Write about REPL module and demo program in development log

LOG.md

@@ -129,3 +129,17 @@ serialisations.
 The printing logic could then be packaged up in an implementation of
 the Rust standard library's `Display` trait, which is for types that
 have a textual representation.
+
+## REPL
+
+At this point, with the reading and printing implemented, it made
+sense to create the REPL logic and demo binary so that a full flow
+through the program could be demonstrated.  I first created an
+`Evaluator` trait to capture the future behaviour of the evaluator,
+and then wrote a generic REPL module that's able to work with any
+expression type implementing the `FromStr` and `Display` traits.
+
+I was then able to create a demo program by defining a dummy evaluator
+that simply returns the input expression unmodified, and running a
+REPL using that, connected to stdin and stdout.  This demonstrated a
+full round trip through the parsing and printing procedures.