Add model plugin (#163)

* Add 'model' plugin.

* Fix misc issues

model/Makefile

@@ -6,4 +6,4 @@
 GOPATH=$(shell go env GOPATH)
 include ../plugins.mk
 
-gen:
+gen:

model/expr/root.go

@@ -38,10 +38,10 @@ type (
 
 // Register design root with eval engine.
 func init() {
+	InitRoot()
 	if err := eval.Register(Root); err != nil {
 		panic(err)
 	}
-	InitRoot()
 }
 
 // InitRoot initializes the root expression.

model/validate.go

@@ -26,13 +26,12 @@ func Validate(_ string, roots []eval.Root, files []*codegen.File) ([]*codegen.Fi
 	var r *modelexpr.RootExpr
 	var ok bool
 	for _, root := range roots {
-		if r, ok := root.(*expr.RootExpr); ok {
+		if re, ok := root.(*expr.RootExpr); ok {
 			// Note: this root is guaranteed to be appear before the model root
-			services = r.Services
+			services = re.Services
 			continue
 		}
-		r, ok = root.(*modelexpr.RootExpr)
-		if ok {
+		if r, ok = root.(*modelexpr.RootExpr); ok {
 			break
 		}
 	}
@@ -152,7 +151,7 @@ func Validate(_ string, roots []eval.Root, files []*codegen.File) ([]*codegen.Fi
 			pluralize(len(noService)), strings.Join(noService, ", "), pluralVerb(len(noService)))
 	}
 
-	return nil, nil
+	return files, nil
 }
 
 // Helper functions to handle singular/plural