LangChain Message

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我正在阅读  langchain message 子包的源代码，注意到 message 包内，有三个 base 的抽象类:
1. BaseMessage
2. BaseMessageChunk
3. BaseDataContentBlock
请给我从语义上解释一下这三个类的的作用

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class BaseMessage:
    content: Union[str, List[BaseContentBlock]]
    type: str  # "human", "ai", "system", etc.
    name: Optional[str]  # 发送者名称，函数调用时有用

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class BaseMessageChunk:
    content: Union[str, List[BaseContentBlock]]
    type: str

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class BaseMessage(Serializable):
    """Base abstract message class.

    Messages are the inputs and outputs of ChatModels.
    """

    content: Union[str, list[Union[str, dict]]]
    """The string contents of the message."""

    additional_kwargs: dict = Field(default_factory=dict)
    """Reserved for additional payload data associated with the message.

    For example, for a message from an AI, this could include tool calls as
    encoded by the model provider.
    """

    response_metadata: dict = Field(default_factory=dict)
    """Response metadata. For example: response headers, logprobs, token counts, model
    name."""

    type: str
    """The type of the message. Must be a string that is unique to the message type.

    The purpose of this field is to allow for easy identification of the message type
    when deserializing messages.
    """

    name: Optional[str] = None
    """An optional name for the message.

    This can be used to provide a human-readable name for the message.

    Usage of this field is optional, and whether it's used or not is up to the
    model implementation.
    """

    id: Optional[str] = Field(default=None, coerce_numbers_to_str=True)
    """An optional unique identifier for the message. This should ideally be
    provided by the provider/model which created the message."""

    def text(self) -> str:
        """Get the text content of the message.

        Returns:
            The text content of the message.
        """
        pass


    def __add__(self, other: Any) -> ChatPromptTemplate:
        """Concatenate this message with another message."""
        from langchain_core.prompts.chat import ChatPromptTemplate

        prompt = ChatPromptTemplate(messages=[self])
        return prompt + other

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    def __add__(self, other: Any) -> BaseMessageChunk:  # type: ignore[override]
        
        if isinstance(other, BaseMessageChunk):
            # If both are (subclasses of) BaseMessageChunk,
            # concat into a single BaseMessageChunk

            return self.__class__(
                id=self.id,
                type=self.type,
                content=merge_content(self.content, other.content),
                additional_kwargs=merge_dicts(
                    self.additional_kwargs, other.additional_kwargs
                ),
                response_metadata=merge_dicts(
                    self.response_metadata, other.response_metadata
                ),
            )
        if isinstance(other, list) and all(
            isinstance(o, BaseMessageChunk) for o in other
        ):
            content = merge_content(self.content, *(o.content for o in other))
            additional_kwargs = merge_dicts(
                self.additional_kwargs, *(o.additional_kwargs for o in other)
            )
            response_metadata = merge_dicts(
                self.response_metadata, *(o.response_metadata for o in other)
            )
            return self.__class__(  # type: ignore[call-arg]
                id=self.id,
                content=content,
                additional_kwargs=additional_kwargs,
                response_metadata=response_metadata,
            )
        msg = (
            'unsupported operand type(s) for +: "'
            f"{self.__class__.__name__}"
            f'" and "{other.__class__.__name__}"'
        )
        raise TypeError(msg)

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class BaseDataContentBlock(TypedDict, total=False):
    """Base class for data content blocks."""

    mime_type: NotRequired[str]
    """MIME type of the content block (if needed)."""


class URLContentBlock(BaseDataContentBlock):
    """Content block for data from a URL."""

    type: Literal["image", "audio", "file"]
    """Type of the content block."""
    source_type: Literal["url"]
    """Source type (url)."""
    url: str
    """URL for data."""


class Base64ContentBlock(BaseDataContentBlock):
    """Content block for inline data from a base64 string."""

    type: Literal["image", "audio", "file"]
    """Type of the content block."""
    source_type: Literal["base64"]
    """Source type (base64)."""
    data: str
    """Data as a base64 string."""

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{
  "role": "user",
  "content": [
    {"type": "text", "text": "请分析这张图片"},
    {
      "type": "image_url",
      "image_url": {
        "url": "data:image/jpeg;base64,...",
        "detail": "auto"
      }
    }
  ]
}

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{
  "type": "image_url",
  "image_url": {
    "url": "<image URL 或 base64>",
    "detail": "auto" | "low" | "high"
  }
}

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{
  "type": "file_data",
  "file_data": {
    "file_id": "file-abc123"
  }
}

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我正在阅读  langchain Prompts 子包的源代码，注意到 Message 定义了如下 Message 类:
1. FunctionMessage
4. ToolMessage
2. HumanMessage
3. AIMessage
5. SystemMessage
6. RemoveMessage
7. ChatMessage
请给我从语义上解释一下这些类的的作用

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class ChatMessage(BaseMessage):
    """Message that can be assigned an arbitrary speaker (i.e. role)."""

    role: str
    """The speaker / role of the Message."""

    type: Literal["chat"] = "chat"
    """The type of the message (used during serialization). Defaults to "chat"."""


class HumanMessage(BaseMessage):
    example: bool = False
    """Use to denote that a message is part of an example conversation.

    At the moment, this is ignored by most models. Usage is discouraged.
    Defaults to False.
    """

    type: Literal["human"] = "human"
    """The type of the message (used for serialization). Defaults to "human"."""

class RemoveMessage(BaseMessage):
    """Message responsible for deleting other messages."""

    type: Literal["remove"] = "remove"
    """The type of the message (used for serialization). Defaults to "remove"."""


class SystemMessage(BaseMessage):
    type: Literal["system"] = "system"
    """The type of the message (used for serialization). Defaults to "system"."""

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class FunctionMessage(BaseMessage):
    """Message for passing the result of executing a tool back to a model.

    FunctionMessage are an older version of the ToolMessage schema, and
    do not contain the tool_call_id field.

    The tool_call_id field is used to associate the tool call request with the
    tool call response. This is useful in situations where a chat model is able
    to request multiple tool calls in parallel.
    """

    name: str
    """The name of the function that was executed."""

    type: Literal["function"] = "function"
    """The type of the message (used for serialization). Defaults to "function"."""

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class ToolMessage(BaseMessage, ToolOutputMixin):
    tool_call_id: str
    """Tool call that this message is responding to."""

    type: Literal["tool"] = "tool"
    """The type of the message (used for serialization). Defaults to "tool"."""

    artifact: Any = None
    """Artifact of the Tool execution which is not meant to be sent to the model.

    Should only be specified if it is different from the message content, e.g. if only
    a subset of the full tool output is being passed as message content but the full
    output is needed in other parts of the code.

    .. versionadded:: 0.2.17
    """

    status: Literal["success", "error"] = "success"
    """Status of the tool invocation.

    .. versionadded:: 0.2.24
    """

    additional_kwargs: dict = Field(default_factory=dict, repr=False)
    """Currently inherited from BaseMessage, but not used."""
    response_metadata: dict = Field(default_factory=dict, repr=False)
    """Currently inherited from BaseMessage, but not used."""