BUGS Parser

The macro @bugs produces a Julia Expr object that represents the BUGS model definition.

If the input is a String, it's assumed to be a program in the original BUGS language. In this case, the macro will first convert the program to an equivalent Julia program, then use the Julia parser to parse the program into an Expr object.

Both model definitions written in Julia and those written in the original BUGS and subsequently parsed are now represented as a Julia Expr object. These objects go through syntax checking and post-processing to create the input for the compile function.

Below, we describe how the original BUGS program is translated to an equivalent Julia program and detail the post-processing done to the Expr object.

BUGS to Julia Translation

In this section, we refer to the translation program as the "parser" and the translating process as "parsing". Although the parser doesn't produce a syntax tree, it does follow the form of a recursive descent parser, building a Julia program in the form of a vector of tokens rather than a syntax tree.

This general implementation is heavily inspired by JuliaSyntax.jl, the official parser for Julia since version 1.10.

The BUGS parser implemented here takes a token stream with a recursive descent structure and checks the program's correctness. Here's how it works:

1. Use tokenize to obtain the token vector.
2. Inspect the tokens and build the Julia version of the program as a vector of tokens.
3. Push the token to the Julia version of the program vector when appropriate.
4. Detect errors and make necessary alterations to tokens, such as deletion, combination, or replacement.

During the recursive descent, BUGS syntax tokens will be translated into Julia syntax tokens. Some tokens will remain as they are, while others will be transformed, removed, or new tokens may be added.

The parser will throw an error if it encounters a program that does not adhere to strict BUGS syntax.

Some Notes on Error Recovery

The current error recovery is ad hoc and primarily rudimentary. If the program is correct, it will produce the correct result. If the program is syntactically or semantically incorrect, the token stream will not be pushed forward, resulting in failure.

The failure detection mechanism checks if two errors occur with the same "current token". If they do, the parser stops and reports the error. This ensures that the parser won't incorrectly parse a flawed program.