# How Figaro (Scala) / Anglican are different from PyMC3/Stan?

**URL:** <https://discourse.pymc.io/t/how-figaro-scala-anglican-are-different-from-pymc3-stan/1873>\
**Category:** Questions\
**Created:** [September 6, 2018, 3:28pm UTC](https://discourse.pymc.io/t/how-figaro-scala-anglican-are-different-from-pymc3-stan/1873 "2018-09-06T15:28:23Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![nanounanue](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.pymc.io/nanounanue/32/1098_2.png) [@nanounanue](https://discourse.pymc.io/u/nanounanue)\
**Post date:** [September 6, 2018, 3:28pm UTC](https://discourse.pymc.io/t/how-figaro-scala-anglican-are-different-from-pymc3-stan/1873/1 "2018-09-06T15:28:23Z")

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I just posted this in stackoverflow and the question was rejected given that was too broad. I am searching for answers to this, so, I help this community could help me:

I just started to learn PyMC3, and in general Probabilistic Programming, but after some exploration I have the folllowing question:

Obviously the first thing is that the language in the first 3 are different (Scala, Clojure, Python). The second difference is that (at least) from the examples around the web and books PyMC3 and Stan are _most_ used in Bayesian Hierarchical Models/ Bayesian Inference and the examples (very few) from the former two are (apparently) more general.

In the book _Practical probabilistic programming_ the author describes a  
_Probabilistic reasoning system_ and he describes it as capable of doing: (a) Predict future events, (b) Infer the cause of effects and © Learn from past events to better predict future events (pages 8-9), and he gives examples about this in the book using Figaro. Are PyMC3 and Stan capable of doing this? Are there examples of this? The same goes to [Anglican](https://probprog.github.io/anglican/index.html), in several talks (including [this one](https://www.youtube.com/watch?v=6Lqt07enBGs)) they mention the use of Anglican for systems that shown the capabilities expressed above (in this case for a physical simulation in oil sea platforms).

Maybe a better way of asking this is possible to implement all the examples of the forementioned book in PyMC3 or Stan?

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**Author:** ![junpenglao](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.pymc.io/junpenglao/32/8_2.png) [@junpenglao](https://discourse.pymc.io/u/junpenglao)\
**Post date:** [September 7, 2018, 9:09am UTC](https://discourse.pymc.io/t/how-figaro-scala-anglican-are-different-from-pymc3-stan/1873/2 "2018-09-07T09:09:53Z")

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I think the recent thesis by Tom Rainforth gives a pretty good description, it is a bit bias in favor towards Anglican because he’s from Frank Wood’s team. See Chapter 4:

> **[rainforth2017thesis.pdf](https://www.robots.ox.ac.uk/~twgr/assets/pdf/rainforth2017thesis.pdf)**
>
> 17.04 MB

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**Author:** ![nanounanue](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.pymc.io/nanounanue/32/1098_2.png) [@nanounanue](https://discourse.pymc.io/u/nanounanue)\
**Post date:** [September 7, 2018, 3:38pm UTC](https://discourse.pymc.io/t/how-figaro-scala-anglican-are-different-from-pymc3-stan/1873/3 "2018-09-07T15:38:28Z")

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@junpenglao This is awesome, thank you so much!

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**Author:** ![cscherrer](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.pymc.io/cscherrer/32/592_2.png) [@cscherrer](https://discourse.pymc.io/u/cscherrer)\
**Post date:** [September 15, 2018, 5:18pm UTC](https://discourse.pymc.io/t/how-figaro-scala-anglican-are-different-from-pymc3-stan/1873/4 "2018-09-15T17:18:22Z")

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Hi @nanounanue, I just saw your question. I’ve used Stan and PyMC3 a lot, and I served as technical lead on the evaluation team for PPAML, the DARPA program that funded Figaro and Anglican. I like @junpenglao’s suggestion of checking out Rainforth’s thesis, but I wonder if some discussion could be helpful as well.

There’s an analogy I’ve found helpful in thinking about how these different systems. Say someone vaguely asks you to “drive them to a thing”, and you get to choose between a luxury sedan and one of those hardcore off-road buggy things (there must be a term for those, I’ll go with “buggy”) like they used in Westworld. Which is the better choice?

If you need to be able to go absolutely anywhere, you should take the buggy. If it’s in-town driving, the buggy will be fun for a while and then probably get tiring. And if it’s a highway road trip, it will not be nearly as enjoyable.

By constraining the problem space (“normal” road driving), the sedan can do a better job when it’s in its preferred space. That doesn’t mean the sedan is “better”, or that you couldn’t use the buggy for absolutely everything.

Ok, back to the point. All of the systems you mention satisfy Avi’s description. “Universal” probabilistic programming (as in Anglican) is the buggy. It can represent any model you throw at it. Stan is the other extreme, limited to a fixed-dimensional parameter space over differentiable distributions. Stan can’t represent nearly as many models. Things like mixture models are a bit awkward, because you have to jump through some hoops to sum over the discrete values (weird that they haven’t automated that). But when you do sum them out, you get the benefit of “Rao-Blackwellization”, making inference more efficient. And it generates efficient C++ code, so (after you get past compiler overhead) inference is very fast.

Well, that almost does it. There’s still the potential for general-purpose systems like Anglican to recognize (automatically or through user input) that the model is constrained, and to go with a specialized algorithm. In principle, a universal PPL can give the best of both worlds. In practice, this can come down to the time and energy of the development team, characteristics of the language they’re using for development, and the degree to which they can leverage external tools.

In terms of “representable models”, it’s something like  
Stan \< PyMC3 \< Figaro \< Anglican  
(Figaro has some advantage over PyMC3 because it allows an “open world” - you can introduce new variables as you go).

Overall , these are all great, and I think the primary decision I would use to decide is what language you prefer to work in:  
C++, command line, or R -\> Stan  
Python -\> PyMC3  
Scala -\> Figaro  
Clojure -\> Anglican

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**Author:** ![\_eigenfoo](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.pymc.io/_eigenfoo/32/3059_2.png) [@\_eigenfoo](https://discourse.pymc.io/u/_eigenfoo)\
**Post date:** [September 18, 2018, 4:27am UTC](https://discourse.pymc.io/t/how-figaro-scala-anglican-are-different-from-pymc3-stan/1873/5 "2018-09-18T04:27:09Z")

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@cscherrer That was an amazing overview, thank you!

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**Author:** ![cscherrer](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.pymc.io/cscherrer/32/592_2.png) [@cscherrer](https://discourse.pymc.io/u/cscherrer)\
**Post date:** [September 18, 2018, 4:43am UTC](https://discourse.pymc.io/t/how-figaro-scala-anglican-are-different-from-pymc3-stan/1873/6 "2018-09-18T04:43:37Z")

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That’s very kind, thank you. Glad it was helpful 🙂