Implementing union-find for types (WIP) (apache#38)

* First pass at union-find

* Misc. style

* Simplify union-find interface; will keep the matter of unifying TYPES rather than type vars in a separate visitor

* Whitespace

* Whitespace for lint

* Use pointer equality

* Move union-find to separate files

* Whitespace for linter

* It was actually a mistake to compare the references at all

relay/include/relay/unifier.h

@@ -0,0 +1,45 @@
+/*!
+ *  Copyright (c) 2018 by Contributors
+ * \file nnvm/relay/unifier.h
+ * \brief Type unification data structures
+ */
+#ifndef NNVM_RELAY_UNIFIER_H_
+#define NNVM_RELAY_UNIFIER_H_
+
+#include <nnvm/relay/node.h>
+
+namespace nnvm {
+namespace relay {
+
+/*! \brief a union-find data structure for the type-checker */
+class UnionFind;
+class UnionFindNode : public ExprNode {
+ public:
+  tvm::Map<TypeVar, TypeVar> uf_map;
+
+  UnionFindNode() {}
+
+  void VisitAttrs(tvm::AttrVisitor* v) final {
+    v->Visit("uf_map", &uf_map);
+  }
+
+  TVM_DLL static UnionFind make(tvm::Map<TypeVar, TypeVar> uf_map);
+
+  // insert v into UF
+  void insert(const TypeVar& v);
+
+  // infers that v1 and v2 must be of the smae type
+  void unify(const TypeVar& v1, const TypeVar& v2);
+
+  // returns representative of v's UF-group
+  TypeVar find(const TypeVar& v);
+
+  static constexpr const char* _type_key = "nnvm.UnionFind";
+  TVM_DECLARE_NODE_TYPE_INFO(UnionFindNode, ExprNode);
+};
+
+TVM_DEFINE_NODE_REF(UnionFind, UnionFindNode);
+
+}  // namespace relay
+}  // namespace nnvm
+#endif  // NNVM_RELAY_UNIFIER_H_