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Prolog implementation of McCarthy's metacircular lisp interpreter, following pg's explication
:- use_module(library(reif)).
axiom([quote, X], X).
axiom([atom, X], R) :-
axiom(X, XR),
((atomic(XR), R = t, !) ;
(compound(XR), R = [])).
axiom([eq, X, Y], R) :-
axiom(X, XR),
axiom(Y, YR),
if_((XR = YR), R = t, R = []).
axiom([car, X], R) :-
axiom(X, [R|_]).
axiom([cdr, X], R) :-
axiom(X, [_|R]).
axiom([cons, X, More], R) :-
axiom(X, XR),
axiom(More, MoreR),
R = [XR|MoreR].
axiom([cond, [Test, Expr] | More], R) :-
axiom(Test, TestR),
if_((TestR = t),
axiom(Expr, R),
axiom([cond | More], R)).
/* These are the base forms! Now we move on to derived ones. */
/* Null testing */
axiom([null, X], R) :-
axiom([eq, X, [quote, []]], R).
/* Boolean logic */
axiom([and, X, Y], R) :-
axiom([cond, [X, [cond, [Y, [quote, t]],
[[quote, t], [quote, []]]]],
[[quote, t], [quote, []]]],
axiom([not, X], R) :-
axiom([cond, [X, [quote, []]],
[[quote, t], [quote, t]]],
/* List construction */
axiom([list, X, Y], R) :-
axiom([cons, X, [cons, Y, [quote, []]]], R).
/* List concat */
axiom([append, X, Y], R) :-
axiom([cond, [[null, X], Y],
[[quote, t], [cons, [car, X], [append, [cdr, X], Y]]]],
/* Zip together lists of as and lists of bs into [[a1 b1] [a2 b2] ...] */
axiom([pair, Xs, Ys], R) :-
axiom([cond, [[and, [null, Xs], [null, Ys]], [quote, []]],
[[and, [not, [atom, Xs]], [not, [atom, Ys]]],
[cons, [list, [car, Xs], [car, Ys]],
[pair, [cdr, Xs], [cdr, Ys]]]]],
/* Look up an element in a pair list */
axiom([assoc, K, Pairs], R) :-
axiom([cond, [[eq, K, [car, [car, Pairs]]], [car, [cdr, [car, Pairs]]]],
[[quote, t], [assoc, K, [cdr, Pairs]]]],
/* Cond evaluator */
axiom([evcon, Cond, Env], R) :-
axiom([cond, [[eval, [car, [car, Cond]], Env],
[eval, [car, [cdr, [car, Cond]]], Env]],
/* Recur */
[[quote, t], [evcon, [cdr, Cond], Env]]],
/* Evaluate each element of a list in the context of an environment*/
axiom([evlis, M, Env], R) :-
axiom([cond, [[null, M], [quote, []]],
[[quote, t], [cons, [eval, [car, M], Env],
[evlis, [cdr, M], Env]]]],
/* Debugger */
axiom([log, X], R) :-
axiom(X, R),
/* :- set_prolog_flag(toplevel_print_options,
[quoted(true), portrayed(true), max_depth(0)]). */
/* E V A L U A T O R */
axiom([eval, Expr, Env], R) :-
/* Symbol dereference in environment */
[[atom, Expr], [assoc, Expr, Env]],
/* (quote expr), (atom expr), etc */
[[atom, [car, Expr]],
[[eq, [car, Expr], [quote, quote]], [car, [cdr, Expr]]],
[[eq, [car, Expr], [quote, log]],
[log, [eval, [car, [cdr, Expr]], Env]]],
[[eq, [car, Expr], [quote, atom]],
[atom, [eval, [car, [cdr, Expr]], Env]]],
[[eq, [car, Expr], [quote, eq]],
[eq, [eval, [car, [cdr, Expr]], Env],
[eval, [car, [cdr, [cdr, Expr]]], Env]]],
[[eq, [car, Expr], [quote, car]],
[car, [eval, [car, [cdr, Expr]], Env]]],
[[eq, [car, Expr], [quote, cdr]],
[cdr, [eval, [car, [cdr, Expr]], Env]]],
[[eq, [car, Expr], [quote, cons]],
[cons, [eval, [car, [cdr, Expr]], Env],
[eval, [car, [cdr, [cdr, Expr]]], Env]]],
[[eq, [car, Expr], [quote, cond]], [evcon, [cdr, Expr], Env]],
/* Look up term in environment and recur */
[[quote, t], [eval, [cons, [assoc, [car, Expr], Env],
[cdr, Expr]],
/* (label var body) */
[[eq, [car, [car, Expr]], [quote, label]],
/* Evaluate body... */
[eval, [cons, [car, [cdr, [cdr, [car, Expr]]]], [cdr, Expr]],
/* With var bound to the fn form */
[cons, [list, [car, [cdr, [car, Expr]]], [car, Expr]], Env]]],
/* (lambda (x y) body) */
[[eq, [car, [car, Expr]], [quote, lambda]],
/* Evaluate body ... */
[eval, [car, [cdr, [cdr, [car, Expr]]]],
/* With an environment plus additional bindings of the argument
list to... */
[append, [pair, [car, [cdr, [car, Expr]]],
[evlis, [cdr, Expr], Env]],
[[quote, t], [quote, "no match"]]],
[test, [lambda, [], [quote, test-success]]],
[do2, [lambda, [a, b], b]],
[do3, [lambda, [a, b, c], c]],
[rectest, [label, rectest, [lambda, [x],
[cond, [[atom, x], x],
[[quote, t], [rectest, [car, x]]]]]]]
lisp(Expr, R) :-
axiom([eval, [quote, Expr], [quote, P]], R).
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