Listen to one output field while streaming

stream_listener() attaches a callback to an output field for run_stream(), like DSPy's StreamListener. The callback receives the field's text as it is produced: chunk by chunk when the field can be streamed, otherwise once with the complete value.

Usage

stream_listener(field, callback)

Arguments

field

The name of the output field, such as "answer".

callback

A function called with one string: each chunk of text, or the complete value of a field that cannot be streamed.

Value

A listener (class dsprrr_stream_listener) for the listeners argument of run_stream().

Details

A field can be streamed chunk by chunk when it is the only output field of its module and is a string. Fields of modules with several outputs, or non-string outputs, arrive once, when the module finishes, because structured output cannot be streamed.

See also

Other execution: concurrency_control(), evaluate(), predict.Module(), run(), run_async(), run_dataset(), run_stream(), stream_async()

Examples

show_answer <- stream_listener("answer", function(chunk) cat(chunk))

if (FALSE) { # \dontrun{
storyteller <- module(signature("question -> answer"))
run_stream(
  storyteller,
  question = "Tell me a short story about a lighthouse.",
  .llm = ellmer::chat_openai(model = "gpt-6-luna"),
  listeners = show_answer
)
} # }