Write about printing implementation in development log

LOG.md

@@ -102,3 +102,30 @@ 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.
+
+## Printer
+
+For expression printing, a simple recursive tree-walking algorithm,
+dispatching on the various kinds of expression was sufficient.
+Satisfyingly, I found I was able to use the same binding power algebra
+I defined for the parser in deciding when to use parentheses, and thus
+got correct associativity handling effectively for free, while also
+keeping all associativity-handling logic well-factored in one place.
+
+The only complication was in the handling of products, as here there
+was some ambiguity as to when to use explicit or implicit
+multiplication (that is `x*y` or `xy`).  Products of symbols with
+other symbols and integers with symbols are more naturally expressed
+with implicit multiplication, but of course this couldn't be used for
+products of integers with other integers as this would result in
+incorrect serialisations (such as "12" for the product of 1 and 2).  I
+ultimately settled on an approach where the right-hand operand is
+first printed into a string, and an explicit multiplication sign is
+used whenever that serialisation begins with a numerical character
+(either a digit or a minus sign).  This nicely handled all of the
+cases I could think of and systematically avoids any erroneous
+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.