core[patch]: allow message utils to work with lcel (#23743)

The functions `convert_to_messages` has had an expansion of the
arguments it can take:

1. Previously, it only could take a `Sequence` in order to iterate over
it. This has been broadened slightly to an `Iterable` (which should have
no other impact).
2. Support for `PromptValue` and `BaseChatPromptTemplate` has been
added. These are generated when combining messages using the overloaded
`+` operator.

Functions which rely on `convert_to_messages` (namely `filter_messages`,
`merge_message_runs` and `trim_messages`) have had the type of their
arguments similarly expanded.

Resolves #23706.

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---------

Signed-off-by: JP-Ellis <josh@jpellis.me>
Co-authored-by: Bagatur <baskaryan@gmail.com>

docs/docs/how_to/merge_message_runs.ipynb

@@ -63,6 +63,38 @@
     "Notice that if the contents of one of the messages to merge is a list of content blocks then the merged message will have a list of content blocks. And if both messages to merge have string contents then those are concatenated with a newline character."
    ]
   },
+  {
+   "cell_type": "markdown",
+   "id": "11f7e8d3",
+   "metadata": {},
+   "source": [
+    "The `merge_message_runs` utility also works with messages composed together using the overloaded `+` operation:"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "b51855c5",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "messages = (\n",
+    "    SystemMessage(\"you're a good assistant.\")\n",
+    "    + SystemMessage(\"you always respond with a joke.\")\n",
+    "    + HumanMessage([{\"type\": \"text\", \"text\": \"i wonder why it's called langchain\"}])\n",
+    "    + HumanMessage(\"and who is harrison chasing anyways\")\n",
+    "    + AIMessage(\n",
+    "        'Well, I guess they thought \"WordRope\" and \"SentenceString\" just didn\\'t have the same ring to it!'\n",
+    "    )\n",
+    "    + AIMessage(\n",
+    "        \"Why, he's probably chasing after the last cup of coffee in the office!\"\n",
+    "    )\n",
+    ")\n",
+    "\n",
+    "merged = merge_message_runs(messages)\n",
+    "print(\"\\n\\n\".join([repr(x) for x in merged]))"
+   ]
+  },
   {
    "cell_type": "markdown",
    "id": "1b2eee74-71c8-4168-b968-bca580c25d18",

libs/core/langchain_core/messages/utils.py

@@ -16,6 +16,7 @@ from typing import (
     Any,
     Callable,
     Dict,
+    Iterable,
     List,
     Literal,
     Optional,
@@ -40,6 +41,7 @@ if TYPE_CHECKING:
     from langchain_text_splitters import TextSplitter
 
     from langchain_core.language_models import BaseLanguageModel
+    from langchain_core.prompt_values import PromptValue
     from langchain_core.runnables.base import Runnable
 
 AnyMessage = Union[
@@ -284,7 +286,7 @@ def _convert_to_message(message: MessageLikeRepresentation) -> BaseMessage:
 
 
 def convert_to_messages(
-    messages: Sequence[MessageLikeRepresentation],
+    messages: Union[Iterable[MessageLikeRepresentation], PromptValue],
 ) -> List[BaseMessage]:
     """Convert a sequence of messages to a list of messages.
 
@@ -294,6 +296,11 @@ def convert_to_messages(
     Returns:
         List of messages (BaseMessages).
     """
+    # Import here to avoid circular imports
+    from langchain_core.prompt_values import PromptValue
+
+    if isinstance(messages, PromptValue):
+        return messages.to_messages()
     return [_convert_to_message(m) for m in messages]
 
 
@@ -329,7 +336,7 @@ def _runnable_support(func: Callable) -> Callable:
 
 @_runnable_support
 def filter_messages(
-    messages: Sequence[MessageLikeRepresentation],
+    messages: Union[Iterable[MessageLikeRepresentation], PromptValue],
     *,
     include_names: Optional[Sequence[str]] = None,
     exclude_names: Optional[Sequence[str]] = None,
@@ -417,7 +424,7 @@ def filter_messages(
 
 @_runnable_support
 def merge_message_runs(
-    messages: Sequence[MessageLikeRepresentation],
+    messages: Union[Iterable[MessageLikeRepresentation], PromptValue],
 ) -> List[BaseMessage]:
     """Merge consecutive Messages of the same type.
 
@@ -506,7 +513,7 @@ def merge_message_runs(
 
 @_runnable_support
 def trim_messages(
-    messages: Sequence[MessageLikeRepresentation],
+    messages: Union[Iterable[MessageLikeRepresentation], PromptValue],
     *,
     max_tokens: int,
     token_counter: Union[

libs/core/tests/unit_tests/messages/test_utils.py

@@ -127,7 +127,6 @@ _MESSAGES_TO_TRIM = [
     HumanMessage("This is a 4 token text.", id="third"),
     AIMessage("This is a 4 token text.", id="fourth"),
 ]
-
 _MESSAGES_TO_TRIM_COPY = [m.copy(deep=True) for m in _MESSAGES_TO_TRIM]