main.c

/*
 * main.c
 *
 *
 *
 * Copyright (C) 2012-10-01 liutos
 */
#include "type.h"
#include <stdio.h>

extern LispObject make_cons(char *);
void print_cons_core(LispObject);
void print_cons(LispObject);

int main(int argc, char *argv[])
{
    print_cons(make_cons("(a bc)"));
    putchar('\n');

    return 0;
}

void print_cons_core(LispObject cons)
{
    while (cons != NULL) {
	if (ATOM == cons->car->type)
	    printf("%s", cons->car->symbol_name);
	else {
	    print_cons(cons->car);
	}
	if (cons->cdr != NULL)
	    putchar(' ');
	cons = cons->cdr;
    }
}

void print_cons(LispObject cons)
{
    putchar('(');
    print_cons_core(cons);
    putchar(')');
}

parse.c

/*
 * parse.c
 *
 *
 *
 * Copyright (C) 2012-10-01 liutos
 */
#include "type.h"
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

char *get_symbol(char *string)
{
    int i;

    i = 0;
    while (string[i] != ' ' && string[i] != '(' &&
	   string[i] != ')' && string[i] != '\0')
	i++;

    return strndup(string, i);
}

LispObject make_cons_core(char *string, int *offset)
{
    int i, sub_offset, step;
    LispObject head, pre, cur, atom;
    char *token;

    i = 0;
    head = malloc(sizeof(struct LispObject));
    head->car = head->cdr = NULL;
    pre = head;
    while (string[i] != '\0') {
	switch (string[i]) {
	case '(':
	    cur = malloc(sizeof(struct LispObject));
	    cur->type = CONS;
	    cur->car = make_cons_core(string + i + 1, &sub_offset);
	    cur->cdr = NULL;
	    step = sub_offset;
	    break;
	case ' ':
	    step = 1;
	    break;
	case ')':
	    *offset = i + 2;
	    return head->cdr;
	    break;
	default :
	    token = get_symbol(string + i);
	    atom = malloc(sizeof(struct LispObject));
	    atom->type = ATOM;
	    atom->symbol_name = token;
	    cur = malloc(sizeof(struct LispObject));
	    cur->type = CONS;
	    cur->car = atom;
	    cur->cdr = NULL;
	    step = strlen(token);
	}
	if (string[i] != ' ') {
	    pre->cdr = cur;
	    pre = cur;
	}
	i += step;
    }

    return head->cdr;
}

LispObject make_cons(char *string)
{
    int i;

    return make_cons_core(string + 1, &i);
}

type.h

#ifndef TYPE_H
#define TYPE_H

typedef enum {
    CONS,
    ATOM,
    SYMBOL,
} LispType;

typedef struct LispObject {
    LispType type;
    union {
	char *symbol_name;
	struct {
	    struct LispObject *car;
	    struct LispObject *cdr;
	};
    };
} *LispObject;

#endif