Add getparameters and setparameters!!

docs/src/api.md

@@ -113,3 +113,144 @@ For chains of this type, AbstractMCMC defines the following two methods.
 AbstractMCMC.chainscat
 AbstractMCMC.chainsstack
 ```
+
+## Interacting with states of samplers
+
+To make it a bit easier to interact with some arbitrary sampler state, we encourage implementations of `AbstractSampler` to implement the following methods:
+```@docs
+AbstractMCMC.getparams
+AbstractMCMC.setparams!!
+```
+These methods can also be useful for implementing samplers which wraps some inner samplers, e.g. a mixture of samplers.
+
+### Example: `MixtureSampler`
+
+In a `MixtureSampler` we need two things:
+- `components`: collection of samplers.
+- `weights`: collection of weights representing the probability of choosing the corresponding sampler.
+
+```julia
+struct MixtureSampler{W,C} <: AbstractMCMC.AbstractSampler
+    components::C
+    weights::W
+end
+```
+
+To implement the state, we need to keep track of a couple of things:
+- `index`: the index of the sampler used in this `step`.
+- `states`: the current states of _all_ the components.
+We need to keep track of the states of _all_ components rather than just the state for the sampler we used previously.
+The reason is that lots of samplers keep track of more than just the previous realizations of the variables, e.g. in `AdvancedHMC.jl` we keep track of the momentum used, the metric used, etc.
+
+
+```julia
+struct MixtureState{S}
+    index::Int
+    states::S
+end
+```
+The `step` for a `MixtureSampler` is defined by the following generative process
+```math
+\begin{aligned}
+i &\sim \mathrm{Categorical}(w_1, \dots, w_k) \\
+X_t &\sim \mathcal{K}_i(\cdot \mid X_{t - 1})
+\end{aligned}
+```
+where ``\mathcal{K}_i`` denotes the i-th kernel/sampler, and ``w_i`` denotes the weight/probability of choosing the i-th sampler.
+[`AbstractMCMC.getparams`](@ref) and [`AbstractMCMC.setparams!!`](@ref) comes into play in defining/computing ``\mathcal{K}_i(\cdot \mid X_{t - 1})`` since ``X_{t - 1}`` could be coming from a different sampler.
+
+If we let `state` be the current `MixtureState`, `i` the current component, and `i_prev` is the previous component we sampled from, then this translates into the following piece of code:
+
+```julia
+# Update the corresponding state, i.e. `state.states[i]`, using
+# the state and transition from the previous iteration.
+state_current = AbstractMCMC.setparams!!(
+    state.states[i], 
+    AbstractMCMC.getparams(state.states[i_prev]),
+)
+
+# Take a `step` for this sampler using the updated state.
+transition, state_current = AbstractMCMC.step(
+    rng, model, sampler_current, sampler_state;
+    kwargs...
+)
+```
+
+The full [`AbstractMCMC.step`](@ref) implementation would then be something like:
+
+```julia
+function AbstractMCMC.step(rng, model::AbstractMCMC.AbstractModel, sampler::MixtureSampler, state; kwargs...)
+    # Sample the component to use in this `step`.
+    i = rand(Categorical(sampler.weights))
+    sampler_current = sampler.components[i]
+
+    # Update the corresponding state, i.e. `state.states[i]`, using
+    # the state and transition from the previous iteration.
+    i_prev = state.index
+    state_current = AbstractMCMC.setparams!!(  
+        state.states[i], 
+        AbstractMCMC.getparams(state.states[i_prev]),  
+    )
+
+    # Take a `step` for this sampler using the updated state.
+    transition, state_current = AbstractMCMC.step(
+        rng, model, sampler_current, state_current;
+        kwargs...
+    )
+
+    # Create the new states.
+    # NOTE: Code below will result in `states_new` being a `Vector`.
+    # If we wanted to allow usage of alternative containers, e.g. `Tuple`,
+    # it would be better to use something like `@set states[i] = state_current`
+    # where `@set` is from Setfield.jl.
+    states_new = map(1:length(state.states)) do j
+        if j == i
+            # Replace the i-th state with the new one.
+            state_current
+        else
+            # Otherwise we just carry over the previous ones.
+            state.states[j]
+        end
+    end
+
+    # Create the new `MixtureState`.
+    state_new = MixtureState(i, states_new)
+
+    return transition, state_new
+end
+```
+
+And for the initial [`AbstractMCMC.step`](@ref) we have:
+
+```julia
+function AbstractMCMC.step(rng, model::AbstractMCMC.AbstractModel, sampler::MixtureSampler; kwargs...)
+    # Initialize every state.
+    transitions_and_states = map(sampler.components) do spl
+        AbstractMCMC.step(rng, model, spl; kwargs...)
+    end
+
+    # Sample the component to use this `step`.
+    i = rand(Categorical(sampler.weights))
+    # Extract the corresponding transition.
+    transition = first(transitions_and_states[i])
+    # Extract states.
+    states = map(last, transitions_and_states)
+    # Create new `MixtureState`.
+    state = MixtureState(i, states)
+
+    return transition, state
+end
+```
+
+Suppose we then wanted to use this with some of the packages which implements AbstractMCMC.jl's interface, e.g. [`AdvancedMH.jl`](https://github.com/TuringLang/AdvancedMH.jl), then we'd simply have to implement `getparams` and `setparams!!`.
+
+
+To use `MixtureSampler` with two samplers `sampler1` and `sampler2` from `AdvancedMH.jl` as components, we'd simply do
+
+```julia
+sampler = MixtureSampler([sampler1, sampler2], [0.1, 0.9])
+transition, state = AbstractMCMC.step(rng, model, sampler)
+while ...
+    transition, state = AbstractMCMC.step(rng, model, sampler, state)
+end
+```

src/AbstractMCMC.jl

@@ -80,6 +80,26 @@ The `MCMCSerial` algorithm allows users to sample serially, with no thread or pr
 """
 struct MCMCSerial <: AbstractMCMCEnsemble end
 
+"""
+    getparams(state[; kwargs...])
+
+Retrieve the values of parameters from the sampler's `state` as a `Vector{<:Real}`.
+"""
+function getparams end
+
+"""
+    setparams!!(state, params)
+
+Set the values of parameters in the sampler's `state` from a `Vector{<:Real}`. 
+
+This function should follow the `BangBang` interface: mutate `state` in-place if possible and 
+return the mutated `state`. Otherwise, it should return a new `state` containing the updated parameters.
+
+Although not enforced, it should hold that `setparams!!(state, getparams(state)) == state`. In another
+word, the sampler should implement a consistent transformation between its internal representation
+and the vector representation of the parameter values.
+"""
+function setparams!! end
 include("samplingstats.jl")
 include("logging.jl")
 include("interface.jl")