Write about reader implementation in development log

LOG.md

@@ -73,3 +73,32 @@ provided in the base language, which is convenient for doing TDD.
 ### LLM / AI Usage
 
 I will not be using any LLM coding assistants for this project.
+
+## Reader
+
+### Lexer
+
+I first implemented the lexer with a very simple finite state machine
+consisting of two states, `Init` and `Integer`.  This was
+straightforward as the input language is very simple.
+
+### Parser
+
+Since the input grammar consists of terms, parentheses and infix
+operators with varying precedence, Pratt parsing is the logical choice
+for the implementation of a parser.
+
+I find the usual way of handling associativity in precedence climbing
+parsers (wherein a small amount is added to the binding power on one
+side) somewhat inelegant, so I decided to implement a simple algebra
+for binding powers that handles associativity using custom comparison
+semantics.
+
+Implementing the parser itself was straightforward as I have
+implemented Pratt parsing many times before (though not in Rust
+specifically).
+
+To tie together the lexer and parser, and to provide a convenient
+means of decoupling other components from them for testing purposes, I
+then implemented the Rust standard library's `FromStr` trait, which is
+for types that can be parsed from a string.