Fix MH

Author: penelopeysm

src/mcmc/mh.jl

@@ -157,6 +157,8 @@ end
 # Utility functions #
 #####################
 
+# TODO(DPPL0.37/penelopeysm): This function should no longer be needed
+# once InitContext is merged.
 """
     set_namedtuple!(vi::VarInfo, nt::NamedTuple)
 
@@ -181,17 +183,15 @@ function set_namedtuple!(vi::DynamicPPL.VarInfoOrThreadSafeVarInfo, nt::NamedTup
     end
 end
 
-"""
-    MHLogDensityFunction
-
-A log density function for the MH sampler.
-
-This variant uses the `set_namedtuple!` function to update the `VarInfo`.
-"""
-const MHLogDensityFunction{M<:Model,S<:Sampler{<:MH},V<:AbstractVarInfo} =
-    DynamicPPL.LogDensityFunction{M,V,AD} where {AD}
-
-function LogDensityProblems.logdensity(f::MHLogDensityFunction, x::NamedTuple)
+# NOTE(penelopeysm): MH does not conform to the usual LogDensityProblems
+# interface in that it gets evaluated with a NamedTuple. Hence we need this
+# method just to deal with MH.
+# TODO(DPPL0.37/penelopeysm): Check the extent to which this method is actually
+# needed. If it's still needed, replace this with `init!!(f.model, f.varinfo,
+# ParamsInit(x))`. Much less hacky than `set_namedtuple!` (hopefully...).
+# In general, we should much prefer to either (1) conform to the
+# LogDensityProblems interface or (2) use VarNames anyway.
+function LogDensityProblems.logdensity(f::LogDensityFunction, x::NamedTuple)
     vi = deepcopy(f.varinfo)
     set_namedtuple!(vi, x)
     vi_new = last(DynamicPPL.evaluate!!(f.model, vi, f.context))