Add the ability to dynamically look up variables

mustache.go

@@ -499,6 +499,26 @@ Outer:
 				return v, nil
 			}
 			switch av := v; av.Kind() {
+			case reflect.Func:
+				typ := av.Type()
+				// Ensure the function takes one argument and it's a string.
+				if typ.NumIn() != 1 || typ.In(0).Kind() != reflect.String {
+					continue Outer
+				}
+				out := av.Call([]reflect.Value{reflect.ValueOf(name)})
+
+				// We can have either 1 or 2 return values.
+				if len(out) < 1 || len(out) > 2 {
+					continue Outer
+				}
+
+				// If we have 2 return values, the second must be a bool and
+				// represents if the variable exists or not.
+				if len(out) == 2 && (out[1].Kind() != reflect.Bool || !out[1].Bool()) {
+					continue Outer
+				}
+
+				return out[0], nil
 			case reflect.Ptr:
 				v = av.Elem()
 			case reflect.Interface:

mustache_test.go

@@ -350,6 +350,68 @@ func TestMultiContext(t *testing.T) {
 	}
 }
 
+func TestDynamic(t *testing.T) {
+	tmpl := `{{hello}}{{something}}`
+	lookupFunc := func(name string) (interface{}, bool) {
+		if name == "hello" {
+			return "world", true
+		}
+
+		return nil, false
+	}
+
+	output, err := Render(tmpl, lookupFunc)
+	if err != nil {
+		t.Fatal(err)
+	}
+	expect := "world"
+	if output != expect {
+		t.Fatalf("TestLambda expected %q got %q", expect, output)
+	}
+}
+
+func TestSimpleDynamic(t *testing.T) {
+	tmpl := `{{hello}}{{something}}`
+	lookupFunc := func(name string) string {
+		if name == "hello" {
+			return "world"
+		}
+
+		return ""
+	}
+
+	output, err := Render(tmpl, lookupFunc)
+	if err != nil {
+		t.Fatal(err)
+	}
+	expect := "world"
+	if output != expect {
+		t.Fatalf("TestLambda expected %q got %q", expect, output)
+	}
+}
+
+func TestComplexDynamic(t *testing.T) {
+	tmpl := `{{hello.world}}{{something}}`
+	lookupFunc := func(name string) interface{} {
+		if name == "hello" {
+			return map[string]string{
+				"world": "some value",
+			}
+		}
+
+		return ""
+	}
+
+	output, err := Render(tmpl, lookupFunc)
+	if err != nil {
+		t.Fatal(err)
+	}
+	expect := "some value"
+	if output != expect {
+		t.Fatalf("TestLambda expected %q got %q", expect, output)
+	}
+}
+
 func TestLambda(t *testing.T) {
 	tmpl := `{{#lambda}}Hello {{name}}. {{#sub}}{{.}} {{/sub}}{{^negsub}}nothing{{/negsub}}{{/lambda}}`
 	data := map[string]interface{}{