Synopsis A lisp interpreter in Rascal.
Description Writing a Lisp interpreter is a classical challenge. Popular word has that all large applications evolve until they include a Lisp interpreter. (A variant says the same about including an email client in every large application).

We will closely follow and reuse parts of Peter Norvig's excellent page on Lispy, a Lisp interpreter written in Python. The Lisp variant to be implemented is the following subset of the Scheme language:

FormSyntaxSemantics and Example
variable reference varA symbol is interpreted as a variable name; its value is the variable's value.
Example: x
constant literal numberA number evaluates to itself.
Example: 12
quotation (quote exp) Return the exp literally; do not evaluate it. Example: (quote (a b c)) ⇒ (a b c)
conditional (if test conseq alt) Evaluate test; if true, evaluate and return conseq; otherwise evaluate and return alt.
Example: (if (< 10 20) (+ 1 1) (+ 3 3)) ⇒ 2
assignment (set! var exp) Evaluate exp and assign that value to var, which must have been previously defined (with a define or as a parameter to an enclosing procedure).
Example: (set! x2 (* x x))
definition (define var exp) Define a new variable in the innermost environment and give it the value of evaluating the expression exp.
Examples: (define r 3) or (define square (lambda (x) (* x x))).
procedure (lambda (var...) exp)Create a procedure with parameter(s) named var... and the expression as the body.
Example: (lambda (r) (* r r))
sequencing (begin exp...) Evaluate each of the expressions in left-to-right order, and return the final value.
Example: (begin (set! x 1) (set! x (+ x 1)) (* x 2)) ⇒ 4
procedure call (proc exp...) If proc is anything other than one of the symbols if, set!, define, lambda, begin, or quote then it is treated as a procedure. It is evaluated using the same rules defined here. All the expressions are evaluated as well, and then the procedure is called with the list of expressions as arguments. Example: (square 12) ⇒ 144

In this table, var must be a symbol--an identifier such as x or square--and number must be an integer number, while the other italicized words can be any expression. The notation exp... means zero or more repetitions of exp.

A Lisp interpreter consists of the following parts:
  • A parser that reads a Lisp program in text form and converts it to a runtime representation that is suitable for the interpreter.
  • The interpreter itself that executes the program in runtime representation and computes its outcome.
  • A pretty printer that converts the outcome in internal representation back to text.
  • Finally, an interactive console is needed that interact with the user.
We discuss all these aspects:

  • Runtime: The runtime representation of Lisp programs and data.
  • Eval: A Lisp interpreter.
  • Syntax: The textual syntax of Lisp.
  • Parse: Parsing a Lisp expression.
  • Pretty: A Lisp pretty printer.
  • Test: Tests for the Lisp interpreter.
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