1 files changed, 28 insertions, 0 deletions
diff --git a/receme/src/coralgebra.rs b/receme/src/coralgebra.rs
new file mode 100644
index 0000000..a4a73d2
--- /dev/null
+++ b/receme/src/coralgebra.rs
@@ -0,0 +1,28 @@
+//! This file defines the behaviours of R-co-algebras.
+//!
+//! For a functor F, an R-co-algebra is a natural transformation from
+//! the identity functor to F1, where F1 is the functor that sends T
+//! to F(F(T) + T), where the + is the categorical co-product,
+//! represented in Rust as `Result`.  And the F(T) variant means to
+//! short-circuit the expansion.
+
+use crate::functor::Functor;
+
+/// An R-co-algebra is a function from T to F(Result<F(T), T>).
+///
+/// This is a "trait alias".  Since Rust does not support trait alias
+/// yet, we define an empty trait with an automatic implementation.
+pub trait Coralgebra<T, F, G>: FnMut(T) -> G
+where
+    F: Functor<T>,
+    G: Functor<Result<T, F>>,
+{
+}
+
+impl<T, F, G, R> Coralgebra<T, F, G> for R
+where
+    F: Functor<T>,
+    G: Functor<Result<T, F>>,
+    R: FnMut(T) -> G,
+{
+}