Variable going out of scope when other variables are defined

[sanitgupta]

var P = function() {
  return flip(theta) ?  'a' : 'b';
}

var fn = function() {
  return sample(Beta({a: 1, b: 1}))
}

//var x = sample(Beta({a: 1, b: 1}));
var theta = 0.1;
theta;

var dist = Infer(
  {method: 'MCMC', kernel: 'MH', samples: 1000},
  P);

viz.auto(dist)

This code runs fine but when I uncomment the line initializing x, the code gives an error 'theta is not defined' . I am not sure how/why this is happening. Apologies if it isn't a bug.

[null-a]

Thanks for taking the time to open this issue!

The way to make this work regardless of whether var x = ... is commented out or not, is to move var theta = ... to the top of the program. (Before theta is mentioned in P.)

I'm not sure whether we expect it to be possible to write var theta = ... after defining P and have things work, but that fact that this sometimes works and sometimes doesn't seems like a bug either way.

I am not sure how/why this is happening.

As part of compilation, the program is transformed into continuation passing style. When var x = sample(...) is present, everything after it appears inside a new function, and because that new function introduces a new scope theta isn't visible from P.

In other words, without the call to sample, the program generates JS something like this:

var P = function() {
  return theta;
};
var theta = 100;
P();

... but with the call to sample we get something like:

var P = function() {
  return theta;
};
(function() {
  var theta = 100;
  P();
})();

[sanitgupta]

Thanks so much for the explanation!