Add support for dynamic partial names

Uses the subexpression syntax to allow for dynamic partial lookups. Ex:

```
{{> (helper) }}
```

Fixes #933

9 files changed, 92 insertions, 9 deletions

diff --git a/docs/compiler-api.md b/docs/compiler-api.md
index 74af672..4a0ec01 100644
--- a/docs/compiler-api.md
+++ b/docs/compiler-api.md
@@ -115,6 +115,17 @@ interface SubExpression <: Expression {
 
 `isHelper` is not required and is used to disambiguate between cases such as `{{foo}}` and `(foo)`, which have slightly different call behaviors.
 
+```java
+interface PartialExpression <: Expression {
+    type: "PartialExpression";
+    name: PathExpression | SubExpression;
+    params: [ Expression ];
+    hash: Hash;
+}
+```
+
+`path` may be a `SubExpression` when tied to a dynamic partial, i.e. `{{> (foo) }}`
+
 ##### Paths
 
 ```java
@@ -221,6 +232,8 @@ The `Handlebars.JavaScriptCompiler` object has a number of methods that may be c
   - `name` is the current path component
   - `type` is the type of name being evaluated. May be one of `context`, `data`, `helper`, or `partial`.
 
+  Note that this does not impact dynamic partials, which implementors need to be aware of. Overriding `VM.resolvePartial` may be required to support dynamic cases.
+
 - `depthedLookup(name)`
   Used to generate code that resolves parameters within any context in the stack. Is only used in `compat` mode. 
 
diff --git a/lib/handlebars/compiler/ast.js b/lib/handlebars/compiler/ast.js
index 113c03e..c193460 100644
--- a/lib/handlebars/compiler/ast.js
+++ b/lib/handlebars/compiler/ast.js
@@ -63,6 +63,15 @@ var AST = {
     this.hash = hash;
   },
 
+  PartialExpression: function(name, params, hash, locInfo) {
+    this.loc = locInfo;
+
+    this.type = 'PartialExpression';
+    this.name = name;
+    this.params = params || [];
+    this.hash = hash;
+  },
+
   PathExpression: function(data, depth, parts, original, locInfo) {
     this.loc = locInfo;
     this.type = 'PathExpression';
diff --git a/lib/handlebars/compiler/compiler.js b/lib/handlebars/compiler/compiler.js
index 95fe101..524f9aa 100644
--- a/lib/handlebars/compiler/compiler.js
+++ b/lib/handlebars/compiler/compiler.js
@@ -145,7 +145,6 @@ Compiler.prototype = {
   },
 
   PartialStatement: function(partial) {
-    var partialName = partial.sexpr.path.original;
     this.usePartial = true;
 
     var params = partial.sexpr.params;
@@ -155,6 +154,12 @@ Compiler.prototype = {
       params.push({type: 'PathExpression', parts: [], depth: 0});
     }
 
+    var partialName = partial.sexpr.name.original,
+        isDynamic = partial.sexpr.name.type === 'SubExpression';
+    if (isDynamic) {
+      this.accept(partial.sexpr.name);
+    }
+
     this.setupFullMustacheParams(partial.sexpr, undefined, undefined, true);
 
     var indent = partial.indent || '';
@@ -162,7 +167,8 @@ Compiler.prototype = {
       this.opcode('appendContent', indent);
       indent = '';
     }
-    this.opcode('invokePartial', partialName, indent);
+
+    this.opcode('invokePartial', isDynamic, partialName, indent);
     this.opcode('append');
   },
 
diff --git a/lib/handlebars/compiler/javascript-compiler.js b/lib/handlebars/compiler/javascript-compiler.js
index 75f9960..a027edb 100644
--- a/lib/handlebars/compiler/javascript-compiler.js
+++ b/lib/handlebars/compiler/javascript-compiler.js
@@ -644,17 +644,26 @@ JavaScriptCompiler.prototype = {
   //
   // This operation pops off a context, invokes a partial with that context,
   // and pushes the result of the invocation back.
-  invokePartial: function(name, indent) {
+  invokePartial: function(isDynamic, name, indent) {
     var params = [],
         options = this.setupParams(name, 1, params, false);
 
+    if (isDynamic) {
+      name = this.popStack();
+      delete options.name;
+    }
+
     if (indent) {
       options.indent = JSON.stringify(indent);
     }
     options.helpers = 'helpers';
     options.partials = 'partials';
 
-    params.unshift(this.nameLookup('partials', name, 'partial'));
+    if (!isDynamic) {
+      params.unshift(this.nameLookup('partials', name, 'partial'));
+    } else {
+      params.unshift(name);
+    }
 
     if (this.options.compat) {
       options.depths = 'depths';
diff --git a/lib/handlebars/compiler/printer.js b/lib/handlebars/compiler/printer.js
index ad276f1..448331c 100644
--- a/lib/handlebars/compiler/printer.js
+++ b/lib/handlebars/compiler/printer.js
@@ -75,7 +75,7 @@ PrintVisitor.prototype.BlockStatement = function(block) {
 
 PrintVisitor.prototype.PartialStatement = function(partial) {
   var sexpr = partial.sexpr,
-      content = 'PARTIAL:' + sexpr.path.original;
+      content = 'PARTIAL:' + sexpr.name.original;
   if(sexpr.params[0]) {
     content += ' ' + this.accept(sexpr.params[0]);
   }
diff --git a/lib/handlebars/compiler/visitor.js b/lib/handlebars/compiler/visitor.js
index 3fb37fb..03af915 100644
--- a/lib/handlebars/compiler/visitor.js
+++ b/lib/handlebars/compiler/visitor.js
@@ -92,6 +92,11 @@ Visitor.prototype = {
     this.acceptArray(sexpr.params);
     this.acceptKey(sexpr, 'hash');
   },
+  PartialExpression: function(partial) {
+    this.acceptRequired(partial, 'name');
+    this.acceptArray(partial.params);
+    this.acceptKey(partial, 'hash');
+  },
 
   PathExpression: function(/* path */) {},
 
diff --git a/lib/handlebars/runtime.js b/lib/handlebars/runtime.js
index 2b9f474..4e8c33a 100644
--- a/lib/handlebars/runtime.js
+++ b/lib/handlebars/runtime.js
@@ -39,10 +39,8 @@ export function template(templateSpec, env) {
     if (options.hash) {
       context = Utils.extend({}, context, options.hash);
     }
-    if (!partial) {
-      partial = options.partials[options.name];
-    }
 
+    partial = env.VM.resolvePartial.call(this, partial, context, options);
     var result = env.VM.invokePartial.call(this, partial, context, options);
 
     if (result == null && env.compile) {
@@ -187,6 +185,17 @@ export function program(container, i, fn, data, declaredBlockParams, blockParams
   return prog;
 }
 
+export function resolvePartial(partial, context, options) {
+  if (!partial) {
+    partial = options.partials[options.name];
+  } else if (!partial.call && !options.name) {
+    // This is a dynamic partial that returned a string
+    options.name = partial;
+    partial = options.partials[partial];
+  }
+  return partial;
+}
+
 export function invokePartial(partial, context, options) {
   options.partial = true;
 
diff --git a/spec/partials.js b/spec/partials.js
index b150942..0c9e0f6 100644
--- a/spec/partials.js
+++ b/spec/partials.js
@@ -8,6 +8,32 @@ describe('partials', function() {
     shouldCompileToWithPartials(string, [hash, {}, {dude: partial},,false], true, 'Dudes: Yehuda (http://yehuda) Alan (http://alan) ');
   });
 
+  it('dynamic partials', function() {
+    var string = 'Dudes: {{#dudes}}{{> (partial)}}{{/dudes}}';
+    var partial = '{{name}} ({{url}}) ';
+    var hash = {dudes: [{name: 'Yehuda', url: 'http://yehuda'}, {name: 'Alan', url: 'http://alan'}]};
+    var helpers = {
+      partial: function() {
+        return 'dude';
+      }
+    };
+    shouldCompileToWithPartials(string, [hash, helpers, {dude: partial}], true, 'Dudes: Yehuda (http://yehuda) Alan (http://alan) ');
+    shouldCompileToWithPartials(string, [hash, helpers, {dude: partial},,false], true, 'Dudes: Yehuda (http://yehuda) Alan (http://alan) ');
+  });
+  it('failing dynamic partials', function() {
+    var string = 'Dudes: {{#dudes}}{{> (partial)}}{{/dudes}}';
+    var partial = '{{name}} ({{url}}) ';
+    var hash = {dudes: [{name: 'Yehuda', url: 'http://yehuda'}, {name: 'Alan', url: 'http://alan'}]};
+    var helpers = {
+      partial: function() {
+        return 'missing';
+      }
+    };
+    shouldThrow(function() {
+      shouldCompileToWithPartials(string, [hash, helpers, {dude: partial}], true, 'Dudes: Yehuda (http://yehuda) Alan (http://alan) ');
+    }, Handlebars.Exception, 'The partial missing could not be found');
+  });
+
   it("partials with context", function() {
     var string = "Dudes: {{>dude dudes}}";
     var partial = "{{#this}}{{name}} ({{url}}) {{/this}}";
diff --git a/src/handlebars.yy b/src/handlebars.yy
index 27985b8..6ca32d9 100644
--- a/src/handlebars.yy
+++ b/src/handlebars.yy
@@ -77,7 +77,13 @@ mustache
   ;
 
 partial
-  : OPEN_PARTIAL sexpr CLOSE -> new yy.PartialStatement($2, yy.stripFlags($1, $3), yy.locInfo(@$))
+  : OPEN_PARTIAL partial_expr CLOSE -> new yy.PartialStatement($2, yy.stripFlags($1, $3), yy.locInfo(@$))
+  ;
+
+partial_expr
+  : helperName param* hash? -> new yy.PartialExpression($1, $2, $3, yy.locInfo(@$))
+  | dataName -> new yy.PartialExpression($1, null, null, yy.locInfo(@$))
+  | OPEN_SEXPR sexpr CLOSE_SEXPR param* hash? -> new yy.PartialExpression($2, $4, $5, yy.locInfo(@$))
   ;
 
 sexpr