README

This is the repo for National Lisp Interpreter Month: a project to write a Lisp
interpreter every day in the month of November, in a different language each
day.

As a result, it is full of code in languages I don't know, written in an
extreme rush and with sloppy style. If you judge it, please do so in context.
;-)

You can find my blog posts about the project at

http://nalintmo.livejournal.com/ 

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Datatypes:

- Number (at least integer)
- Symbol
- List (dotted lists are not necessarily supported)
- Closure (primitive or user-defined function)

Supported special forms: (When we say "foo is a bar" here, we mean syntactically.)

- (progn e_1 ... e_n)
  - e_k are expressions, n >= 0
  - Evaluates the e_k in order, returning the last.
  - (progn) => nil.
- (lambda (a_1 ... a_m) e_1 ... e_n)
  - a_j are symbols, e_k are expressions, m >= 0, n >= 0
  - Creates a closure taking m arguments
  - When applied, binds the a_j to the supplied arguments in the local
    environment, and evaluates
    (progn e_1 ... e_n).
- (define k e)
  - k is a symbol, e is an expression
  - Evaluates e, and sets k to the result in the global environment. (This
    creates a binding if needed, or mutates an existing binding if present.)
- (define (f a_1 ... a_n) e_1 ... e_n)
  - Evalutes (define f (lambda (a_1 ... a_n) e_1 ... e_n)).
- (quote e)
  - e is any expression
  - Evalutes to e (literally, without evaluating e.)
- (cond (c_1 ... c_n))
  - c_k are clauses: lists of length >= 1.
  - Evaluates the first element of each c_k in turn. When it finds one that is
    non-nil, evaluates the remaining elements of c_k as though wrapped in
    progn.
    - If c_k is of length 1, returns the result of evaluating the first element
      of c_k instead.
  - If all of the c_k evaluate to nil, or there are no c_k, returns nil.
- (setq k e)
  - k is a symbol, e is an expression
  - Searches first the local, then the global environment for a binding of k,
    and mutates that binding to have the value resulting from evaluating e.
  - If no binding is found, signals an error.


Supported primitive functions: (When we say "foo is a bar" here, we mean after
evaulation.)

- (car c)
  - c is a list
  - Returns the first element of c.
- (cdr c)
  - c is a list
  - Returns the list of all but the first element of c.
- (eq a b)
  - a and b are any values
  - Returns nil if a != b, and any other value otherwise.
  - Not guaranteed to consider objects the same unless they are identical; i.e.
    not required to perform a deep comparison (but free to do so).
  - Not guaranteed to consider any two functions the same, ever, even if they
    are identical.
- (cons a b)
  - a is any value; b is a list
  - Returns a list whose first element is a, and remaining elements are the
    elements of b.
- (set k v)
  - Description exactly the same as the setq special form; but note that since
    it is a function, the first argument is evaluated to a symbol rather than
    being a literal symbol.
- (+ e_1 ... e_n)
  - e_k are numbers
  - Returns the sum of the e_k, or zero if n = 0.
- (- e_0 ... e_n)
  - e_k are numbers; note that e_0 is required.
  - Returns e_0 minus the sum of e_k for k >= 1, or -e_0 if n = 0.
- (* e_1 ... e_n)
  - e_k are numbers
  - Returns the product of the e_k, or one if n = 0.