Use and syntax of "observe"

[nicolaspoisson42-oss]

Hi !
I'm currently working on the translation of Problang in WebPPL to Memo, in order to familiarize myself with the language, but I'm stuck on a seemingly very simple problem, could you help me understand the syntax issue?

I'm translating this block:

var listener = function(utterance) 
{ Infer({model: function(){ 
var prevalence = sample(prevalencePrior) 
var theta = uniform(0, 1) 
condition(meaning(utterance,prevalence,theta)) 
return prevalence }}) } 

After defining the correct Prevalence and Theta distributions, prevalence prior and meaning function, I define the following listener:

@memo
def L0(u_: Utt,):
    listener: knows(u_)
    listener: thinks[
        world: given(p in P, wpp=prevalencePrior()),
        world: given(t in T, wpp=1)
    ]
    listener: observes meaning(
        U(u_),
        Prevalence[p],
        Theta[t]
    ) is True
    return p + 1e-10

"listener: observes meaning(...) is True" is ungrammatical, but I don't know any other way of conditioning properly in Memo. Could you explain me what I'm missing ?

Thanks a lot

[kach]

Try using observes_that to condition on a fact about the world (rather than an observation of a particular variable, which is what observes is for).

listener: observes_that[ meaning(u_, p, t) ]