Confusion regarding .~

[torfjelde]

Ref: TuringLang/Turing.jl#2390 (comment)

MWE:

julia> @model demo(x) = x .~ Normal()
demo (generic function with 2 methods)

julia> demo(missing)()
ERROR: MethodError: no method matching dot_assume(::Random.TaskLocalRNG, ::SampleFromPrior, ::Normal{…}, ::VarName{…}, ::Missing, ::UntypedVarInfo{…})
[...]

julia> demo([missing])()
ERROR: MethodError: no method matching loglikelihood(::Normal{Float64}, ::Vector{Union{Missing, Float64}})
[...]

Even if you use condition, you still run into a similar issue when specifying components, i.e.

julia> @model function demo()
           x = Vector{Float64}(undef, 2)
           x .~ Normal()
       end
demo (generic function with 4 methods)

julia> (demo() | (@varname(x[1]) => 1.0,))()  # not conditioned
2-element Vector{Float64}:
 1.6841936984140546
 0.3645020534551503

The reason why we do this is because of performance: we want .~ to be faster than just iterating over the entire broadcast expression and calling tilde_assume!! and tilde_observe!! separately. However, this is very, very confusing for users, and I think it's very understandable.

Sooo the question is how to fix this. A few immediate thoughts:

1. Do we need .~? Does it just make things confusing?
2. Should we just convert it into a loop over the broadcasted expression and drop the perf benefits?

[mhauru]

How significant are the perf benefits? Could we benchmark this?

Either of the proposed two solutions would also simplify the tilde pipeline code a lot, improving maintainability. It's a source of a lot of corner cases and code complexity.

[mhauru]

We could ask on Slack/Discourse whether our users are making heavy use of .~, to gauge how much effort we should put into keeping it around and performant.

[yebai]

1. Do we need .~? Does it just make things confusing?
2. Should we just convert it into a loop over the broadcasted expression and drop the perf benefits?

I lean towards option 2 or something similar if it simplifies the implementation. The tilde pipeline is becoming a bit too (deep and) complex to reason about.