diff --git a/docs/CHANGELOG.md b/docs/CHANGELOG.md
index 27aa5460..828a850d 100644
--- a/docs/CHANGELOG.md
+++ b/docs/CHANGELOG.md
@@ -1,5 +1,10 @@
 # Change Log
 
+## 0.5.2
+
+- 📝 Refactor codebase: unify type annotations and import future features
+- 🐛 Allow methods decorated by @ProcGroup.add_proc to return None
+
 ## 0.5.1
 
 - 🚑 Remove remaining more-itertools
diff --git a/pipen/channel.py b/pipen/channel.py
index 8760f131..7277fec3 100644
--- a/pipen/channel.py
+++ b/pipen/channel.py
@@ -1,7 +1,9 @@
 """Provide some function for creating and modifying channels (dataframes)"""
+from __future__ import annotations
+
 from glob import glob
 from os import path
-from typing import Any, List, Union
+from typing import Any, List
 
 import pandas
 from pandas import DataFrame
@@ -14,7 +16,7 @@ class Channel(DataFrame):
     """A DataFrame wrapper with creators"""
 
     @classmethod
-    def create(cls, value: Union[DataFrame, List[Any]]) -> DataFrame:
+    def create(cls, value: DataFrame | List[Any]) -> DataFrame:
         """Create a channel from a list.
 
         The second dimension is identified by tuple. if all elements are tuple,
@@ -151,7 +153,7 @@ def from_table(cls, *args, **kwargs):
 @register_verb(DataFrame)
 def expand_dir(
     data: DataFrame,
-    col: Union[str, int] = 0,
+    col: str | int = 0,
     pattern: str = "*",
     ftype: str = "any",
     sortby: str = "name",
@@ -189,7 +191,7 @@ def expand_dir(
 
 
 @register_verb(DataFrame)
-def collapse_files(data: DataFrame, col: Union[str, int] = 0) -> DataFrame:
+def collapse_files(data: DataFrame, col: str | int = 0) -> DataFrame:
     """Collapse a Channel according to the files in <col>,
     other cols will use the values in row 0.
 
diff --git a/pipen/cli/version.py b/pipen/cli/version.py
index 06f7520b..ee43a0c8 100644
--- a/pipen/cli/version.py
+++ b/pipen/cli/version.py
@@ -51,5 +51,5 @@ def exec_command(self, args: Namespace) -> None:
             ver = versions[key]
             verlines = ver.splitlines()
             print(f"{key.ljust(keylen)}: {verlines.pop(0)}")
-            for verline in verlines:
+            for verline in verlines:  # pragma: no cover
                 print(f"{' ' * keylen}  {verline}")
diff --git a/pipen/defaults.py b/pipen/defaults.py
index 1954ba1b..e860be8a 100644
--- a/pipen/defaults.py
+++ b/pipen/defaults.py
@@ -59,16 +59,16 @@
 
 # Just the total width of the terminal
 # when logging with a rich.Panel()
-CONSOLE_WIDTH_WITH_PANEL: int = 100
+CONSOLE_WIDTH_WITH_PANEL = 100
 # The width of the terminal when the width cannot be detected,
 # we are probably logging into a file
-CONSOLE_DEFAULT_WIDTH: int = 256
+CONSOLE_DEFAULT_WIDTH = 256
 # [05/16/22 11:46:40] I
 # v0.3.4:
 # 05-16 11:11:11 I
 # The markup code is included
 # Don't modify this unless the logger formatter is changed
-CONSOLE_WIDTH_SHIFT: int = 25
+CONSOLE_WIDTH_SHIFT = 25
 # For pipen scheduler plugins
 SCHEDULER_ENTRY_GROUP = "pipen_sched"
 # For pipen template plugins
diff --git a/pipen/job.py b/pipen/job.py
index ca35a30f..06557d95 100644
--- a/pipen/job.py
+++ b/pipen/job.py
@@ -1,10 +1,12 @@
 """Provide the Job class"""
+from __future__ import annotations
+
 import logging
 import shlex
 from os import PathLike
 from pathlib import Path
 import shutil
-from typing import TYPE_CHECKING, Any, Dict, Mapping, Union
+from typing import TYPE_CHECKING, Any, Dict, Mapping
 
 from diot import OrderedDiot
 from xqute import Job as XquteJob
@@ -210,7 +212,7 @@ def template_data(self) -> Mapping[str, Any]:
 
     def log(
         self,
-        level: Union[int, str],
+        level: int | str,
         msg: str,
         *args,
         limit: int = 3,
diff --git a/pipen/pipen.py b/pipen/pipen.py
index f0cbf90e..7db3acdd 100644
--- a/pipen/pipen.py
+++ b/pipen/pipen.py
@@ -1,11 +1,13 @@
 """Main entry module, provide the Pipen class"""
+from __future__ import annotations
+
 import asyncio
 from itertools import chain
 from os import PathLike
 from pathlib import Path
 import pprint
 import textwrap
-from typing import ClassVar, Iterable, List, Sequence, Type, Union
+from typing import ClassVar, Iterable, List, Sequence, Type
 
 from diot import Diot
 from rich import box
@@ -64,7 +66,7 @@ def __init__(
         self,
         name: str = None,
         desc: str = None,
-        outdir: PathLike = None,
+        outdir: str | PathLike = None,
         **kwargs,
     ) -> None:
         """Constructor"""
@@ -215,7 +217,7 @@ def set_data(self, *indata: Iterable) -> "Pipen":
 
     def set_starts(
         self,
-        *procs: Union[Type[Proc], Sequence[Type[Proc]]],
+        *procs: Type[Proc] | Sequence[Type[Proc]],
         clear: bool = True,
     ):
         """Set the starts
diff --git a/pipen/pluginmgr.py b/pipen/pluginmgr.py
index f8c1b051..d2587f27 100644
--- a/pipen/pluginmgr.py
+++ b/pipen/pluginmgr.py
@@ -1,7 +1,8 @@
 """Define hooks specifications and provide plugin manager"""
-import signal
+from __future__ import annotations
+
 from pathlib import Path
-from typing import Any, Dict, Optional, Union, TYPE_CHECKING
+from typing import Any, Dict, TYPE_CHECKING
 
 from simplug import Simplug, SimplugResult
 from xqute import JobStatus, Scheduler
@@ -11,6 +12,7 @@
 
 
 if TYPE_CHECKING:  # pragma: no cover
+    import signal
     from xqute import Xqute
     from .job import Job
     from .proc import Proc
@@ -35,7 +37,7 @@ def on_setup(config: Dict[str, Any]) -> None:
 
 
 @plugin.spec
-async def on_init(pipen: "Pipen") -> None:
+async def on_init(pipen: Pipen) -> None:
     """When the pipeline is initialized, and default configs are loaded
 
     Args:
@@ -44,7 +46,7 @@ async def on_init(pipen: "Pipen") -> None:
 
 
 @plugin.spec
-async def on_start(pipen: "Pipen") -> None:
+async def on_start(pipen: Pipen) -> None:
     """Right before the pipeline starts running.
 
     Process relationships are inferred.
@@ -55,7 +57,7 @@ async def on_start(pipen: "Pipen") -> None:
 
 
 @plugin.spec
-async def on_complete(pipen: "Pipen", succeeded: bool):
+async def on_complete(pipen: Pipen, succeeded: bool):
     """The the pipeline is completed.
 
     Args:
@@ -65,7 +67,7 @@ async def on_complete(pipen: "Pipen", succeeded: bool):
 
 
 @plugin.spec
-def on_proc_init(proc: "Proc"):
+def on_proc_init(proc: Proc):
     """Called before proc get instantiated.
 
     Enables plugins to modify the default attributes of processes
@@ -76,7 +78,7 @@ def on_proc_init(proc: "Proc"):
 
 
 @plugin.spec
-def on_proc_input_computed(proc: "Proc"):
+def on_proc_input_computed(proc: Proc):
     """Called after process input data is computed.
 
     Args:
@@ -85,7 +87,7 @@ def on_proc_input_computed(proc: "Proc"):
 
 
 @plugin.spec
-async def on_proc_start(proc: "Proc"):
+async def on_proc_start(proc: Proc):
     """When a process is starting
 
     Args:
@@ -94,7 +96,7 @@ async def on_proc_start(proc: "Proc"):
 
 
 @plugin.spec(result=SimplugResult.TRY_ALL_FIRST_AVAIL)
-def on_proc_shutdown(proc: "Proc", sig: Optional[signal.Signals]) -> None:
+def on_proc_shutdown(proc: Proc, sig: signal.Signals) -> None:
     """When pipeline is shutting down, by Ctrl-c for example.
 
     Return False to stop shutting down, but you have to shut it down
@@ -109,7 +111,7 @@ def on_proc_shutdown(proc: "Proc", sig: Optional[signal.Signals]) -> None:
 
 
 @plugin.spec
-async def on_proc_done(proc: "Proc", succeeded: Union[bool, str]) -> None:
+async def on_proc_done(proc: Proc, succeeded: bool | str) -> None:
     """When a process is done
 
     Args:
@@ -120,7 +122,7 @@ async def on_proc_done(proc: "Proc", succeeded: Union[bool, str]) -> None:
 
 
 @plugin.spec
-async def on_job_init(proc: "Proc", job: "Job"):
+async def on_job_init(proc: Proc, job: Job):
     """When a job is initialized
 
     Args:
@@ -130,7 +132,7 @@ async def on_job_init(proc: "Proc", job: "Job"):
 
 
 @plugin.spec
-async def on_job_queued(proc: "Proc", job: "Job"):
+async def on_job_queued(proc: Proc, job: Job):
     """When a job is queued in xqute. Note it might not be queued yet in
     the scheduler system.
 
@@ -141,7 +143,7 @@ async def on_job_queued(proc: "Proc", job: "Job"):
 
 
 @plugin.spec(result=SimplugResult.TRY_ALL_FIRST_AVAIL)
-async def on_job_submitting(proc: "Proc", job: "Job") -> Optional[bool]:
+async def on_job_submitting(proc: Proc, job: Job) -> bool:
     """When a job is submitting.
 
     The first plugin (based on priority) have this hook return False will
@@ -157,7 +159,7 @@ async def on_job_submitting(proc: "Proc", job: "Job") -> Optional[bool]:
 
 
 @plugin.spec
-async def on_job_submitted(proc: "Proc", job: "Job"):
+async def on_job_submitted(proc: Proc, job: Job):
     """When a job is submitted in the scheduler system.
 
     Args:
@@ -167,7 +169,7 @@ async def on_job_submitted(proc: "Proc", job: "Job"):
 
 
 @plugin.spec
-async def on_job_running(proc: "Proc", job: "Job"):
+async def on_job_running(proc: Proc, job: Job):
     """When a job starts to run in then scheduler system.
 
     Args:
@@ -177,7 +179,7 @@ async def on_job_running(proc: "Proc", job: "Job"):
 
 
 @plugin.spec(result=SimplugResult.TRY_ALL_FIRST_AVAIL)
-async def on_job_killing(proc: "Proc", job: "Job") -> Optional[bool]:
+async def on_job_killing(proc: Proc, job: Job) -> bool:
     """When a job is being killed.
 
     The first plugin (based on priority) have this hook return False will
@@ -193,7 +195,7 @@ async def on_job_killing(proc: "Proc", job: "Job") -> Optional[bool]:
 
 
 @plugin.spec
-async def on_job_killed(proc: "Proc", job: "Job"):
+async def on_job_killed(proc: Proc, job: Job):
     """When a job is killed
 
     Args:
@@ -203,7 +205,7 @@ async def on_job_killed(proc: "Proc", job: "Job"):
 
 
 @plugin.spec
-async def on_job_succeeded(proc: "Proc", job: "Job"):
+async def on_job_succeeded(proc: Proc, job: Job):
     """When a job completes successfully.
 
     Args:
@@ -213,7 +215,7 @@ async def on_job_succeeded(proc: "Proc", job: "Job"):
 
 
 @plugin.spec
-async def on_job_failed(proc: "Proc", job: "Job"):
+async def on_job_failed(proc: Proc, job: Job):
     """When a job is done but failed.
 
     Args:
@@ -229,7 +231,7 @@ class PipenMainPlugin:
     name = "main"
 
     @plugin.impl
-    def on_proc_shutdown(self, proc: "Proc", sig: Optional[signal.Signals]):
+    def on_proc_shutdown(self, proc: Proc, sig: signal.Signals):
         """When a process is shutting down"""
         if sig:  # pragma: no cover
             proc.log(
@@ -239,17 +241,17 @@ def on_proc_shutdown(self, proc: "Proc", sig: Optional[signal.Signals]):
             )
 
     @plugin.impl
-    async def on_job_submitted(self, proc: "Proc", job: "Job"):
+    async def on_job_submitted(self, proc: Proc, job: Job):
         """Update the progress bar when a job is submitted"""
         proc.pbar.update_job_submitted()
 
     @plugin.impl
-    async def on_job_running(self, proc: "Proc", job: "Job"):
+    async def on_job_running(self, proc: Proc, job: Job):
         """Update the progress bar when a job starts to run"""
         proc.pbar.update_job_running()
 
     @plugin.impl
-    async def on_job_succeeded(self, proc: "Proc", job: "Job"):
+    async def on_job_succeeded(self, proc: Proc, job: Job):
         """Cache the job and update the progress bar when a job is succeeded"""
         # now the returncode is 0, however, we need to check if output files
         # have been created or not, this makes sure job.cache not fail
@@ -271,7 +273,7 @@ async def on_job_succeeded(self, proc: "Proc", job: "Job"):
             proc.pbar.update_job_succeeded()
 
     @plugin.impl
-    async def on_job_failed(self, proc: "Proc", job: "Job"):
+    async def on_job_failed(self, proc: Proc, job: Job):
         """Update the progress bar when a job is failed"""
         proc.pbar.update_job_failed()
         if job.status == JobStatus.RETRYING:
@@ -279,7 +281,7 @@ async def on_job_failed(self, proc: "Proc", job: "Job"):
             proc.pbar.update_job_retrying()
 
     @plugin.impl
-    async def on_job_killed(self, proc: "Proc", job: "Job"):
+    async def on_job_killed(self, proc: Proc, job: Job):
         """Update the status of a killed job"""
         # instead of FINISHED to force the whole pipeline to quit
         job.status = JobStatus.FAILED  # pragma: no cover
@@ -296,37 +298,37 @@ class XqutePipenPlugin:
     name = "xqute.pipen"
 
     @xqute_plugin.impl
-    def on_shutdown(self, xqute: "Xqute", sig: Optional[signal.Signals]):
+    def on_shutdown(self, xqute: Xqute, sig: signal.Signals):
         """When a process is shutting down"""
         return plugin.hooks.on_proc_shutdown(xqute.proc, sig)
 
     @xqute_plugin.impl
-    async def on_job_init(self, scheduler: Scheduler, job: "Job"):
+    async def on_job_init(self, scheduler: Scheduler, job: Job):
         """When a job is initialized"""
         await plugin.hooks.on_job_init(job.proc, job)
 
     @xqute_plugin.impl
-    async def on_job_queued(self, scheduler: Scheduler, job: "Job"):
+    async def on_job_queued(self, scheduler: Scheduler, job: Job):
         """When a job is queued"""
         await plugin.hooks.on_job_queued(job.proc, job)
 
     @xqute_plugin.impl
-    async def on_job_submitting(self, scheduler: Scheduler, job: "Job"):
+    async def on_job_submitting(self, scheduler: Scheduler, job: Job):
         """When a job is being submitted"""
         return await plugin.hooks.on_job_submitting(job.proc, job)
 
     @xqute_plugin.impl
-    async def on_job_submitted(self, scheduler: Scheduler, job: "Job"):
+    async def on_job_submitted(self, scheduler: Scheduler, job: Job):
         """When a job is submitted"""
         await plugin.hooks.on_job_submitted(job.proc, job)
 
     @xqute_plugin.impl
-    async def on_job_running(self, scheduler: Scheduler, job: "Job"):
+    async def on_job_running(self, scheduler: Scheduler, job: Job):
         """When a job starts to run"""
         await plugin.hooks.on_job_running(job.proc, job)
 
     @xqute_plugin.impl
-    async def on_job_killing(self, scheduler: Scheduler, job: "Job"):
+    async def on_job_killing(self, scheduler: Scheduler, job: Job):
         """When a job is being killed"""
         return await plugin.hooks.on_job_killing(  # pragma: no cover
             job.proc,
@@ -334,7 +336,7 @@ async def on_job_killing(self, scheduler: Scheduler, job: "Job"):
         )
 
     @xqute_plugin.impl
-    async def on_job_killed(self, scheduler: Scheduler, job: "Job"):
+    async def on_job_killed(self, scheduler: Scheduler, job: Job):
         """When a job is killed"""
         await plugin.hooks.on_job_killed(  # pragma: no cover
             job.proc,
@@ -342,12 +344,12 @@ async def on_job_killed(self, scheduler: Scheduler, job: "Job"):
         )
 
     @xqute_plugin.impl
-    async def on_job_succeeded(self, scheduler: Scheduler, job: "Job"):
+    async def on_job_succeeded(self, scheduler: Scheduler, job: Job):
         """When a job is succeeded"""
         await plugin.hooks.on_job_succeeded(job.proc, job)
 
     @xqute_plugin.impl
-    async def on_job_failed(self, scheduler: Scheduler, job: "Job"):
+    async def on_job_failed(self, scheduler: Scheduler, job: Job):
         """When a job is failed"""
         await plugin.hooks.on_job_failed(job.proc, job)
 
diff --git a/pipen/proc.py b/pipen/proc.py
index 3218ef63..a1f2a046 100644
--- a/pipen/proc.py
+++ b/pipen/proc.py
@@ -303,7 +303,7 @@ def __init_subclass__(cls) -> None:
         )
         cls.__procgroup__ = None
 
-    def __init__(self, pipeline: "Pipen" = None) -> None:
+    def __init__(self, pipeline: Pipen = None) -> None:
         """Constructor
 
         This is called only at runtime.
diff --git a/pipen/procgroup.py b/pipen/procgroup.py
index b6095a4d..7d13cdcc 100644
--- a/pipen/procgroup.py
+++ b/pipen/procgroup.py
@@ -120,10 +120,15 @@ def add_proc(
             a cached property that returns the process class
         """
         if isinstance(self_or_method, ProcGroup):
+            # Called as self.add_proc or pg.add_proc
+            if proc is None:
+                return self_or_method.add_proc  # type: ignore
+
             if proc.name in self_or_method.__class__.PRESERVED:
                 raise ValueError(
                     f"Process name `{proc.name}` is reserved for ProcGroup"
                 )
+
             setattr(self_or_method, proc.name, proc)
             proc.__procgroup__ = self_or_method
             if not proc.requires:
@@ -134,6 +139,19 @@ def add_proc(
         @wraps(self_or_method)
         def wrapper(self):
             proc = self_or_method(self)
+
+            if proc is None:
+                return None
+
+            if (
+                not isinstance(proc, Proc)
+                and (not isinstance(proc, type) or not issubclass(proc, Proc))
+            ):
+                raise ValueError(
+                    f"`{proc}` is not a process "
+                    "(either a subclass or an instance of Proc)"
+                )
+
             proc.__procgroup__ = self
             if not proc.requires:
                 self.starts.append(proc)
@@ -144,9 +162,9 @@ def wrapper(self):
 
     def as_pipen(
         self,
-        name: str = None,
-        desc: str = None,
-        outdir: PathLike = None,
+        name: str | None = None,
+        desc: str | None = None,
+        outdir: str | PathLike | None = None,
         **kwargs,
     ) -> Pipen:
         """Convert the pipeline to a Pipen instance
diff --git a/pipen/progressbar.py b/pipen/progressbar.py
index 08ba9d46..3ca29cfa 100644
--- a/pipen/progressbar.py
+++ b/pipen/progressbar.py
@@ -1,4 +1,6 @@
 """Provide the PipelinePBar and ProcPBar classes"""
+from __future__ import annotations
+
 from typing import TYPE_CHECKING
 
 from .utils import truncate_text
@@ -16,7 +18,7 @@ class ProcPBar:
     """The progress bar for processes"""
 
     def __init__(
-        self, manager: "enlighten.Manager", proc_size: int, proc_name: str
+        self, manager: enlighten.Manager, proc_size: int, proc_name: str
     ) -> None:
         self.submitted_counter = manager.counter(
             total=proc_size,
diff --git a/pipen/scheduler.py b/pipen/scheduler.py
index b86f4984..79ab245c 100644
--- a/pipen/scheduler.py
+++ b/pipen/scheduler.py
@@ -1,5 +1,7 @@
 """Provide builting schedulers"""
-from typing import Type, Union
+from __future__ import annotations
+
+from typing import Type
 
 from xqute import Scheduler
 from xqute.schedulers.local_scheduler import LocalJob as XquteLocalJob
@@ -39,7 +41,7 @@ class SgeScheduler(XquteSgeScheduler):
     job_class = SgeJob
 
 
-def get_scheduler(scheduler: Union[str, Type[Scheduler]]) -> Type[Scheduler]:
+def get_scheduler(scheduler: str | Type[Scheduler]) -> Type[Scheduler]:
     """Get the scheduler by name of the scheduler class itself
 
     Args:
diff --git a/pipen/template.py b/pipen/template.py
index bee21b5e..0a84fc2e 100644
--- a/pipen/template.py
+++ b/pipen/template.py
@@ -1,6 +1,8 @@
 """Template adaptor for pipen"""
+from __future__ import annotations
+
 from abc import ABC, abstractmethod
-from typing import Any, Mapping, Type, Union
+from typing import Any, Mapping, Type
 
 from liquid import Liquid
 
@@ -116,9 +118,7 @@ def _render(self, data: Mapping[str, Any]) -> str:
         return self.engine.render(data)
 
 
-def get_template_engine(
-    template: Union[str, Type[Template]],
-) -> Type[Template]:
+def get_template_engine(template: str | Type[Template]) -> Type[Template]:
     """Get the template engine by name or the template engine itself
 
     Args:
diff --git a/pipen/utils.py b/pipen/utils.py
index 216f4acc..ef92b84e 100644
--- a/pipen/utils.py
+++ b/pipen/utils.py
@@ -1,4 +1,6 @@
 """Provide some utilities"""
+from __future__ import annotations
+
 import logging
 import textwrap
 from itertools import groupby
@@ -17,24 +19,8 @@
     Mapping,
     Tuple,
     Type,
-    Union,
 )
 
-if TYPE_CHECKING:  # pragma: no cover
-    import pandas
-
-try:  # pragma: no cover
-    from functools import cached_property
-except ImportError:  # pragma: no cover
-    # python 3.7
-    from cached_property import cached_property
-
-try:  # pragma: no cover
-    from importlib import metadata as importlib_metadata
-except ImportError:  # pragma: no cover
-    # python 3.7
-    import importlib_metadata
-
 from rich.console import Console
 from rich.logging import RichHandler as _RichHandler
 from rich.table import Table
@@ -51,7 +37,20 @@
 from .pluginmgr import plugin
 from .version import __version__
 
+try:  # pragma: no cover
+    from functools import cached_property
+except ImportError:  # pragma: no cover
+    # python 3.7
+    from cached_property import cached_property
+
+try:  # pragma: no cover
+    from importlib import metadata as importlib_metadata
+except ImportError:  # pragma: no cover
+    # python 3.7
+    import importlib_metadata
+
 if TYPE_CHECKING:  # pragma: no cover
+    import pandas
     from rich.console import RenderableType
 
 
@@ -98,7 +97,7 @@ def get_level_text(self, record: logging.LogRecord) -> Text:
 
 def get_logger(
     name: str = LOGGER_NAME,
-    level: Union[str, int] = None,
+    level: str | int = None,
 ) -> logging.LoggerAdapter:
     """Get the logger by given plugin name
 
@@ -299,8 +298,8 @@ def get_plugin_context(plugins: List[Any]) -> SimplugContext:
 
 
 def log_rich_renderable(
-    renderable: "RenderableType",
-    color: str,
+    renderable: RenderableType,
+    color: str | None,
     logfunc: Callable,
     *args: Any,
     **kwargs: Any,
@@ -348,7 +347,7 @@ def brief_list(blist: List[int]) -> str:
     return ", ".join(ret)
 
 
-def pipen_banner() -> "RenderableType":
+def pipen_banner() -> RenderableType:
     """The banner for pipen
 
     Returns:
@@ -441,7 +440,7 @@ def truncate_text(text: str, width: int, end: str = "…") -> str:
     return text[: (width - len(end))] + end
 
 
-def make_df_colnames_unique_inplace(thedf: "pandas.DataFrame") -> None:
+def make_df_colnames_unique_inplace(thedf: pandas.DataFrame) -> None:
     """Make the columns of a data frame unique
 
     Args:
diff --git a/pipen/version.py b/pipen/version.py
index 926158d3..efba4568 100644
--- a/pipen/version.py
+++ b/pipen/version.py
@@ -1,3 +1,3 @@
 """Provide version of pipen"""
 
-__version__ = "0.5.1"
+__version__ = "0.5.2"
diff --git a/poetry.lock b/poetry.lock
index 0b60ff3a..4394233d 100644
--- a/poetry.lock
+++ b/poetry.lock
@@ -1,4 +1,4 @@
-# This file is automatically @generated by Poetry and should not be changed by hand.
+# This file is automatically @generated by Poetry 1.4.0 and should not be changed by hand.
 
 [[package]]
 name = "aiofile"
@@ -149,42 +149,37 @@ files = [
 
 [[package]]
 name = "caio"
-version = "0.9.11"
+version = "0.9.12"
 description = "Asynchronous file IO for Linux MacOS or Windows."
 category = "main"
 optional = false
 python-versions = ">=3.7, <4"
 files = [
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-    {file = "caio-0.9.11.tar.gz", hash = "sha256:33ca49789bf2d55bc3f5def36784c4624fbf16b78e9c299a2c7f6f22ac084aa6"},
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+    {file = "caio-0.9.12-cp39-cp39-macosx_11_0_x86_64.whl", hash = "sha256:55317d2c90479a58108cfbd6816b85e584e61c48b42269c55f69cf4443857068"},
+    {file = "caio-0.9.12-cp39-cp39-manylinux_2_12_x86_64.manylinux2010_x86_64.manylinux_2_17_x86_64.manylinux2014_x86_64.whl", hash = "sha256:b18318d04cad7ef985fc2fb86d43f866ba34c1ee3445385f2370d6f843c05c69"},
+    {file = "caio-0.9.12-cp39-cp39-manylinux_2_17_aarch64.manylinux2014_aarch64.whl", hash = "sha256:73c20d8fc7dfb140b7d57e69e6f17e1637d2ac4a9ebe0f5f8c94b56f87c5c087"},
+    {file = "caio-0.9.12-py3-none-any.whl", hash = "sha256:b81b3271478e91f18e7ac1f3e4f914ba0924364857ba8e27f03b9d0aea915ca8"},
+    {file = "caio-0.9.12.tar.gz", hash = "sha256:d2be553738dd793f8a01a60316f2c5284fbf152219241c0c67ca05f650a37a37"},
 ]
 
 [package.extras]
-develop = ["aiomisc", "pytest", "pytest-cov"]
+develop = ["aiomisc-pytest", "pytest", "pytest-cov"]
 
 [[package]]
 name = "colorama"
@@ -267,14 +262,14 @@ toml = ["tomli"]
 
 [[package]]
 name = "datar"
-version = "0.11.1"
+version = "0.11.2"
 description = "A Grammar of Data Manipulation in python"
 category = "dev"
 optional = false
 python-versions = ">=3.7.1,<4.0.0"
 files = [
-    {file = "datar-0.11.1-py3-none-any.whl", hash = "sha256:b4014405e586b2f4361ae7a6e93aef160433d4d5e36fba1f7caa574af056a24b"},
-    {file = "datar-0.11.1.tar.gz", hash = "sha256:6cb2e8f4c21908394b1ddc7569b40b036017d0a7919fc3b66da5a4216c5377e8"},
+    {file = "datar-0.11.2-py3-none-any.whl", hash = "sha256:1b1789157666869fe7e4752725b6598709d0e7846bbd385987bed88cddb82ebd"},
+    {file = "datar-0.11.2.tar.gz", hash = "sha256:8d4a31b492dd4d18d414364ea821c98bf390d31bb891c03af9328baaa364c9eb"},
 ]
 
 [package.dependencies]
@@ -305,14 +300,14 @@ numpy = ">=1.17,<2.0"
 
 [[package]]
 name = "datar-pandas"
-version = "0.2.0"
+version = "0.2.1"
 description = "The pandas backend for datar"
 category = "dev"
 optional = false
 python-versions = ">=3.7.1,<4.0.0"
 files = [
-    {file = "datar_pandas-0.2.0-py3-none-any.whl", hash = "sha256:6af735a8c2703afe68b53ef33e9c3461142516690b34ed798693add5835b0bf4"},
-    {file = "datar_pandas-0.2.0.tar.gz", hash = "sha256:5be01b58ce55fae9d9bbe4ff30ea78fd555550d1270108d09a3dcec89f6fa7f2"},
+    {file = "datar_pandas-0.2.1-py3-none-any.whl", hash = "sha256:29647a0bb11c1577a691a0304b653f8fe31382ca9f9bc92527c676952c2131a6"},
+    {file = "datar_pandas-0.2.1.tar.gz", hash = "sha256:a4887c161f08c78055ed40b2827ae7acb8267bff2b0a8afa7d8e7c2ce4689412"},
 ]
 
 [package.dependencies]
@@ -873,14 +868,14 @@ plugins = ["importlib-metadata"]
 
 [[package]]
 name = "pytest"
-version = "7.2.1"
+version = "7.2.2"
 description = "pytest: simple powerful testing with Python"
 category = "dev"
 optional = false
 python-versions = ">=3.7"
 files = [
-    {file = "pytest-7.2.1-py3-none-any.whl", hash = "sha256:c7c6ca206e93355074ae32f7403e8ea12163b1163c976fee7d4d84027c162be5"},
-    {file = "pytest-7.2.1.tar.gz", hash = "sha256:d45e0952f3727241918b8fd0f376f5ff6b301cc0777c6f9a556935c92d8a7d42"},
+    {file = "pytest-7.2.2-py3-none-any.whl", hash = "sha256:130328f552dcfac0b1cec75c12e3f005619dc5f874f0a06e8ff7263f0ee6225e"},
+    {file = "pytest-7.2.2.tar.gz", hash = "sha256:c99ab0c73aceb050f68929bc93af19ab6db0558791c6a0715723abe9d0ade9d4"},
 ]
 
 [package.dependencies]
@@ -1006,19 +1001,19 @@ files = [
 
 [[package]]
 name = "rich"
-version = "13.3.1"
+version = "13.3.2"
 description = "Render rich text, tables, progress bars, syntax highlighting, markdown and more to the terminal"
 category = "main"
 optional = false
 python-versions = ">=3.7.0"
 files = [
-    {file = "rich-13.3.1-py3-none-any.whl", hash = "sha256:8aa57747f3fc3e977684f0176a88e789be314a99f99b43b75d1e9cb5dc6db9e9"},
-    {file = "rich-13.3.1.tar.gz", hash = "sha256:125d96d20c92b946b983d0d392b84ff945461e5a06d3867e9f9e575f8697b67f"},
+    {file = "rich-13.3.2-py3-none-any.whl", hash = "sha256:a104f37270bf677148d8acb07d33be1569eeee87e2d1beb286a4e9113caf6f2f"},
+    {file = "rich-13.3.2.tar.gz", hash = "sha256:91954fe80cfb7985727a467ca98a7618e5dd15178cc2da10f553b36a93859001"},
 ]
 
 [package.dependencies]
-markdown-it-py = ">=2.1.0,<3.0.0"
-pygments = ">=2.14.0,<3.0.0"
+markdown-it-py = ">=2.2.0,<3.0.0"
+pygments = ">=2.13.0,<3.0.0"
 typing-extensions = {version = ">=4.0.0,<5.0", markers = "python_version < \"3.9\""}
 
 [package.extras]
diff --git a/pyproject.toml b/pyproject.toml
index 2c030c42..401835e4 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -4,7 +4,7 @@ build-backend = "poetry.masonry.api"
 
 [tool.poetry]
 name = "pipen"
-version = "0.5.1"
+version = "0.5.2"
 description = "A pipeline framework for python"
 authors = [ "pwwang <pwwang@pwwang.com>",]
 license = "MIT"
diff --git a/tests/test_procgroup.py b/tests/test_procgroup.py
index 865d7640..2030455b 100644
--- a/tests/test_procgroup.py
+++ b/tests/test_procgroup.py
@@ -26,7 +26,7 @@ class PG(ProcGroup):
 
     pg = PG()
 
-    @pg.add_proc
+    @pg.add_proc()
     class P1(Proc):
         pass
 
@@ -59,6 +59,7 @@ def p3(self):
             return P3
 
     pg = PG()
+
     assert pg.starts == [P1]
 
     assert pg.p1 is P1
@@ -67,6 +68,20 @@ def p3(self):
     assert pg.procs == {"P1": P1, "P2": P2, "P3": P3}
 
 
+def test_define_proc_wrong_return():
+    class PG(ProcGroup):
+        @ProcGroup.add_proc
+        def p1(self):
+            return None
+
+        @ProcGroup.add_proc
+        def p2(self):
+            return 1
+
+    with pytest.raises(ValueError):
+        PG()
+
+
 def test_as_pipen():
     class PG(ProcGroup):
         """A pipeline group"""