% LISP 1.5 implemented in Prolog
% https://archive.org/details/lisp1.5programmersmanual, p. 13

assoc(A, L, B) :- !, member([A|B], L).

pairlis([], [], A, A).
pairlis([Kh|Kt], [Vh|Vt], A, [[Kh|Vh]|A0]) :-
    pairlis(Kt, Vt, A, A0).
    
apply(car,  [[E|_]], _, E).
apply(cdr,  [[_|E]], _, E).
apply(cons, [A,B], _, [A|B]).
apply(atom, [A], _, R) :- atom(A) -> R = t ; R = [].
apply(eq,   [A,B], _, R) :- A = B -> R = t ; R = [].
apply([lambda, Params, Body], Args, Ctx, R) :-
    pairlis(Params, Args, Ctx, Ctx0),
    eval(Body, Ctx0, R).
apply([label, Name, Val], Args, Ctx, R) :-
    apply(Name, Args, [[Name|Val]|Ctx], R).

eval(A, Ctx, E) :- atom(A), assoc(A, Ctx, E).
eval([quote, A], _, A).
eval([cond | Cs], Ctx, A) :- evcon(Cs, Ctx, A).
eval([H|T], Ctx, E) :- evlist(T, Ctx, A), apply(H, A, Ctx, E).

evcon([], _, []).
evcon([H|T], Ctx, R) :- eval(H, Ctx, R) -> evcon(T, Ctx, R); R \= [].

evlist([], _, []).
evlist([H|T], Ctx, [E|R]) :- eval(H, Ctx, E), evlist(T, Ctx, R).

?- eval([[lambda, [a], [eq, a, [quote, b]]], [quote, b]], [], R).