Add support for dynamic partial names

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

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

Fixes #933

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';

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');
   },
 

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';

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]);
   }

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 */) {},
 

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;