Re-export main types and traits from root module

src/bin/demo.rs

@@ -1,6 +1,6 @@
-use deriv::expression::Expression;
+use deriv::Expression;
-use deriv::eval::Evaluator;
+use deriv::Evaluator;
-use deriv::repl::Repl;
+use deriv::Repl;
 
 struct DummyEvaluator;
 

src/lib.rs

@@ -7,3 +7,7 @@ pub mod eval;
 pub mod print;
 
 pub mod repl;
+
+pub use expression::Expression;
+pub use eval::Evaluator;
+pub use repl::Repl;