# JuliaCon 2020 Birds of a Feather

**URL:** <https://discourse.julialang.org/t/juliacon-2020-birds-of-a-feather/39181>\
**Category:** Probabilistic Programming\
**Created:** [May 9, 2020, 2:11pm UTC](https://discourse.julialang.org/t/juliacon-2020-birds-of-a-feather/39181 "2020-05-09T14:11:51Z")\
**Posts on this page:** 5\
**Page:** 2

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**Author:** ![McCoy](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mccoy/32/13833_2.png) [@McCoy](https://discourse.julialang.org/u/McCoy)\
**Post date:** [August 1, 2020, 9:08pm UTC](https://discourse.julialang.org/t/juliacon-2020-birds-of-a-feather/39181/22 "2020-08-01T21:08:43Z")

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Not-so-shameless (but useless) plug here - I re-designed a portion of Cassette using IRTools so that I could understand the Cassette transform better - I put together a pedagogical package which illustrates this: [https://github.com/femtomc/Mixtape.jl](https://github.com/femtomc/Mixtape.jl)

If you’re curious about compiler metaprogramming, these ~40 lines of code might prove enlightening 🙂

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**Author:** ![phg](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/phg/32/17184_2.png) [@phg](https://discourse.julialang.org/u/phg)\
**Post date:** [August 14, 2020, 7:48am UTC](https://discourse.julialang.org/t/juliacon-2020-birds-of-a-feather/39181/23 "2020-08-14T07:48:19Z")

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I really excited about all these plans! And I’d be very happy to join the meetings.

> So maybe we need “my waking hours in UTC” for each of us, or something like that?

For me in central Europe (UTC+2 during DST, otherwise +1) that would be about 06:00–18:00 UTC currently.

> I agree it would be great to have a general-purpose abstract type for tracing. @alex-lew helped me connect Soss to the trace type needed for Gen, but I have no idea how this relates to Turing’s representation. It would be good to get a better understanding of the `VarInfo` stuff and see if there’s a best-of-both-worlds, or if some tradeoffs are unavoidable.

`VarInfo` is really just one big float vector with a couple of extra arrays to remember where each variable went. The additional difficulty is that we have this `VarName` type, representing any indexed variable, like `@varname(T[1][:])` – basically just a Symbol and a tuple of tuples. Currently indexing with that into `VarInfo` is difficult sometimes (same in AbstractMCMC chains), because information about the “original shape” is not completely preserved. (@mohamed82008, correct me if I have a flaw in my mental model.)

I have lately been thinking about something like Gen’s trace mixed with Turing’s idea: an underlying array of type `T`, of which you can take reshaped views, and on top of it some kind of trie or R-tree structure to allow proper indexing and slicing with `VarName`. This would be an `AbstractDict{VarName, T}` with fully general indexing, but still _one_ array. And a kind of “write-once” data structure, due to the nature of how RVs work. But I stopped trying at some point.

> My big question here is, is there an opportunity for an IR between the model syntax (as input by the user) and this lower-level representation?

Another point I have had some ideas about 😃 I believe there must exist some some possibility to extend existing SSA-based Julia IR with “sampling statements”/tildes (“single static sampling form”?), and maybe some CPS stuff, so that there’s a low level representation of every probabilistic program very similar to the IR of the corresponding Julia program, which is additionally in direct correspondence with a monadic formulation (for each basic block, at least). The difficult part is “temporary RVs” like sampling a scalar and putting it into an array. Maybe some approaches from linear type systems can help with that. But that’s still to be worked out.

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**Author:** ![McCoy](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mccoy/32/13833_2.png) [@McCoy](https://discourse.julialang.org/u/McCoy)\
**Post date:** [August 26, 2020, 2:49pm UTC](https://discourse.julialang.org/t/juliacon-2020-birds-of-a-feather/39181/24 "2020-08-26T14:49:16Z")

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@cscherrer should we start putting together some dates for a call? It’s the end of the month now. Should we aim for a call meeting in the middle of the next month (i.e. a few weeks from now)?

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**Author:** ![cscherrer](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/cscherrer/32/7631_2.png) [@cscherrer](https://discourse.julialang.org/u/cscherrer)\
**Post date:** [August 26, 2020, 3:53pm UTC](https://discourse.julialang.org/t/juliacon-2020-birds-of-a-feather/39181/25 "2020-08-26T15:53:44Z")

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Thanks for following up on this @McCoy. Not sure what the best way is to go about this. I guess the biggest blocker would be conference deadlines, right? Anything coming up to account for?

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**Author:** ![McCoy](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mccoy/32/13833_2.png) [@McCoy](https://discourse.julialang.org/u/McCoy)\
**Post date:** [August 28, 2020, 12:31pm UTC](https://discourse.julialang.org/t/juliacon-2020-birds-of-a-feather/39181/26 "2020-08-28T12:31:48Z")

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Yeah, I’m totally out of the loop on that. I think the answer is that there are a few - but I’m not sure.

Let me start banging on doors on Slack and report what I find.

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