diff --git a/src/parser/mod.rs b/src/parser/mod.rs
index 00eb190..e0c8eb3 100644
--- a/src/parser/mod.rs
+++ b/src/parser/mod.rs
@@ -1,18 +1,15 @@
 mod boxed_parser;
 mod named_parser;
+mod parser_input;
 
 use std::rc::Rc;
 
 pub use boxed_parser::BoxedParser;
 pub use named_parser::NamedParser;
+pub use parser_input::ParserInput;
 
 pub type ParseResult<I, O, E> = Result<(O, I), E>;
 
-pub trait ParserInput: std::fmt::Debug {}
-
-impl ParserInput for &str {}
-impl ParserInput for String {}
-
 pub trait Parser<I, O, E>
 where
     I: ParserInput,
diff --git a/src/parser/parser_input.rs b/src/parser/parser_input.rs
new file mode 100644
index 0000000..0784227
--- /dev/null
+++ b/src/parser/parser_input.rs
@@ -0,0 +1,11 @@
+pub trait ParserInput: std::fmt::Debug {
+    type Output;
+    fn next_token() -> Self::Output;
+}
+
+impl ParserInput for &str {
+    type Output = ();
+    fn next_token() -> Self::Output {
+        ()
+    }
+}