Get expr.zip from the book homepage and unpack it. Using a command prompt, generate (1) the lexer and (2) the parser for expressions by running fslex and fsyacc; then (3) load the expression abstract syntax, the lexer and parser modules, and the expression interpreter and compilers, into an interactive F# session (fsi):

Now try the parser on several example expressions, both well-formed and ill-formed ones, such as these, and some of your own invention: