text to boolean.
+
+    Examples:
+
+        是的 -> True
+        好的呀 -> True
+        不要 -> False
+        不用了 -> False
+        你好呀 -> None
+    """
+    text = text.strip().lower().replace(' ', '') \
+        .rstrip(',.!?~，。！？～了的呢吧呀啊呗啦')
+    if text in {'要', '用', '是', '好', '对', '嗯', '行',
+                'ok', 'okay', 'yeah', 'yep',
+                '当真', '当然', '必须', '可以', '肯定', '没错', '确定', '确认'}:
+        return True
+    if text in {'不', '不要', '不用', '不是', '否', '不好', '不对', '不行', '别',
+                'no', 'nono', 'nonono', 'nope', '不ok', '不可以', '不能',
+                '不可以'}:
+        return False
+    return None
+
+
+def _split_nonempty_lines(text: str) -> List[str]:
+    return list(filter(lambda x: x, text.splitlines()))
+
+
+def _split_nonempty_stripped_lines(text: str) -> List[str]:
+    return list(filter(lambda x: x,
+                       map(lambda x: x.strip(), text.splitlines())))
+
+
+simple_chinese_to_bool = _simple_chinese_to_bool
+split_nonempty_lines = _split_nonempty_lines
+split_nonempty_stripped_lines = _split_nonempty_stripped_lines
diff --git a/nonebot/command/argfilter/extractors.py b/nonebot/command/argfilter/extractors.py
new file mode 100644
index 00000000..6c0e54c2
--- /dev/null
+++ b/nonebot/command/argfilter/extractors.py
@@ -0,0 +1,29 @@
+import re
+from typing import List
+
+from nonebot.message import Message
+from nonebot.typing import Message_T
+
+
+def _extract_text(arg: Message_T) -> str:
+    """Extract all plain text segments from a message-like object."""
+    arg_as_msg = Message(arg)
+    return arg_as_msg.extract_plain_text()
+
+
+def _extract_image_urls(arg: Message_T) -> List[str]:
+    """Extract all image urls from a message-like object."""
+    arg_as_msg = Message(arg)
+    return [s.data['url'] for s in arg_as_msg
+            if s.type == 'image' and 'url' in s.data]
+
+
+def _extract_numbers(arg: Message_T) -> List[float]:
+    """Extract all numbers (integers and floats) from a message-like object."""
+    s = str(arg)
+    return list(map(float, re.findall(r'[+-]?(\d*\.?\d+|\d+\.?\d*)', s)))
+
+
+extract_text = _extract_text
+extract_image_urls = _extract_image_urls
+extract_numbers = _extract_numbers
diff --git a/nonebot/command/argfilter/validators.py b/nonebot/command/argfilter/validators.py
new file mode 100644
index 00000000..373760af
--- /dev/null
+++ b/nonebot/command/argfilter/validators.py
@@ -0,0 +1,101 @@
+import re
+from typing import Callable, Any
+
+from nonebot.command.argfilter import ValidateError
+
+
+class BaseValidator:
+    def __init__(self, message=None):
+        self.message = message
+
+    def raise_failure(self):
+        raise ValidateError(self.message)
+
+
+class not_empty(BaseValidator):
+    """
+    Validate any object to ensure it's not empty (is None or has no elements).
+    """
+
+    def __call__(self, value):
+        if value is None:
+            self.raise_failure()
+        if hasattr(value, '__len__') and value.__len__() == 0:
+            self.raise_failure()
+        return value
+
+
+class fit_size(BaseValidator):
+    """
+    Validate any sized object to ensure the size/length
+    is in a given range [min_length, max_length].
+    """
+
+    def __init__(self, min_length: int = 0, max_length: int = None,
+                 message=None):
+        super().__init__(message)
+        self.min_length = min_length
+        self.max_length = max_length
+
+    def __call__(self, value):
+        length = len(value) if value is not None else 0
+        if length < self.min_length or \
+                (self.max_length is not None and length > self.max_length):
+            self.raise_failure()
+        return value
+
+
+class match_regex(BaseValidator):
+    """
+    Validate any string object to ensure it matches a given pattern.
+    """
+
+    def __init__(self, pattern: str, message=None, *, flags=0,
+                 fullmatch: bool = False):
+        super().__init__(message)
+        self.pattern = re.compile(pattern, flags)
+        self.fullmatch = fullmatch
+
+    def __call__(self, value):
+        if self.fullmatch:
+            if not re.fullmatch(self.pattern, value):
+                self.raise_failure()
+        else:
+            if not re.match(self.pattern, value):
+                self.raise_failure()
+        return value
+
+
+class ensure_true(BaseValidator):
+    """
+    Validate any object to ensure the result of applying
+    a boolean function to it is True.
+    """
+
+    def __init__(self, bool_func: Callable[[Any], bool], message=None):
+        super().__init__(message)
+        self.bool_func = bool_func
+
+    def __call__(self, value):
+        if self.bool_func(value) is not True:
+            self.raise_failure()
+        return value
+
+
+class between_inclusive(BaseValidator):
+    """
+    Validate any comparable object to ensure it's between
+    `start` and `end` inclusively.
+    """
+
+    def __init__(self, start=None, end=None, message=None):
+        super().__init__(message)
+        self.start = start
+        self.end = end
+
+    def __call__(self, value):
+        if self.start is not None and value < self.start:
+            self.raise_failure()
+        if self.end is not None and self.end < value:
+            self.raise_failure()
+        return value