1 files changed, 40 insertions, 11 deletions
diff --git a/semiring/src/lib.rs b/semiring/src/lib.rs
index 7d12d9a..9d096db 100644
--- a/semiring/src/lib.rs
+++ b/semiring/src/lib.rs
@@ -1,14 +1,43 @@
-pub fn add(left: usize, right: usize) -> usize {
-    left + right
+#![warn(missing_docs)]
+//! This crate defines the expected behaviours of semirings and
+//! semiring parsers.  Some standard semirings are also implemented,
+//! such as the boolean, the counting, and the derivation forest
+//! semirings.  Moreover, this crate also defines the behaviours of a
+//! parser, with the formalism of an "item-based description".  See
+//! the paper [Parsing
+//! inside-out](https://arxiv.org/abs/cmp-lg/9805007) by Joshua
+//! Goodman.  To be more precise, it defines the behaviours of an
+//! "item interpreter" that executes "items" in a chosen semiring.
+//! Finally, it provides a default implementation of an item
+//! interpreter.
+//!
+//! The crate *chain* will implement a parser using the item-based
+//! description.  Specifically, it will produce items as it parses the
+//! input string.  Then we can execute these items by any interpreter.
+
+/// A semiring is equipped with two operations: the addition and the
+/// multiplication.  It also has additive and multiplicative
+/// identities.  Also the two operations are supposed to be
+/// associative, and the multiplication is supposed to be distributive
+/// over the addition.  But the addition is not required to be
+/// commutative, and is usually not indeed commutative.
+pub trait Semiring {
+    /// The additive identity.
+    fn zero() -> Self;
+
+    /// The multiplicative identity.
+    fn one() -> Self;
+
+    /// The addition.
+    fn add(&mut self, other: impl AsRef<Self>);
+
+    /// The multiplication.
+    fn mul(&mut self, other: impl AsRef<Self>);
 }
 
+// TODO: Implement boolean semiring
+// TODO: Implement counting semiring
+// TODO: Implement derivation forest semiring
+
 #[cfg(test)]
-mod tests {
-    use super::*;
-
-    #[test]
-    fn it_works() {
-        let result = add(2, 2);
-        assert_eq!(result, 4);
-    }
-}
+mod tests {}