YARLISP

What is it and a brief history.

Yet Another Ruby Lisp.

(I assume that other people have implemented Lisp in ruby so...)

After reading McCarthy's Paper for one of the Boston Software Crafstmanship Meetups I decided to try to expand my limited knowledge of Ruby by implmenting this Lisp.

That attempt failed - I even deleted the GitHub repository for it.

I hope that this attempt works better.

The relevant parts of the paper:

Snippets from McCarthy's paper:

The Elementary S-functions and Predicates. We introduce the following functions and predicates:

atom. atom[x] has the value of T or F according to whether x is an atomic symbol. Thus
```
atom [X] = T
atom [(X . A)] = F
```
eq. eq [x;y] is defined if and only if both x and y are atomic. eq [x; y] = T if x and y are the same symbol, and eq [x; y] = F otherwise. Thus
```
eq [X; X] = T
eq [X; A] = F 
eq [X; (X . A)] is undefined
```
car. car[x] is defined if and only if x is not atomic.
```
car [(e1 . e2)] = e1 
```

Thus car [X] is undefined.

    car [(X . A)] = X
    car [((X . A) . Y )] = (X . A)

cdr. cdr [x] is also defined when x is not atomic. We have cdr [(e1 . e2)]= e2. Thus cdr [X] is undefined.
```
cdr [(X . A)] = A
cdr [((X . A) . Y )] = Y
```
cons. cons [x; y] is defined for any x and y. We have cons [e1; e2] = (e1 . e2). Thus
```
cons [X; A] = (X A)
cons [(X . A); Y ] = ((X . A) Y )
```

car, cdr, and cons are easily seen to satisfy the relations

    car [cons [x; y]] = x
    cdr [cons [x; y]] = y
    cons [car [x]; cdr [x]] = x, provided that x is not atomic.

The S-function apply is defined by

apply[f; args] = eval[cons[f; appq[args]];NIL],

where

appq[m] = [null[m] ! NIL; T ! cons[list[QUOTE; car[m]]; appq[cdr[m]]]]

and

eval[e; a] = [
    atom [e] ! assoc [e; a];
    atom [car [e]] ! [
        eq [car [e]; QUOTE] ! cadr [e];
        eq [car [e]; ATOM] ! atom [eval [cadr [e]; a]];
        eq [car [e]; EQ] ! [eval [cadr [e]; a] = eval [caddr [e]; a]];
        eq [car [e]; COND] ! evcon [cdr [e]; a];
        eq [car [e]; CAR] ! car [eval [cadr [e]; a]];
        eq [car [e]; CDR] ! cdr [eval [cadr [e]; a]];
        eq [car [e]; CONS] ! cons [eval [cadr [e]; a]; eval [caddr [e]; a]]; 
        T ! eval [cons [assoc [car [e]; a]; evlis [cdr [e]; a]]; a]];
    eq [caar [e]; LABEL] ! eval [cons [caddar [e]; cdr [e]]; cons [list [cadar [e]; car [e]; a]];
    eq [caar [e]; LAMBDA] ! eval [caddar [e]; append [pair [cadar [e]; evlis [cdr [e]; a]; a]]]

and

evcon[c; a] = [eval[caar[c]; a] ! eval[cadar[c]; a]; T ! evcon[cdr[c]; a]]

and

evlis[m; a] = [null[m] ! NIL; T ! cons[eval[car[m]; a]; evlis[cdr[m]; a]]]