|---| | [ View source on GitHub](https://github.com/tensorflow/probability/blob/v0.25.0/tensorflow_probability/python/experimental/mcmc/sample.py#L76-L266) | Runs a Markov chain defined by the given `TransitionKernel`. tfp.experimental.mcmc.sample_chain( kernel, num_results, current_state, previous_kernel_results=None, reducer=(), previous_reducer_state=None, trace_fn=_trace_everything, parallel_iterations=10, seed=None, name=None ) This is meant as a (more) helpful frontend to the low-level `TransitionKernel`-based MCMC API, supporting several main features: - Running a batch of multiple independent chains using SIMD parallelism - Tracing the history of the chains, or not tracing it to save memory - Computing reductions over chain history, whether it is also traced or not - Warm (re-)start, including auxiliary state This function samples from a Markov chain at `current_state` whose stationary distribution is governed by the supplied `TransitionKernel` instance (`kernel`). The `current_state` can be represented as a single `Tensor` or a `list` of `Tensors` which collectively represent the current state. This function can sample from multiple chains, in parallel. Whether or not there are multiple chains is dictated by how the `kernel` treats its inputs. Typically, the shape of the independent chains is shape of the result of the `target_log_prob_fn` used by the `kernel` when applied to the given `current_state`. This function can compute reductions over the samples in tandem with sampling, for example to return summary statistics without materializing all the samples. To request reductions, pass a `Reducer` object, or a nested structure of `Reducer` objects, as the `reducer=` argument. In addition to the chain state, this function supports tracing of auxiliary variables used by the kernel, as well as intermediate values of any supplied reductions. The traced values are selected by specifying `trace_fn`. The `trace_fn` must be a callable accepting three arguments: the chain state, the kernel_results of the `kernel`, and the current results of the reductions, if any are supplied. The return value of `trace_fn` (which may be a `Tensor` or a nested structure of `Tensor`s) is accumulated, such that each `Tensor` gains a new outmost dimension representing time in the chain history. Since MCMC states are correlated, it is sometimes desirable to produce additional intermediate states, and then discard them, ending up with a set of states with decreased autocorrelation. See \[Owen (2017)\]\[1\]. Such 'thinning' is made possible by setting `num_steps_between_results > 0`. The chain then takes `num_steps_between_results` extra steps between the steps that make it into the results, or are shown to any supplied reductions. The extra steps are never materialized, and thus do not increase memory requirements.


| ## Args || |---|---| | `kernel` | An instance of [`tfp.mcmc.TransitionKernel`](https://www.tensorflow.org/probability/api_docs/python/tfp/mcmc/TransitionKernel) which implements one step of the Markov chain. | | `num_results` | Integer number of (non-discarded) Markov chain draws to compute. | | `current_state` | `Tensor` or Python `list` of `Tensor`s representing the initial state(s) of the Markov chain(s). | | `previous_kernel_results` | A `Tensor` or a nested collection of `Tensor`s representing internal calculations made within the previous call to this function (or as returned by `bootstrap_results`). | | `reducer` | A (possibly nested) structure of `Reducer`s to be evaluated on the `kernel`'s samples. If no reducers are given (`reducer=None`), their states will not be passed to any supplied `trace_fn`. | | `previous_reducer_state` | A (possibly nested) structure of running states corresponding to the structure in `reducer`. For resuming streaming reduction computations begun in a previous run. | | `trace_fn` | A callable that takes in the current chain state, the current auxiliary kernel state, and the current result of any reducers, and returns a `Tensor` or a nested collection of `Tensor`s that is then traced. If `None`, nothing is traced. | | `parallel_iterations` | The number of iterations allowed to run in parallel. It must be a positive integer. See [`tf.while_loop`](https://www.tensorflow.org/api_docs/python/tf/while_loop) for more details. | | `seed` | PRNG seed; see [`tfp.random.sanitize_seed`](https://www.tensorflow.org/probability/api_docs/python/tfp/random/sanitize_seed) for details. | | `name` | Python `str` name prefixed to Ops created by this function. Default value: `None` (i.e., 'mcmc_sample_chain'). |



| ## Returns || |---|---| | `result` | A `SampleChainResults` instance containing information about the sampling run. Main fields are `trace`, the history of outputs of `trace_fn`, and `reduction_results`, the final outputs of all supplied `Reducer`s. See `SampleChainResults` for contents of other fields. |