Confusing error when sampling from empty model

[penelopeysm]

julia> using Turing

julia> @model function f(x)
           x ~ Normal()
       end
f (generic function with 2 methods)

julia> sample(f(1.0), NUTS(), 100)
Sampling 100%|██████████████████████████████████████████| Time: 0:00:00
ERROR: ArgumentError: reducing over an empty collection is not allowed; consider supplying `init` to the reducer
[...]

I think check_model() should probably error if the model is empty.

[mhauru]

Or maybe VarInfo should be able to handle this without erroring, return a chain with one dimension being zero? I don't see a reason why we should ban likelihood-only models, even if they are a bit silly. Might come around as a special case in some system of models someone is developing or somesuch.

[penelopeysm]

DynamicPPL is actually quite fine with it, it's getparams that errors so in that case this would be a Turing bug. Happy to move it, I opened it here because if it was check_model it would be here.

[penelopeysm]

We might run into this though

julia> m = Array{Float64}(undef, 0, 0, 0)
0×0×0 Array{Float64, 3}

julia> Chains(m, [])
ERROR: StackOverflowError:
Stacktrace:
     [1] Chains(::Array{Float64, 3}, ::Vector{Any}; kwargs::@Kwargs{})
       @ MCMCChains ~/.julia/packages/MCMCChains/RIFg4/src/chains.jl:24
     [2] Chains(::Array{Float64, 3}, ::Vector{Any})
       @ MCMCChains ~/.julia/packages/MCMCChains/RIFg4/src/chains.jl:18
--- the above 2 lines are repeated 26655 more times ---
 [53313] Chains(::Array{Float64, 3}, ::Vector{Any}; kwargs::@Kwargs{})
       @ MCMCChains ~/.julia/packages/MCMCChains/RIFg4/src/chains.jl:24