Python interface about coq-serapi

[ejgallego]

This took some time, but it is currently "solved" by https://github.com/ejgallego/pycoq

However, due to toolchain issues, python support has not yet landed in the main branches, so we will keep this issue open until indeed python support is part of an official release.

[brando90]

I am interested in a Python way to use SerAPI. At what stage is this?

I have 4 questions/ideas:

• 1. Is there python interface version I can try?
• 2. Does one for a specific use case already exists that I may start trying? i.e. someone has already integrated Serapi with python?
• 3. I could be the first one to do it? Can I help build it or something?
• 4. Or is the language agnostic way the only/best way to do it now?

[ejgallego]

I am curious, why are RPCs not flexible?

You are limited by the RPC interface, in this sense you have several limits: for example, depend on the chosen serialization format; usually, you must serialize full calls to in the RPC, which is very costly, or implement complex call back mechanism, which bring their own gargabe-collection issues. Also, you depend on the public RPC interface, so if the API doesn't suit you, bad luck.

Agreed! Perhaps a better advertisement in the readme would be useful?

Maybe! If you have any suggestion please submit a PR!

What do you mean by idiomatic here? Like that it looks like "the idioms of python" - pythonic?

See for example issue ejgallego/pycoq#4

Indeed, it would be nice if we could generate a better object interface than the current, low level one.

This makes sense. I guess the main reason is likely because we are skipping serapi alltogether and talking to coq directly?

We are replacing printing and parsing for direct object serialization, you can bench, but it is for sure faster.

Looking forward to feedback!

[ejgallego]

I am interested in a Python way to use SerAPI. At what stage is this?

I have thought a lot about ways to do it well, however not having a Python expert close meant that I moved it down on my priority list. But I am personally very interested in making it happen so I'd be glad to help.

First the questions:

• 1. Is there python interface version I can try?
• 1. Does one for a specific use case already exists that I may start trying? i.e. someone has already integrated Serapi with python?

Unfortunately not.

• 1. I could be the first one to do it? Can I help build it or something?

Sure, I'd be able to help a lot too!

• 1. Or is the language agnostic way the only/best way to do it now?

I see a huge value in having a small Python library that exposes the methods of the protocol; the thing is, what'd be the best way to do it.

Interfacing Python with SerAPI directly could prove costly. One key goal of SerAPI is to have easy maintenance and some parts of the serialization are heavy. Despite that, I think that a good solution is doable, the two main challenges are:

• Async: interaction with Coq is by nature async, what is the best Python idiom to handle async programming? Once we figure this out, it should be a matter of hours to have something working.
• Data representation: presenting Coq objects as python objects may be pretty heavy. But we could go two routes: have the Python API present objects as SEXPS, or indeed write a serializer from SEXP to objects. This can be done at different levels, for example in https://github.com/ejgallego/ocaml-ts there is a shim that connects OCaml's AST to a printer [for example a SEXP to Python object transformer]

[palmskog]

@brando90 I would suggest starting really minimalistically, e.g., a proof with 1-2 lines of proof scripts from here or here.

It may also help if you understand how multi-goal tactic systems work, one good source is a short note by Arnaud Spiwack.

[brando90]

@ejgallego how is that different from this version of pycoq? https://github.com/IBM/pycoq

[ejgallego]

Hey @brando90 ; oh, I totally missed that project when doing some search ; maybe I saw it back in March when I started PyCoq' :S would be helpful if people would add their tools to https://github.com/ejgallego/coq-serapi#clients-and-users so we keep track of them and can coordinate.

From a technical point of view, the projects are very different, IBM's PyCoq relies on serapi to access Coq, and this involves RPC and is not very flexible.

Our own PyCoq does instead create a Python .so object with the full Coq implementation, and doesn't rely on RPC, but links to Coq directly. This has several advantages, in particular, it should be possible to make the API much more idiomatic [as outlined in the issues] and avoiding RPC may matter in a lot of scenarios where for example performance is important. Maintenance should be also easier, tho that's still to see as the PyML stack is a heavy one too.

[ejgallego]

I am curious, why is it faster? I guess I am still struggling to form a model of how this is different from RPCs. Both have to eventually or somehow parse Coq - no?

We don't parse the protocol when using the non-RPC model, we build a Python object in direct style, using https://github.com/thierry-martinez/pyml , so that's faster and way more flexible in terms of sharing and delaying serialization.

Some people is really sending huge objects, and they reported parsing / printing as an slowdown.

[ejgallego]

cc #17

[brando90]

Sounds good! I'm happy to get together to help and chat about the interface/API/design etc. (if thats helpful/useful).

As a selfish comment, I am interesting to keeping in mind some of the functionality of TCoq in mind (#109), specially exposing compund tactics and their intermediate proof states. Just leaving this comment here so I don't forget (could be low priority etc).

Yes, I do think having Python objects will be useful for the useful.

[ejgallego]

Sounds good! I'm happy to get together to help and chat about the interface/API/design etc. (if thats helpful/useful).

That's great!

As a selfish comment, I am interesting to keeping in mind some of the functionality of TCoq in mind (#109), specially exposing compund tactics and their intermediate proof states. Just leaving this comment here so I don't forget (could be low priority etc).

I'd suggest we start with an example then; please write a small proof script and what you would expect from it. Then, we can first see if the generic protocol can handle it; and if yes, we could see what the most convenient Python interface would be.

[brando90]

this theorem might be more advance than my knowledge of coq but I had in mind something like this (example from http://geocoq.github.io/GeoCoq/html/GeoCoq.Highschool.midpoint_thales.html):

Lemma midpoint_thales_reci :
  ∀ a b c o: Tpoint,
   Per a c b →
   Midpoint o a b →
   Cong o a o b ∧ Cong o b o c.
Proof.
intros.

induction (Col_dec a b c).

induction (l8_9 a c b H);
treat_equalities;assert_congs_perm;try split;finish.
assert_diffs.
assert_congs_perm.
split.
Cong.
assert(Hmid := midpoint_existence a c).
destruct Hmid.
assert(Hpar : Par c b x o).
apply (triangle_mid_par c b a o x);finish.
assert(Hper : Perp c b c a)
 by (apply perp_left_comm;apply per_perp;Perp).
assert(HH := par_perp_perp c b x o c a Hpar Hper).
assert(Hper2 : Perp c x o x).
  apply (perp_col c a o x x).
  assert_diffs.
  finish.
  Perp.
  assert_cols;Col.
assert_diffs.
assert (Per o x c)
 by (apply perp_per_2;Perp).

unfold Per in H8.
destruct H8.
spliter.
apply l7_2 in H8.
assert(HmidU := l7_9 a x0 x c H2 H8).
subst.
unfold Midpoint in H2.
spliter.
eCong.
Qed.

steps like: treat_equalities;assert_congs_perm;try split;finish. are treated like a single compound tactic that affects multiple proof states (context+goal) in the proof tree. Ideally I'd like to control the granularity of this (getting hidden proof states at any granularity if I want to). So be able to break it down into its atomic (or sub compunded) tactics and perhaps more importantly, their proof states (and proof tree).

[brando90]

This is one of the difference with TCoq (that I am aware of). TCoq records the intermediate proof states encountered as opposed to being an API for machine-machine or human-machine interaction (as far as I know).

[ejgallego]

steps like: treat_equalities; assert_congs_perm; try split; finish. are treated like a single compound tactic that affects multiple proof states (context+goal) in the proof tree. Ideally I'd like to control the granularity of this (getting hidden proof states at any granularity if I want to). So be able to break it down into its atomic (or sub compunded) tactics and perhaps more importantly, their proof states (and tree).

Here we'd need a more concrete specification of what you mean by intermediate states. I see that TCoq instruments LTAC in a particular way, however it doesn't instrument proofview so the kind of traces you get would seem to be a bit arbitrary. [I didn't look at tcoq in depth tho] Note that the notion of state is a bit trickier than expected due to the "multi-goal" model of the 8.5+ proof engine.

I suggest we start with a very simple example with tactics a;b and we see what you would like to get.

As for now, SerAPI does allow to get the state before and after a;b execution. We could get the state just after a [just by running a speculatively], but if you instrument you may be many checkpoints depending on what the definition of a is.

[brando90]

One idea I had in mind was to try to reimplement something similar to TCoq but with a better-maintained library like SerAPI. Though I think you guys are right. I should think clearly what I want first and then we can discuss.

Yes, I will check something simple first and make sure I understand multi-goal tactic systems by reading what you posted. Thanks!

Hope to chat soon in the WG. :)

[brando90]

Thanks for the reply Emilio! A few questions I am curious about (+some comments):

Hey @brando90 ; oh, I totally missed that project when doing some search ; maybe I saw it back in March when I started PyCoq' :S would be helpful if people would add their tools to https://github.com/ejgallego/coq-serapi#clients-and-users so we keep track of them and can coordinate.

Agreed! Perhaps a better advertisement in the readme would be useful?

From a technical point of view, the projects are very different, IBM's PyCoq relies on serapi to access Coq, and this involves RPC and is not very flexible.

I am curious, why are RPCs not flexible? In what ways are they not flexible? Can't they do everything the server (serapi) does? So if it's not flexible would it be because Coq or serapi aren't flexible? Curious to understand (and hopefully these convs help us build a better tool for all of us)

Our own PyCoq does instead create a Python .so object with the full Coq implementation, and doesn't rely on RPC, but links to Coq directly. This has several advantages, in particular, it should be possible to make the API much more idiomatic [as outlined in the issues]

What do you mean by idiomatic here? Like that it looks like "the idioms of python" - pythonic?

and avoiding RPC may matter in a lot of scenarios where for example performance is important.

This makes sense. I guess the main reason is likely because we are skipping serapi alltogether and talking to coq directly? But perhaps some benchmarking would be nice and perhaps with things like asynchronous code (e.g. python 's asyncio's async and await might remove that issue in python).

Maintenance should be also easier, tho that's still to see as the PyML stack is a heavy one too.

Thanks in advance! Hope to try them soon and hopefully provide good feedback.

[brando90]

I am curious, why are RPCs not flexible?

You are limited by the RPC interface, in this sense you have several limits: for example, depend on the chosen serialization format; usually, you must serialize full calls to in the RPC, which is very costly, or implement complex call back mechanism, which bring their own gargabe-collection issues. Also, you depend on the public RPC interface, so if the API doesn't suit you, bad luck.

Agreed! Perhaps a better advertisement in the readme would be useful?

Maybe! If you have any suggestion please submit a PR!

What do you mean by idiomatic here? Like that it looks like "the idioms of python" - pythonic?

See for example issue ejgallego/pycoq#4

Indeed, it would be nice if we could generate a better object interface than the current, low level one.

This makes sense. I guess the main reason is likely because we are skipping serapi alltogether and talking to coq directly?

We are replacing printing and parsing for direct object serialization, you can bench, but it is for sure faster.

I am curious, why is it faster? I guess I am still struggling to form a model of how this is different from RPCs. Both have to eventually or somehow parse Coq - no?

Looking forward to feedback!

:)

[brando90]

as a side comment, my understanding is that ibm pycoq uses serapi as a backend but it's possible to change ithe backend to this pycoq too, so perhaps they aren't competing tools but rather complementary.

The name clashing is rather unfortunate though.