LISP_in_templates/main.cpp

#include<iostream>

namespace S { //S-Expressions
	template<int Name> struct Atom {const static int name = Name;};
#define name(N,S) template<> struct Atom<-N> {const static int name = -N; char* str = #S;}; Atom<-N> typedef S;
	name( 1,NIL)
	name( 2,CONS)
	name( 3,ATOM)
	name( 4,T)
	name( 5,F)
	name( 6,CAR)
	name( 7,CDR)
	name( 8,LAMBDA)
	name( 9,LABEL)
	name(10,QUOTE)
	name(11,COND)
	name(12,EQ)
	name(13,EQUAL)
	name(14,LIST)
#undef name
//	cons
	template<typename L, typename R>
	struct Cons {};
//	bool to Symbolic Bool conversion
	template<bool B>
	struct ToSbool {};
	template<>
	struct ToSbool<true> {
		T typedef value;
	};
	template<>
	struct ToSbool<false> {
		F typedef value;
	};
//	Sybolic Bool to bool conversion
	template<typename B>
	struct ToBool {};
	template<>
	struct ToBool<T> {
		const static bool value = true;
	};
	template<>
	struct ToBool<F> {
		const static bool value = false;
	};
}

namespace M { //M-Expressions
//	atom
	template<typename E>
	struct Atom {
		S::F typedef value;
	};
	template<int I>
	struct Atom<S::Atom<I> > {
		S::T typedef value;
	};
//	car
	template<typename E>
	struct Car {};
	template<typename A, typename D>
	struct Car<S::Cons<A, D> > {
		A typedef value;
	};
//	cdr
	template<typename E>
	struct Cdr {};
	template<typename A, typename D>
	struct Cdr<S::Cons<A, D> > {
		D typedef value;
	};
//	eq
	template<typename E1, typename E2>
	struct Eq {};
	template<int I1, int I2>
	struct Eq<S::Atom<I1>, S::Atom<I2> > {
		S::F typedef value;
	};
	template<int I>
	struct Eq<S::Atom<I>, S::Atom<I> > {
		S::T typedef value;
	};
//	equal
	template<typename E1, typename E2>
	struct Equal {
		S::F typedef value;
	};
	template<typename A1, typename A2, typename D1, typename D2>
	struct Equal<S::Cons<A1, D1>, S::Cons<A2, D2> > {
		typename Equal<A1, A2>::value typedef EA;
		typename Equal<D1, D2>::value typedef ED;
		typename S::ToSbool<
			S::ToBool<EA>::value && S::ToBool<ED>::value
		>::value typedef value;
	};
	template<int I1, int I2>
	struct Equal<S::Atom<I1>, S::Atom<I2> > {
		typename Eq<S::Atom<I1>, S::Atom<I2> >::value typedef value;
	};
//	eval (prototype)
	template<typename X, typename E>
	struct Eval {};
//	evlist
	template<typename L, typename E>
	struct Evlist {};
	template<typename E>
	struct Evlist<S::NIL, E> {
		S::NIL typedef value;
	};
	template<typename H, typename R, typename E>
	struct Evlist<S::Cons<H, R>, E> {
		S::Cons<typename Eval<H, E>::value, typename Evlist<R, E>::value> typedef value;
	};
//	assoc
	template<typename X, typename E>
	struct Assoc {};
	template<typename X, typename V, typename R>
	struct Assoc<X, S::Cons<S::Cons<X, V>, R> > {
		V typedef value;
	};
	template<typename X, typename WX, typename WV, typename R>
	struct Assoc<X, S::Cons<S::Cons<WX, WV>, R> > {
		typename Assoc<X, R>::value typedef value;
	};
//	evcond
	template<typename C, typename E>
	struct Evcond {};
	template<typename V, typename R, typename E>
	struct Evcond<S::Cons<S::Cons<S::T, S::Cons<V, S::NIL> >, R>, E> {
		typename Eval<V, E>::value typedef value;
	};
	template<typename V, typename R, typename E>
	struct Evcond<S::Cons<S::Cons<S::F, S::Cons<V, S::NIL> >, R>, E> {
		typename Evcond<R, E>::value typedef value;
	};
	template<typename Tr, typename V, typename R, typename E>
	struct Evcond<S::Cons<S::Cons<Tr, S::Cons<V, S::NIL> >, R>, E> {
		typename Evcond<S::Cons<S::Cons<typename Eval<Tr, E>::value, S::Cons<V, S::NIL> >, R>, E>::value typedef value;
	};
//	pairlist
	template<typename As, typename Ds, typename E>
	struct Pairlist {};
	template<typename Ds, typename E>
	struct Pairlist<S::NIL, Ds, E> {
		E typedef value;
	};
	template<typename A, typename RA, typename D, typename RD, typename E>
	struct Pairlist<S::Cons<A, RA>, S::Cons<D, RD>, E> {
		S::Cons<S::Cons<A, D>, typename Pairlist<RA, RD, E>::value> typedef value;
	};
//	eval
	template<int I, typename E>
	struct Eval<S::Atom<I>, E> {
		typename Assoc<S::Atom<I>, E>::value typedef value;
	};
	template<typename R, typename E>
	struct Eval<S::Cons<S::QUOTE, R>, E> {
		typename Car<R>::value typedef value;
	};
	template<typename R, typename E>
	struct Eval<S::Cons<S::ATOM, R>, E> {
		typename Atom<typename Eval<typename Car<R>::value, E>::value>::value typedef value;
	};
	template<typename R, typename E>
	struct Eval<S::Cons<S::COND, R>, E> {
		typename Evcond<R, E>::value typedef value;
	};
	template<typename R, typename E>
	struct Eval<S::Cons<S::EQ, R>, E> {
		typename Eq<
			typename Eval<typename Car<R>::value, E>::value,
			typename Eval<typename Car<typename Cdr<R>::value>::value, E>::value
		>::value typedef value;
	};
	template<typename R, typename E>
	struct Eval<S::Cons<S::EQUAL, R>, E> {
		typename Equal<
			typename Eval<typename Car<R>::value, E>::value,
			typename Eval<typename Car<typename Cdr<R>::value>::value, E>::value
		>::value typedef value;
	};
	template<typename R, typename E>
	struct Eval<S::Cons<S::CAR, R>, E> {
		typename Car<
			typename Eval<typename Car<R>::value, E>::value
		>::value typedef value;
	};
	template<typename R, typename E>
	struct Eval<S::Cons<S::CDR, R>, E> {
		typename Cdr<
			typename Eval<typename Car<R>::value, E>::value
		>::value typedef value;
	};
	template<typename R, typename E>
	struct Eval<S::Cons<S::CONS, R>, E> {
		S::Cons<
			typename Eval<typename Car<R>::value, E>::value,
			typename Eval<typename Car<typename Cdr<R>::value>::value, E>::value
		> typedef value;
	};
	template<typename R, typename E>
	struct Eval<S::Cons<S::LIST, R>, E> {
		typename Evlist<R, E>::value typedef value;
	};
	template<int N, typename R, typename E>
	struct Eval<S::Cons<S::Atom<N>, R>, E> {
		typename Eval<S::Cons<typename Eval<S::Atom<N>, E>::value, R>, E>::value typedef value;
	};
	template<typename Names, typename Body, typename R, typename E>
	struct Eval<S::Cons<S::Cons<S::LAMBDA, S::Cons<Names, S::Cons<Body, S::NIL> > >, R>, E> {
		typename Eval<
			Body,
			typename Pairlist<Names, typename Evlist<R, E>::value, E>::value
		>::value typedef value;
	};
	template<int N, typename Body, typename R, typename E>
	struct Eval<S::Cons<S::Cons<S::LABEL, S::Cons<S::Atom<N>, S::Cons<Body, S::NIL> > >, R>, E> {
		typename Eval<
			S::Cons<Body, R>,
			S::Cons<S::Cons<S::Atom<N>, Body>, E>
		>::value typedef value;
	};
}

int main() {
/* Human-readable version of the following expression
Eval[
	((LABEL Atom0
		(LAMBDA (Atom1 Atom2) (COND
			((EQUAL Atom1 'NIL) Atom2)
			(T (CONS (CAR Atom1) (Atom0 (CDR Atom1) Atom2)))
		)))
		'(Atom3 Atom4)
		'(Atom5 Atom6));
	NIL
]
*/
	typename M::Eval<
		S::Cons<
			S::Cons<
				S::LABEL,
			S::Cons<
				S::Atom<0>,//concat
			S::Cons<
				S::Cons<
					S::LAMBDA,
				S::Cons<
					S::Cons<
						S::Atom<1>,
					S::Cons<
						S::Atom<2>,
					S::NIL> >,
				S::Cons<
					S::Cons<
						S::COND,
					S::Cons<
						S::Cons<
							S::Cons<
								S::EQUAL,
							S::Cons<
								S::Atom<1>,
							S::Cons<
								S::Cons<
									S::QUOTE,
								S::Cons<
									S::NIL,
								S::NIL> >,
							S::NIL> > >,
						S::Cons<
							S::Atom<2>,
						S::NIL> >,
					S::Cons<
						S::Cons<
							S::T,
						S::Cons<
							S::Cons<
								S::CONS,
							S::Cons<
								S::Cons<
									S::CAR,
								S::Cons<
									S::Atom<1>,
								S::NIL> >,
							S::Cons<
								S::Cons<
									S::Atom<0>,
								S::Cons<
									S::Cons<
										S::CDR,
									S::Cons<
										S::Atom<1>,
									S::NIL> >,
								S::Cons<
									S::Atom<2>,
								S::NIL> > >,
							S::NIL> > >,
						S::NIL> >,
					S::NIL> > >,
				S::NIL> > >,
			S::NIL> > >,
		S::Cons<
			S::Cons<
				S::QUOTE,
			S::Cons<
				S::Cons<
					S::Atom<3>,
				S::Cons<
					S::Atom<4>,
				S::NIL> >,
			S::NIL> >,
		S::Cons<
			S::Cons<
				S::QUOTE,
			S::Cons<
				S::Cons<
					S::Atom<5>,
				S::Cons<
					S::Atom<6>,
				S::NIL> >,
			S::NIL> >,
		S::NIL> > >,
		S::NIL
	>::value typedef X; //X is the result of the computation
	X::print_by_error; //print_by_error doesn't exist, so a compiler with good feedback will represent the type "X" in an error
	return 0;
}