save messages (#1653)

@yakigac this is my alternative to
https://github.com/hwchase17/langchain/pull/1648 - thoughts?

langchain/schema.py

@@ -40,24 +40,75 @@ class BaseMessage(BaseModel):
     content: str
     additional_kwargs: dict = Field(default_factory=dict)
 
+    @property
+    @abstractmethod
+    def type(self) -> str:
+        """Type of the message, used for serialization."""
+
 
 class HumanMessage(BaseMessage):
     """Type of message that is spoken by the human."""
 
+    @property
+    def type(self) -> str:
+        """Type of the message, used for serialization."""
+        return "human"
+
 
 class AIMessage(BaseMessage):
     """Type of message that is spoken by the AI."""
 
+    @property
+    def type(self) -> str:
+        """Type of the message, used for serialization."""
+        return "ai"
+
 
 class SystemMessage(BaseMessage):
     """Type of message that is a system message."""
 
+    @property
+    def type(self) -> str:
+        """Type of the message, used for serialization."""
+        return "system"
+
 
 class ChatMessage(BaseMessage):
     """Type of message with arbitrary speaker."""
 
     role: str
 
+    @property
+    def type(self) -> str:
+        """Type of the message, used for serialization."""
+        return "chat"
+
+
+def _message_to_json(message: BaseMessage) -> dict:
+    return {"type": message.type, "data": message.dict()}
+
+
+def messages_to_json(messages: List[BaseMessage]) -> List[dict]:
+    return [_message_to_json(m) for m in messages]
+
+
+def _message_from_json(message: dict) -> BaseMessage:
+    _type = message["type"]
+    if _type == "human":
+        return HumanMessage(**message["data"])
+    elif _type == "ai":
+        return AIMessage(**message["data"])
+    elif _type == "system":
+        return SystemMessage(**message["data"])
+    elif _type == "chat":
+        return ChatMessage(**message["data"])
+    else:
+        raise ValueError(f"Got unexpected type: {_type}")
+
+
+def messages_from_json(messages: List[dict]) -> List[BaseMessage]:
+    return [_message_from_json(m) for m in messages]
+
 
 class ChatGeneration(Generation):
     """Output of a single generation."""