Does a fit object carry usable posterior draws?

Detect the degenerate "no draws" case after a fit, so a caller can fail loudly instead of returning an empty fit. This is backend-aware: rstan returns a mode-2 stanfit with an empty @sim when the sampler fails to initialize (rather than erroring), while cmdstanr exposes its draws through $draws(). Unrecognized objects (e.g. test mocks) are treated as having draws so they pass through untouched.

Usage

backend_has_draws(raw_fit)

Arguments

raw_fit

a backend-native fit object (an rstan stanfit or a cmdstanr CmdStanMCMC).

Value

logical; TRUE if the fit carries usable draws.