diff --git a/Project.toml b/Project.toml
index cc7d8b8d..7c40ca91 100644
--- a/Project.toml
+++ b/Project.toml
@@ -3,7 +3,7 @@ uuid = "80f14c24-f653-4e6a-9b94-39d6b0f70001"
 keywords = ["markov chain monte carlo", "probabilistic programming"]
 license = "MIT"
 desc = "A lightweight interface for common MCMC methods."
-version = "5.8.2"
+version = "5.9.0"
 
 [deps]
 BangBang = "198e06fe-97b7-11e9-32a5-e1d131e6ad66"
diff --git a/src/AbstractMCMC.jl b/src/AbstractMCMC.jl
index e103d5a5..185082a3 100644
--- a/src/AbstractMCMC.jl
+++ b/src/AbstractMCMC.jl
@@ -106,7 +106,14 @@ See also [`sample`](@ref).
 struct MCMCSerial <: AbstractMCMCEnsemble end
 
 """
-    getparams([model::AbstractModel, ]state)
+    requires_unconstrained_space(sampler::AbstractSampler)::Bool
+
+Return `true` if the given sampler must run in unconstrained space. Defaults to true.
+"""
+requires_unconstrained_space(::AbstractSampler) = true
+
+"""
+    getparams([model::AbstractModel, ]state)::Vector{<:Real}
 
 Retrieve the values of parameters from the sampler's `state` as a `Vector{<:Real}`.
 """
@@ -116,6 +123,13 @@ function getparams(model::AbstractModel, state)
     return getparams(state)
 end
 
+"""
+    getstats(state)::NamedTuple
+
+Retrieve sampler statistics from the sampler's `state` as a `NamedTuple`.
+"""
+function getstats end
+
 """
     setparams!!([model::AbstractModel, ]state, params)