> ## Documentation Index
> Fetch the complete documentation index at: https://docs.langchain.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Trace generator functions

In most LLM applications, you will want to stream outputs to minimize the time to the first token seen by the user.

LangSmith's tracing functionality natively supports streamed outputs via `generator` functions. Below is an example.

<CodeGroup>
  ```python Python theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
  from langsmith import traceable
  @traceable
  def my_generator():
    for chunk in ["Hello", "World", "!"]:
        yield chunk
  # Stream to the user
  for output in my_generator():
    print(output)
  # It also works with async functions
  import asyncio
  @traceable
  async def my_async_generator():
    for chunk in ["Hello", "World", "!"]:
        yield chunk
  # Stream to the user
  async def main():
    async for output in my_async_generator():
        print(output)
  asyncio.run(main())
  ```

  ```typescript TypeScript theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
  import { traceable } from "langsmith/traceable";
  const myGenerator = traceable(function* () {
    for (const chunk of ["Hello", "World", "!"]) {
        yield chunk;
    }
  });
  for (const output of myGenerator()) {
    console.log(output);
  }
  ```
</CodeGroup>

## Aggregate results[ ](#aggregate-results "Direct link to aggregate results")

By default, the `outputs` of the traced function are aggregated into a single array in LangSmith. If you want to customize how it is stored (for instance, concatenating the outputs into a single string), you can use the `aggregate` option (`reduce_fn` in python). This is especially useful for aggregating streamed LLM outputs.

<Note>
  Aggregating outputs **only** impacts the traced representation of the outputs. It doesn not alter the values returned by your function.
</Note>

<CodeGroup>
  ```python Python theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
  from langsmith import traceable
  def concatenate_strings(outputs: list):
    return "".join(outputs)
  @traceable(reduce_fn=concatenate_strings)
  def my_generator():
    for chunk in ["Hello", "World", "!"]:
        yield chunk
  # Stream to the user
  for output in my_generator():
    print(output)
  # It also works with async functions
  import asyncio
  @traceable(reduce_fn=concatenate_strings)
  async def my_async_generator():
    for chunk in ["Hello", "World", "!"]:
        yield chunk
  # Stream to the user
  async def main():
    async for output in my_async_generator():
        print(output)
  asyncio.run(main())
  ```

  ```typescript TypeScript theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
  import { traceable } from "langsmith/traceable";
  const concatenateStrings = (outputs: string[]) => outputs.join("");
  const myGenerator = traceable(function* () {
    for (const chunk of ["Hello", "World", "!"]) {
        yield chunk;
    }
  }, { aggregator: concatenateStrings });
  for (const output of await myGenerator()) {
    console.log(output);
  }
  ```
</CodeGroup>

***

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