Allow undefined high order function

This probably was discussed before but I can’t find anything. Does it make sense to allow undefined high order functions? Something like:

matrix foo(func bar, vector x);
matrix bar(vector x);
#include "foobar_impl.hpp" // let's assume we can include hpp
model {
vector b;
matrix a = foo(bar, b);

It implies we need to

  • allow including hpp like what one can do in rstan
  • allow new type “func”

I think doing this helps when someone wants to retrofit sophisticated c/c++ functions and use them in Stan.


I agree that better support for including external functions.

The support rstan offers is what I think we want to support everywhere with a more general approach.
CmdStan’s approach is much less useful and more or less not transfarable as shareing and including a C++ function (regardless if its a HOF or not) in different models is impossible without always touching the C++ code.

The related issues are support arbitrary data type in function block's function signatures · Issue #2572 · stan-dev/stan · GitHub and Including .hpp files and namespaces · Issue #712 · stan-dev/stanc3 · GitHub.

The stanc3 issue would have to be done first. The easiest solution would be that if the compiler sees #include ".../file.hpp" it would simply paste that include to the generated code. This is what rstan does just on the already generated C++ code which works but inserting stuff in generated code is not something we want interfaces doing.

Then we can figure out how to support arbitrary C++ external functions.

Would this similarly allow functions to be defined with gradients?

In C++? Yes.

For gradients in written in Stan thats a different story that is also circling around.

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Thanks, I wasn’t aware of the 2nd thread. No surprise this has a bigger scope of package system. In that regard, perhaps we can do a better job than #include. I like the option suggested in the thread that we can do

import FooPackage // equal to #include <FooPackage.hpp>
matrix FooPackage::foo(func bar, vector x);
matrix FooPackage::bar(vector x);

as it gives us chance to allow

import FooPackage // equal to #include <FooPackage.hpp>
import BarPackage // when we have package system ready, BarPackage would be a Stan package.
matrix FooPackage::foo(func bar, vector x); // we may not even need to declare foo or bar.
matrix BarPackage::bar(vector x);

Then the syntax would be consistent.

For that to work, first we have to go extra mile beyond rstan, to add keyword import & :: in stanc3.

EDIT: related conversion Helpful function repository , in particular Helpful function repository - #7 by bbbales2

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