trait definition and struct-tuple example

src/instances.rs

@@ -0,0 +1,21 @@
+use crate::generic::{Field, Generic, Sum};
+
+impl<A, B> Generic for (A, B) {
+    type Representation = Sum<Field<A>, Field<B>>;
+
+    fn generalize(self) -> Self::Representation {
+        let (l, r) = self;
+        Sum {
+            left: Field { value: l },
+            right: Field { value: r }
+        }
+    }
+
+    fn specialize(rep: Self::Representation) -> Self {
+        let Sum {
+            left: Field { value: a },
+            right: Field { value: b },
+        } = rep;
+        (a, b)
+    }
+}