#ifndef _l_h_
#define _l_h_

/*
   l.h - Common header for a minimalistic Lisp implementation.
   Samuel A. Rebelsky
   Version 0.1 of 3 March 2007
 */

#include <stdio.h>

/*********************************************************************
 * Design Notes *
 ****************/

/*

  On the Intel architecture (for which this is designed), we are
  fortunate that pointers, ints, and floats all take four eight-bit
  bytes.  Hence, we can convert between the types easily. (We won't
  deal with floats in this implementation, though.)

  When a pair stores a symbol, we will set the car to L_SYMBOL and
    the cdr to the string representation of that symbol.
  When a pair stores a string, we will set the car to L_STRING and
    the cdr to the string.
  When a pair stores an integer, we will set the car to L_INT and
    the cdr to the value (not a pointer to the value).

 */
 
/*********************************************************************
 * Types *
 *********/

typedef void *pointer;

/* The primary building block of Lisp programs. */
typedef struct pair {
  pointer car;
  pointer cdr;
} *pair;


/*********************************************************************
 * Constants *
 *************/

/* We use small numbers, unlikely to be real pointers, to indicate the
 * various types.
 */
#define L_SYMBOL ((pointer) 1)
#define L_STRING ((pointer) 2)
#define L_INT ((pointer) 3)
#define L_MAX_CONSTANT 3

/* We will allocate the following constant symbols separately 
 */
extern pair L_CAR;
extern pair L_CDR;
extern pair L_COND;
extern pair L_CONS;
extern pair L_DEFINE;
extern pair L_EQ;
extern pair L_ERROR;
extern pair L_FALSE;
extern pair L_LABEL;
extern pair L_LAMBDA;
extern pair L_NIL;
extern pair L_QUOTE;
extern pair L_TRUE;


/*********************************************************************
 * Macros *
 **********/

#define CAR(PAIR) (((pair) PAIR)->car)
#define CDR(PAIR) (((pair) PAIR)->cdr)


/*********************************************************************
 * Procedures *
 **************/

/* From l_io.c */

extern void l_print(FILE *stream, pair root);
extern pair l_parse(char *str);

/* From l_eval.c */

extern void init_env();
extern pair l_eval(pair exp);
extern int l_eq(pair left, pair right);

/* From l_types.c */

extern pair make_int(int val);
extern pair make_string(char *val);
extern pair make_symbol(char *val);

extern int is_nil(pair val);
extern int is_int(pair val);
extern int is_string(pair val);
extern int is_symbol(pair val);
extern int is_pair(pair val);

/* From l_sym.c */

extern void init_sym();
extern pair insert(char *sym);

/* From l_mem.c */

extern void init_mem(int size);
extern pair allocate_pair(pointer left, pointer right);

#endif /* _l_h_ */