refactor kind logic, add enum, tuple, dict, union, intersection

src/error.js

@@ -8,14 +8,15 @@
 class StructError extends TypeError {
 
   constructor(attrs) {
-    const { data, value, type, path = [] } = attrs
-    const message = `Expected a value of type "${type}" ${path.length ? `for \`${path.join('.')}\`` : ''} but received \`${value}\`.`
+    const { data, value, type, path, errors = [] } = attrs
+    const message = `Expected a value of type \`${type}\`${path.length ? ` for \`${path.join('.')}\`` : ''} but received \`${value}\`.`
     super(message)
     this.data = data
     this.path = path
     this.value = value
     this.type = type
-    this.errors = [this]
+    this.errors = errors
+    if (!errors.length) errors.push(this)
     Error.captureStackTrace(this, this.constructor)
   }
 

src/is-struct.js

@@ -0,0 +1,28 @@
+
+/**
+ * A private string to identify structs by.
+ *
+ * @type {String}
+ */
+
+const IS_STRUCT = '@@__STRUCT__@@'
+
+/**
+ * Check if a `value` is a struct.
+ *
+ * @param {Any} value
+ * @return {Boolean}
+ */
+
+function isStruct(value) {
+  return !!(value && value[IS_STRUCT])
+}
+
+/**
+ * Export.
+ *
+ * @type {Function}
+ */
+
+export default isStruct
+export { IS_STRUCT }