Function to add to logp within @model Turing model

[jonasmac16]

Following from a discussion on Slack #turing I am opening this issue to request the possibility to add to logp of sample from within Turing model defined with the macro @model and to discuss what the best implementation would look like.

Two suggestions so far where

1. Directly adding to the logp variable like this logp += log(1.0)
2. Having a function that handles the adding e.g. acclogpdf!(log(1.0))

@trappmartin suggested the second to be potentially the better solution as it is independent of how the logp value is reflected internally and you don't add to an "undefined" variable like in option 1.

[mohamed82008]

@xukai92 suggested before to have some way to get the VarInfo inside the model. I think we can some function __getvarinfo__() for this purpose. So at the beginning of the model call function body generated by the macro, we can have:

__getvarinfo__() = $vi_sym

where vi_sym is the generated symbol for vi::VarInfo. acclogp! is defined for VarInfo.

[mohamed82008]

This is also useful for #703 where the user can use their own try-catch block or any more performant error avoidance mechanism, e.g. checking for signs of individual failure modes, and then if a failure mode is detected, logp can be set to -Inf and return can be called to interrupt.

[trappmartin]

@xukai92 suggested before to have some way to get the VarInfo inside the model. I think we can some function __getvarinfo__() for this purpose. So at the beginning of the model call function body generated by the macro, we can have:

__getvarinfo__() = $vi_sym

where vi_sym is the generated symbol for vi::VarInfo. acclogp! is defined for VarInfo.

I was thinking about replacing acclogp! and every $lp_ += ... statement with a macro call, e.g. @acclogp ... which would remove the need of passing a VarInfo object. I think it would be the best if the user doesn't need to pass a VarInfo object and doesn't need to know about the vi_sym or similar.

In the macro, we would still manipulate the varinfo object but without the user having to know about it.

What do you think?

[trappmartin]

This is also useful for #703 where the user can use their own try-catch block or any more performant error avoidance mechanism, e.g. checking for signs of individual failure modes, and then if a failure mode is detected, logp can be set to -Inf and return can be called to interrupt.

A @acclogp -Inf macro call would be all we need for this case.

[mohamed82008]

Hmm, you mean @acclogp -Inf will be replaced in @model with vi.logp += -Inf? This also seems like a nice solution.

[mohamed82008]

#965 now provides a way to access the VarInfo using @vi() in the model.

[mohamed82008]

This was solved by #965.