AIS now throws an error when a hard factor statement is encountered.

docs/functions/other.rst

@@ -141,18 +141,29 @@ Other
    ``model``. This is not an unbiased estimator, rather it is a
    stochastic lower bound. [grosse16]_
 
+   The sequence of intermediate distributions used by AIS is obtained
+   by scaling the contribution to the overall score made by the
+   ``factor`` statements in ``model``.
+
+   When a model includes hard factors (e.g. ``factor(-Infinity)``,
+   ``condition(bool)``) this approach does not produce an estimate of
+   the expected quantity. Hence, to avoid confusion, an error is
+   generated by ``AIS`` if a hard factor is encountered in the model.
+
+   The length of the sequence of distributions is given by the
+   ``steps`` option. At step ``k`` the score given by each ``factor``
+   is scaled by ``k / steps``.
+
+
    The MCMC transition operator used is based on the :ref:`MH kernel
    <mh>`.
 
+
    The following options are supported:
 
    .. describe:: steps
 
-      The length of the sequence of intermediate distributions used by
-      AIS. This sequence is obtained by scaling the contribution to
-      the overall score made by the ``factor`` statements in
-      ``model``. At step ``k`` the score given by each ``factor`` is
-      scaled by ``k / steps``.
+      The length of the sequence of intermediate distributions.
 
       Default: ``20``
 

src/inference/ais.js

@@ -44,7 +44,10 @@ module.exports = function(env){
         var mhStepKernel = function(k, trace) {
           weight += increment * trace.scoreAllFactors();
           curStep += 1;
-          return MHKernel(k, trace, {factorCoeff: curStep * increment});
+          return MHKernel(k, trace, {
+            factorCoeff: curStep * increment,
+            allowHardFactors: false
+          });
         };
 
         var mhChainKernel = kernels.repeat(options.steps, mhStepKernel);

src/inference/mhkernel.js

@@ -31,14 +31,16 @@ module.exports = function(env) {
     runOpts = util.mergeDefaults(runOpts, {
       proposalBoundary: 0,
       exitFactor: 0,
-      factorCoeff: 1
+      factorCoeff: 1,
+      allowHardFactors: true
     });
 
     this.proposalBoundary = runOpts.proposalBoundary;
     this.exitFactor = runOpts.exitFactor;
 
     this.factorCoeff = runOpts.factorCoeff;
     assert.ok(0 <= this.factorCoeff && this.factorCoeff <= 1);
+    this.allowHardFactors = runOpts.allowHardFactors;
 
     this.cont = cont;
     this.oldTrace = oldTrace;
@@ -65,6 +67,9 @@ module.exports = function(env) {
   MHKernel.prototype.factor = function(s, k, a, score) {
     // Optimization: Bail early if we know acceptProb will be zero.
     if (ad.value(score) === -Infinity) {
+      if (!this.allowHardFactors) {
+        throw new Error('Hard factor statements are not allowed.');
+      }
       return this.finish(this.oldTrace, false);
     }
     this.trace.numFactors += 1;