Fork off version 13 of the compiler.

code/compiler/13/error.hpp

@@ -0,0 +1,21 @@
+#pragma once
+#include <exception>
+#include "type.hpp"
+
+struct type_error : std::exception {
+    std::string description;
+
+    type_error(std::string d)
+        : description(std::move(d)) {}
+
+    const char* what() const noexcept override;
+};
+
+struct unification_error : public type_error {
+    type_ptr left;
+    type_ptr right;
+
+    unification_error(type_ptr l, type_ptr r)
+        : left(std::move(l)), right(std::move(r)), 
+            type_error("failed to unify types") {}
+};