TuringLang / JuliaBUGS.jl / 8308493870

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Committed by web-flow

Commit Message

Refactor graph creation and node function handling (#163)

This PR introduces several enhancements and refactors the graph creation
process and node function handling in JuliaBUGS. The main changes
include:

* Sharing node function expressions across nodes:

Previously, each node in the graph had its own unique node function
expression. This PR modifies the behavior so that nodes originating from
the same statement in the model definition share the same node function
expression.

  Example:
  ```julia
  @bugs begin
      for i in 1:2
          x[i] ~ dnorm(0, 1)
          y[i] ~ dnorm(x[i], i)
      end
  end
  ````
In the previous version, the nodes for x[1], x[2], y[1], and y[2] would
have separate node function expressions. Now, they will share the
expressions dnorm(0, 1) and dnorm(x[i], i) based on the corresponding
statements.

The function `replace_constants_in_expr` used to plugin all the scalar
values into the node function expr, now the function is removed. The new
node function is a function takes all the variable on the RHS as
arguments (including loop variables).
  E.g., 
  ```julia
  function (;x::AbstractArray{Float64}, i::Int)
      return dnorm(x[1], i)
  end
  ```
The binding of loop variables to values are stored at nodes, and used
when evaluating node function.

This change reduces memory usage and paves the way for potentially
evaluating node functions once and using compiled functions during model
evaluation.

* Simplifying the graph creation process:
  The graph building algorithm has been overhauled for clarity.

  Example:
  ```julia
  @bugs begin
      x[1:2] ~ dmnorm(...)
      x[3] ~ dnorm(0, 1)
      y ~ dnorm(sum(x[2:3]), 1)
  end
  ```
In the previous version, temporary nodes were created for variables used
on the RHS that were not explicitly defined in the model, such as `x[1],
x[2], x[2:3]`. These temporary nodes needed to be removed later, adding
complexity to the... (continued)

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