Return values of Probabilistic Programs Syntax and semantics section of this paper on probabilistic programming mentions that the return expression of the program is a function f satisfying  $
f : \sum \rightarrow \mathbb{R}^{\inf}_{\geqslant0}$ where $\sum$ is the set of states. I do no not see the need of $\geqslant0$. Why can't the return values be negative?
The function $f$ is a probability sub-distribution, which means that it's a like a probability distribution, only its integral can be less than $1$ (due to the observe statements). The return statement normalizes $f$ back to a bona fide probability distribution.

Outside of quantum computing, probabilities are non-negative.