Semantics of define differs between the clojure macros and the metacircular interpreter

[jar398]

Here is a bit of technical debt to pay off some time. I don't think this is likely to bite anyone soon, but eventually it will, so I wanted to record the issue.

The issue is that the clojure-metaprob block macro (with its define keyword) only approximates what python-metaprob did, using the primitives available in clojure (let and letfn). The python-metaprob equivalent was internal definitions written using =, which augments the lexical environment by mutation. But the meta-circular interpreter is faithful to python-metaprob. This leads to differences in behavior, for example:

> (define loser 
    (gen []
      (define bar 13)
      (define foo (gen [] bar))
      (define bar 17)
      (foo)))
#'metaprob.examples.all/loser
> (loser)
13
> (infer :procedure loser :output-trace? true)
[17 {} 0]

The block/define macro is pretty hairy, and creates its illusion of environment modification by segmenting the sequence of definitions into let bindings and letfn bindings - very different from creating an environment up front and mutating it with each succeeding define.

I'm not sure what to recommend regarding a fix. Implementing the python-metaprob semantics in clojure would be extremely painful and would create implementation and documentation complexity far in excess of its utility. But implementing the block macro's semantics in the metacircular interpreter will also be difficult (albeit much less so). A third alternative would be to abandon block/define in favor of clojure let and letfn, with changes to the meta-circular interpreter to implement the clojure syntax.

[zane]

Fixed by #123.