RePhraseQuery is a simple retriever that applies an LLM between the user input and the query passed by the retriever. It can be used to pre-process the user input in any way.

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Example

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Setting up

Create a vector store. 
import logging

from langchain.retrievers import RePhraseQueryRetriever
from langchain_chroma import Chroma
from langchain_community.document_loaders import WebBaseLoader
from langchain_openai import ChatOpenAI, OpenAIEmbeddings
from langchain_text_splitters import RecursiveCharacterTextSplitter

logging.basicConfig()
logging.getLogger("langchain.retrievers.re_phraser").setLevel(logging.INFO)

loader = WebBaseLoader("https://lilianweng.github.io/posts/2023-06-23-agent/")
data = loader.load()

text_splitter = RecursiveCharacterTextSplitter(chunk_size=500, chunk_overlap=0)
all_splits = text_splitter.split_documents(data)

vectorstore = Chroma.from_documents(documents=all_splits, embedding=OpenAIEmbeddings())

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Using the default prompt

The default prompt used in the from_llm classmethod: 
DEFAULT_TEMPLATE = """You are an assistant tasked with taking a natural language \
query from a user and converting it into a query for a vectorstore. \
In this process, you strip out information that is not relevant for \
the retrieval task. Here is the user query: {question}"""

llm = ChatOpenAI(temperature=0)
retriever_from_llm = RePhraseQueryRetriever.from_llm(
    retriever=vectorstore.as_retriever(), llm=llm
)

docs = retriever_from_llm.invoke(
    "Hi I'm Lance. What are the approaches to Task Decomposition?"
)

INFO:langchain.retrievers.re_phraser:Re-phrased question: The user query can be converted into a query for a vectorstore as follows:

"approaches to Task Decomposition"

docs = retriever_from_llm.invoke(
    "I live in San Francisco. What are the Types of Memory?"
)

INFO:langchain.retrievers.re_phraser:Re-phrased question: Query for vectorstore: "Types of Memory"

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Custom prompt

from langchain.chains import LLMChain
from langchain_core.prompts import PromptTemplate

QUERY_PROMPT = PromptTemplate(
    input_variables=["question"],
    template="""You are an assistant tasked with taking a natural languge query from a user
    and converting it into a query for a vectorstore. In the process, strip out all 
    information that is not relevant for the retrieval task and return a new, simplified
    question for vectorstore retrieval. The new user query should be in pirate speech.
    Here is the user query: {question} """,
)
llm = ChatOpenAI(temperature=0)
llm_chain = LLMChain(llm=llm, prompt=QUERY_PROMPT)

retriever_from_llm_chain = RePhraseQueryRetriever(
    retriever=vectorstore.as_retriever(), llm_chain=llm_chain
)

docs = retriever_from_llm_chain.invoke(
    "Hi I'm Lance. What is Maximum Inner Product Search?"
)

INFO:langchain.retrievers.re_phraser:Re-phrased question: Ahoy matey! What be Maximum Inner Product Search, ye scurvy dog?