Merge pull request #842 from null-a/optional-dist-params

Support optional distribution parameters

docs/primitive-distributions.txt

@@ -29,7 +29,7 @@
   * ps: probabilities (can be unnormalized) *(vector or real array [0, Infinity))*
   * vs: support *(any array)*
 
-  Distribution over elements of ``vs`` with ``P(vs[i])`` proportional to ``ps[i]``
+  Distribution over elements of ``vs`` with ``P(vs[i])`` proportional to ``ps[i]``. ``ps`` may be omitted, in which case a uniform distribution over ``vs`` is returned.
 
   `Wikipedia entry <https://en.wikipedia.org/wiki/Categorical_distribution>`__
 

src/dists.ad.js

@@ -26,6 +26,19 @@
 //
 // - All distributions of a particular type should share the same set
 //   of parameters.
+//
+// Optional parameters:
+//
+// The default Dist constructor checks that all parameters described
+// in a distribution's definition are present. The `optional` flag can
+// be set on a parameter to skip this check. In such cases a custom
+// constructor must be defined to fill in the value of the parameter
+// when omitted. More specifically, the constructor should extend the
+// `params` object to include an appropriate default value. This
+// ensures that the condition described above (regarding all types
+// sharing the same set of parameters) is met. Care should also be
+// taken not to modify the `params` object originally passed by the
+// user.
 
 'use strict';
 
@@ -176,26 +189,25 @@ function makeDistributionType(options) {
     }
     return param.type;
   });
+  var parameterOptionalFlags = _.map(options.params, 'optional');
   var extraConstructorFn = options.constructor;
 
   // Note that Chrome uses the name of this local variable in the
   // output of `console.log` when it's called on a distribution that
   // uses the default constructor.
   var dist = function(params) {
-
-    if (params === undefined && parameterNames.length > 0) {
-      throw new Error('Parameters not supplied to ' + this.meta.name + ' distribution.');
-    }
+    params = params || {};
     parameterNames.forEach(function(p, i) {
-      if (!params.hasOwnProperty(p)) {
-        throw new Error('Parameter \"' + p + '\" missing from ' + this.meta.name + ' distribution.');
-      }
-
-      var type = parameterTypes[i];
-      if (type && !type.check(ad.valueRec(params[p]))) {
-        throw new Error('Parameter \"' + p + '\" should be of type "' + type.desc + '".');
+      if (params.hasOwnProperty(p)) {
+        var type = parameterTypes[i];
+        if (type && !type.check(ad.valueRec(params[p]))) {
+          throw new Error('Parameter \"' + p + '\" should be of type "' + type.desc + '".');
+        }
+      } else {
+        if (!parameterOptionalFlags[i]) {
+          throw new Error('Parameter \"' + p + '\" missing from ' + this.meta.name + ' distribution.');
+        }
       }
-
     }, this);
     this.params = params;
     if (extraConstructorFn !== undefined) {
@@ -1506,14 +1518,23 @@ function printMarginal(dist) {
 
 var Categorical = makeDistributionType({
   name: 'Categorical',
-  desc: 'Distribution over elements of ``vs`` with ``P(vs[i])`` proportional to ``ps[i]``',
+  desc: 'Distribution over elements of ``vs`` with ``P(vs[i])`` proportional to ``ps[i]``. ' +
+    '``ps`` may be omitted, in which case a uniform distribution over ``vs`` is returned.',
   params: [
-    {name: 'ps', desc: 'probabilities (can be unnormalized)', type: types.nonNegativeVectorOrRealArray},
+    {name: 'ps', desc: 'probabilities (can be unnormalized)',
+      type: types.nonNegativeVectorOrRealArray, optional: true},
     {name: 'vs', desc: 'support', type: types.array(types.any)}],
   wikipedia: true,
   mixins: [finiteSupport],
   constructor: function() {
     'use ad';
+    // Add default for ps when omitted.
+    if (this.params.ps === undefined) {
+      this.params = {
+        ps: _.fill(Array(this.params.vs.length), 1),
+        vs: this.params.vs
+      };
+    }
     var ps = this.params.ps;
     var vs = this.params.vs;
     if (vs.length !== ad.value(ps).length) {

tests/test-data/deterministic/expected/expectation.json

@@ -1,3 +1,3 @@
 {
-  "result": [0.9, 0.9, 0.9, 0.9]
+  "result": [0.9, 0.9, 0.9, 0.9, 0.5]
 }

tests/test-data/deterministic/models/expectation.wppl

@@ -3,5 +3,6 @@
   expectation(Discrete({ps: [3, 5, 2]}), idF),
   expectation(Discrete({ps: Vector([3, 5, 2])}), idF),
   expectation(Categorical({ps: [3, 5, 2], vs: [0, 1, 2]}), idF),
-  expectation(Categorical({ps: Vector([3, 5, 2]), vs: [0, 1, 2]}), idF)
+  expectation(Categorical({ps: Vector([3, 5, 2]), vs: [0, 1, 2]}), idF),
+  expectation(Categorical({vs: [0, 1]}), idF)
 ];