Quoted Expressions

In this chapter we’ll implement a new type of Lisp Value called a Q-Expression.

This stands for quoted expression, and is a type of Lisp Expression that is not evaluated by the standard Lisp mechanics. When encountered by the evaluation function Q-expressions are left exactly as they are. This makes them ideal for a number of purposes. We can use them to store and manipulate other Lisp values such as numbers, symbols, or other S-Expressions themselves.

After we’ve added Q-Expressions we are going to implement a concise set of operators to manipulate them. Like the arithmetic operators these will prove fundamental in how we think about and play with expressions.

The syntax for Q-Expressions is very similar to that of S-Expressions. The only difference is that instead of parenthesis () Q-Expressions are surrounded by curly brackets {}. We can add this to our grammar as follows.

I’ve never heard of Q-Expressions.

Q-Expressions don’t exist in other Lisps. Other Lisps use Macros to stop evaluation. These look like normal functions, but they do not evaluate their arguments. A special Macro called quote ' exists, which can be used to stop the evaluation of almost anything. This is the inspiration for Q-Expressions, which are unique to our Lisp, and will be used instead of Macros for doing all the same tasks and more.

The way I’ve used S-Expression and Q-Expression in this book is a slight abuse of terminology, but I hope this misdemeanor makes the behaviour of our Lisp clearer.

  1. mpc_parser_t* Number = mpc_new("number");
  2. mpc_parser_t* Symbol = mpc_new("symbol");
  3. mpc_parser_t* Sexpr  = mpc_new("sexpr");
  4. mpc_parser_t* Qexpr  = mpc_new("qexpr");
  5. mpc_parser_t* Expr   = mpc_new("expr");
  6. mpc_parser_t* Lispy  = mpc_new("lispy");
  8. mpca_lang(MPCA_LANG_DEFAULT,
  9.   "                                                    \
  10.     number : /-?[0-9]+/ ;                              \
  11.     symbol : '+' | '-' | '*' | '/' ;                   \
  12.     sexpr  : '(' <expr>* ')' ;                         \
  13.     qexpr  : '{' <expr>* '}' ;                         \
  14.     expr   : <number> | <symbol> | <sexpr> | <qexpr> ; \
  15.     lispy  : /^/ <expr>* /$/ ;                         \
  16.   ",
  17.   Number, Symbol, Sexpr, Qexpr, Expr, Lispy);

We also must remember to update our cleanup function to deal with the new rule we’ve added.

  1. mpc_cleanup(6, Number, Symbol, Sexpr, Qexpr, Expr, Lispy);