Unify `unflatten`, `replace_value`, and others

[mhauru]

As noted in https://github.com/TuringLang/DynamicPPL.jl/pull/793/files#r1936255984 and the comment that it's on, unflatten, replace_value, one of the constructors for VarInfo, and set_values!! seem very similar, especially once indexing by samplers has been removed. Some or all of them should be unified into a single function.

[penelopeysm]

these look similar ...

DynamicPPL.jl/src/varinfo.jl

Lines 231 to 244 in e4fa7f2

	@generated function unflatten_metadata(
	metadata::NamedTuple{names}, x::AbstractVector
	) where {names}
	exprs = []
	offset = :(0)
	for f in names
	mdf = :(metadata.$f)
	len = :(sum(length, $mdf.ranges))
	push!(exprs, :($f = unflatten_metadata($mdf, x[($offset + 1):($offset + $len)])))
	offset = :($offset + $len)
	end
	length(exprs) == 0 && return :(NamedTuple())
	return :($(exprs...),)
	end

DynamicPPL.jl/src/varinfo.jl

Lines 270 to 281 in e4fa7f2

	@generated function replace_values(metadata::NamedTuple{names}, x) where {names}
	exprs = []
	offset = :(0)
	for f in names
	mdf = :(metadata.$f)
	len = :(sum(length, $mdf.ranges))
	push!(exprs, :($f = replace_values($mdf, x[($offset + 1):($offset + $len)])))
	offset = :($offset + $len)
	end
	length(exprs) == 0 && return :(NamedTuple())
	return :($(exprs...),)
	end

DynamicPPL.jl/src/varinfo.jl

Lines 713 to 723 in e4fa7f2

	@generated function _setall!(metadata::NamedTuple{names}, val) where {names}
	expr = Expr(:block)
	start = :(1)
	for f in names
	length = :(sum(length, metadata.$f.ranges))
	finish = :($start + $length - 1)
	push!(expr.args, :(copyto!(metadata.$f.vals, 1, val, $start, $length)))
	start = :($start + $length)
	end
	return expr
	end

edit- they're the same except that one mutates and the other two don't! 😄