cvs/stack/stack.c

/* printf, sscanf, fgets, fprintf */
#include <stdio.h>
/* exit, EXIT_SUCCESS, malloc, free */
#include <stdlib.h>
/* NULL */
#include <stddef.h>
/* dlopen, dlsym, dlerror */
#include <dlfcn.h>
/* strcmp, strcpy, strlen, strcat, strdup */
#include <string.h>

#define HASHTBLSIZE 65536

/* First, define some types. */

/* A value of some type */
typedef struct {
  enum {
    integer,
    string,
    func,                       /* Function pointer */
    symb, 
    list
  } type;                       /* Type of stack element */

  union {
    void *ptr;                  /* Pointer to the content */
    int val;                    /* ...or an integer */
  } content;                    /* Stores a pointer or an integer */

  int refcount;                 /* Reference counter */

} value;

/* A symbol with a name and possible value */
/* (These do not need reference counters, they are kept unique by
   hashing.) */
typedef struct symbol_struct {
  char *id;                     /* Symbol name */
  value *val;                   /* The value (if any) bound to it */
  struct symbol_struct *next;   /* In case of hashing conflicts, a */
} symbol;                       /* symbol is a kind of stack item. */

/* A type for a hash table for symbols */
typedef symbol *hashtbl[HASHTBLSIZE]; /* Hash table declaration */

/* An item (value) on a stack */
typedef struct stackitem_struct
{
  value *item;                  /* The value on the stack */
                                /* (This is never NULL) */
  struct stackitem_struct *next; /* Next item */
} stackitem;

/* An environment; gives access to the stack and a hash table of
   defined symbols */
typedef struct {
  stackitem *head;              /* Head of the stack */
  hashtbl symbols;              /* Hash table of all variable bindings */
  int err;                      /* Error flag */
  int non_eval_flag;
} environment;

/* A type for pointers to external functions */
typedef void (*funcp)(environment *); /* funcp is a pointer to a void
                                         function (environment *) */

/* Initialize a newly created environment */
void init_env(environment *env)
{
  int i;

  env->err= 0;
  env->non_eval_flag= 0;
  for(i= 0; i<HASHTBLSIZE; i++)
    env->symbols[i]= NULL;
}

void printerr(const char* in_string) {
  fprintf(stderr, "Err: %s\n", in_string);
}

/* Throw away a value */
void free_val(value *val){
  stackitem *item, *temp;

  val->refcount--;              /* Decrease the reference count */
  if(val->refcount == 0){
    switch (val->type){         /* and free the contents if necessary */
    case string:
      free(val->content.ptr);
      break;
    case list:                  /* lists needs to be freed recursively */
      item=val->content.ptr;
      while(item != NULL) {     /* for all stack items */
        free_val(item->item);   /* free the value */
        temp=item->next;        /* save next ptr */
        free(item);             /* free the stackitem */
        item=temp;              /* go to next stackitem */
      }
      free(val);                /* Free the actual list value */
      break;
    case integer:
    case func:
    case symb:
      break;
    }
  }
}

/* Discard the top element of the stack. */
extern void toss(environment *env)
{
  stackitem *temp= env->head;

  if((env->head)==NULL) {
    printerr("Too Few Arguments");
    env->err=1;
    return;
  }
  
  free_val(env->head->item);    /* Free the value */
  env->head= env->head->next;   /* Remove the top stack item */
  free(temp);                   /* Free the old top stack item */
}

/* Returns a pointer to a pointer to an element in the hash table. */
symbol **hash(hashtbl in_hashtbl, const char *in_string)
{
  int i= 0;
  unsigned int out_hash= 0;
  char key= '\0';
  symbol **position;
  
  while(1){                     /* Hash in_string */
    key= in_string[i++];
    if(key=='\0')
      break;
    out_hash= out_hash*32+key;
  }

  out_hash= out_hash%HASHTBLSIZE;
  position= &(in_hashtbl[out_hash]);

  while(1){
    if(*position==NULL)         /* If empty */
      return position;
    
    if(strcmp(in_string, (*position)->id)==0) /* If match */
      return position;

    position= &((*position)->next); /* Try next */
  }
}

/* Generic push function. */
void push(stackitem** stack_head, stackitem* in_item)
{
  in_item->next= *stack_head;
  *stack_head= in_item;
}

/* Push a value onto the stack */
void push_val(stackitem **stack_head, value *val)
{
  stackitem *new_item= malloc(sizeof(stackitem));
  new_item->item= val;
  val->refcount++;
  push(stack_head, new_item);
}

/* Push an integer onto the stack. */
void push_int(stackitem **stack_head, int in_val) 
{
  value *new_value= malloc(sizeof(value));
  stackitem *new_item= malloc(sizeof(stackitem));
  new_item->item= new_value;
  
  new_value->content.val= in_val;
  new_value->type= integer;
  new_value->refcount=1;

  push(stack_head, new_item);
}

/* Copy a string onto the stack. */
void push_cstring(stackitem **stack_head, const char *in_string)
{
  value *new_value= malloc(sizeof(value));
  stackitem *new_item= malloc(sizeof(stackitem));
  new_item->item=new_value;

  new_value->content.ptr= malloc(strlen(in_string)+1);
  strcpy(new_value->content.ptr, in_string);
  new_value->type= string;
  new_value->refcount=1;

  push(stack_head, new_item);
}

/* Mangle a symbol name to a valid C identifier name */
char *mangle_str(const char *old_string){
  char validchars[]
    ="0123456789abcdef";
  char *new_string, *current;

  new_string=malloc((strlen(old_string)*2)+4);
  strcpy(new_string, "sx_");    /* Stack eXternal */
  current=new_string+3;
  while(old_string[0] != '\0'){
    current[0]=validchars[(unsigned char)(old_string[0])/16];
    current[1]=validchars[(unsigned char)(old_string[0])%16];
    current+=2;
    old_string++;
  }
  current[0]='\0';

  return new_string;            /* The caller must free() it */
}

extern void mangle(environment *env){
  value *new_value;
  char *new_string;

  if((env->head)==NULL) {
    printerr("Too Few Arguments");
    env->err=1;
    return;
  }

  if(env->head->item->type!=string) {
    printerr("Bad Argument Type");
    env->err=2;
    return;
  }

  new_string= mangle_str((const char *)(env->head->item->content.ptr));

  toss(env);
  if(env->err) return;

  new_value= malloc(sizeof(value));
  new_value->content.ptr= new_string;
  new_value->type= string;
  new_value->refcount=1;

  push_val(&(env->head), new_value);
}

/* Push a symbol onto the stack. */
void push_sym(environment *env, const char *in_string)
{
  stackitem *new_item;          /* The new stack item */
  /* ...which will contain... */
  value *new_value;             /* A new symbol value */
  /* ...which might point to... */
  symbol **new_symbol;          /* (if needed) A new actual symbol */
  /* ...which, if possible, will be bound to... */
  value *new_fvalue;            /* (if needed) A new function value */
  /* ...which will point to... */
  void *funcptr;                /* A function pointer */

  static void *handle= NULL;    /* Dynamic linker handle */
  const char *dlerr;            /* Dynamic linker error */
  char *mangled;                /* Mangled function name */

  /* Create a new stack item containing a new value */
  new_item= malloc(sizeof(stackitem));
  new_value= malloc(sizeof(value));
  new_item->item=new_value;

  /* The new value is a symbol */
  new_value->type= symb;
  new_value->refcount= 1;

  /* Look up the symbol name in the hash table */
  new_symbol= hash(env->symbols, in_string);
  new_value->content.ptr= *new_symbol;

  if(*new_symbol==NULL) { /* If symbol was undefined */

    /* Create a new symbol */
    (*new_symbol)= malloc(sizeof(symbol));
    (*new_symbol)->val= NULL;   /* undefined value */
    (*new_symbol)->next= NULL;
    (*new_symbol)->id= malloc(strlen(in_string)+1);
    strcpy((*new_symbol)->id, in_string);

    /* Intern the new symbol in the hash table */
    new_value->content.ptr= *new_symbol;

    /* Try to load the symbol name as an external function, to see if
       we should bind the symbol to a new function pointer value */
    if(handle==NULL)            /* If no handle */
      handle= dlopen(NULL, RTLD_LAZY);

    funcptr= dlsym(handle, in_string); /* Get function pointer */
    dlerr=dlerror();
    if(dlerr != NULL) {         /* If no function was found */
      mangled=mangle_str(in_string);
      funcptr= dlsym(handle, mangled); /* try mangling it */
      free(mangled);
      dlerr=dlerror();
    }
    if(dlerr==NULL) {           /* If a function was found */
      new_fvalue= malloc(sizeof(value)); /* Create a new value */
      new_fvalue->type=func;    /* The new value is a function pointer */
      new_fvalue->content.ptr=funcptr; /* Store function pointer */
      (*new_symbol)->val= new_fvalue; /* Bind the symbol to the new
                                         function value */
      new_fvalue->refcount= 1;
    }
  }
  push(&(env->head), new_item);
}

/* Print newline. */
extern void nl()
{
  printf("\n");
}

/* Gets the type of a value */
extern void type(environment *env){
  int typenum;

  if((env->head)==NULL) {
    printerr("Too Few Arguments");
    env->err=1;
    return;
  }
  typenum=env->head->item->type;
  toss(env);
  switch(typenum){
  case integer:
    push_sym(env, "integer");
    break;
  case string:
    push_sym(env, "string");
    break;
  case symb:
    push_sym(env, "symbol");
    break;
  case func:
    push_sym(env, "function");
    break;
  case list:
    push_sym(env, "list");
    break;
  default:
    push_sym(env, "unknown");
    break;
  }
}    

/* Prints the top element of the stack. */
void print_h(stackitem *stack_head)
{
  switch(stack_head->item->type) {
  case integer:
    printf("%d", stack_head->item->content.val);
    break;
  case string:
    printf("\"%s\"", (char*)stack_head->item->content.ptr);
    break;
  case symb:
    printf("%s", ((symbol *)(stack_head->item->content.ptr))->id);
    break;
  case func:
    printf("#<function %p>", (funcp)(stack_head->item->content.ptr));
    break;
  case list:
    /* A list is just a stack, so make stack_head point to it */
    stack_head=(stackitem *)(stack_head->item->content.ptr);
    printf("[ ");
    while(stack_head != NULL) {
      print_h(stack_head);
      printf(" ");
      stack_head=stack_head->next;
    }
    printf("]");
    break;
  default:
    printf("#<unknown %p>", (stack_head->item->content.ptr));
    break;
  }
}

extern void print_(environment *env) {
  if(env->head==NULL) {
    printerr("Too Few Arguments");
    env->err=1;
    return;
  }
  print_h(env->head);
}

/* Prints the top element of the stack and then discards it. */
extern void print(environment *env)
{
  print_(env);
  if(env->err) return;
  toss(env);
}

/* Only to be called by function printstack. */
void print_st(stackitem *stack_head, long counter)
{ 
  if(stack_head->next != NULL)
    print_st(stack_head->next, counter+1);
  printf("%ld: ", counter);
  print_h(stack_head);
  nl();
}

/* Prints the stack. */
extern void printstack(environment *env)
{
  if(env->head == NULL) {
    return;
  }
  print_st(env->head, 1);
  nl();
}

/* Swap the two top elements on the stack. */
extern void swap(environment *env)
{
  stackitem *temp= env->head;
  
  if(env->head==NULL || env->head->next==NULL) {
    printerr("Too Few Arguments");
    env->err=1;
    return;
  }

  env->head= env->head->next;
  temp->next= env->head->next;
  env->head->next= temp;
} 

/* Rotate the first three elements on the stack. */
extern void rot(environment *env)
{
  stackitem *temp= env->head;
  
  if(env->head==NULL || env->head->next==NULL
      || env->head->next->next==NULL) {
    printerr("Too Few Arguments");
    env->err=1;
    return;
  }

  env->head= env->head->next->next;
  temp->next->next= env->head->next;
  env->head->next= temp;
}

/* Recall a value from a symbol, if bound */
extern void rcl(environment *env)
{
  value *val;

  if(env->head == NULL) {
    printerr("Too Few Arguments");
    env->err=1;
    return;
  }

  if(env->head->item->type!=symb) {
    printerr("Bad Argument Type");
    env->err=2;
    return;
  }

  val=((symbol *)(env->head->item->content.ptr))->val;
  if(val == NULL){
    printerr("Unbound Variable");
    env->err=3;
    return;
  }
  toss(env);            /* toss the symbol */
  if(env->err) return;
  push_val(&(env->head), val); /* Return its bound value */
}

void stack_read(environment*, char*);

/* If the top element is a symbol, determine if it's bound to a
   function value, and if it is, toss the symbol and execute the
   function. */
extern void eval(environment *env)
{
  funcp in_func;
  value* temp_val;
  stackitem* iterator;
  char* temp_string;

  if(env->head==NULL) {
    printerr("Too Few Arguments");
    env->err=1;
    return;
  }

  switch(env->head->item->type) {
    /* if it's a symbol */
  case symb:
    rcl(env);                   /* get its contents */
    if(env->err) return;
    if(env->head->item->type!=symb){ /* don't recurse symbols */
      eval(env);                        /* evaluate the value */
      return;
    }
    break;

    /* If it's a lone function value, run it */
  case func:
    in_func= (funcp)(env->head->item->content.ptr);
    toss(env); 
    if(env->err) return;
    (*in_func)(env);
    break;

    /* If it's a list */
  case list:
    temp_val= env->head->item;
    env->head->item->refcount++;
    toss(env);
    if(env->err) return;
    iterator= (stackitem*)temp_val->content.ptr;
    while(iterator!=NULL && iterator->item!=NULL) {
      push_val(&(env->head), iterator->item);
      if(env->head->item->type==symb
        && strcmp(";", ((symbol*)(env->head->item->content.ptr))->id)==0) {
        toss(env);
        if(env->err) return;
        eval(env);
        if(env->err) return;
      }
      iterator= iterator->next;
    }
    free_val(temp_val);
    break;

    /* If it's a string */
  case string:
    temp_val= env->head->item;
    env->head->item->refcount++;
    toss(env);
    if(env->err) return;
    temp_string= malloc(strlen((char*)temp_val->content.ptr)+5);
    strcpy(temp_string, "[ ");
    strcat(temp_string, (char*)temp_val->content.ptr);
    strcat(temp_string, " ]");
    stack_read(env, temp_string);
    eval(env);
    if(env->err) return;
    free_val(temp_val);
    free(temp_string);
    break;

  case integer:
    break;
  }
}

/* Reverse (flip) a list */
extern void rev(environment *env){
  stackitem *old_head, *new_head, *item;

  if((env->head)==NULL) {
    printerr("Too Few Arguments");
    env->err=1;
    return;
  }

  if(env->head->item->type!=list) {
    printerr("Bad Argument Type");
    env->err=2;
    return;
  }

  old_head=(stackitem *)(env->head->item->content.ptr);
  new_head=NULL;
  while(old_head != NULL){
    item=old_head;
    old_head=old_head->next;
    item->next=new_head;
    new_head=item;
  }
  env->head->item->content.ptr=new_head;
}

/* Make a list. */
extern void pack(environment *env)
{
  void* delimiter;
  stackitem *iterator, *temp;
  value *pack;

  delimiter= env->head->item->content.ptr; /* Get delimiter */
  toss(env);

  iterator= env->head;

  if(iterator==NULL || iterator->item->content.ptr==delimiter) {
    temp= NULL;
    toss(env);
  } else {
    /* Search for first delimiter */
    while(iterator->next!=NULL
          && iterator->next->item->content.ptr!=delimiter)
      iterator= iterator->next;
    
    /* Extract list */
    temp= env->head;
    env->head= iterator->next;
    iterator->next= NULL;
    
    if(env->head!=NULL)
      toss(env);
  }

  /* Push list */
  pack= malloc(sizeof(value));
  pack->type= list;
  pack->content.ptr= temp;
  pack->refcount= 1;

  temp= malloc(sizeof(stackitem));
  temp->item= pack;

  push(&(env->head), temp);
  rev(env);
}

/* Parse input. */
void stack_read(environment *env, char *in_line) 
{
  char *temp, *rest;
  int itemp;
  size_t inlength= strlen(in_line)+1;
  int convert= 0;

  temp= malloc(inlength);
  rest= malloc(inlength);

  do {
    /* If comment */
    if((convert= sscanf(in_line, "#%[^\n\r]", rest))) {
      free(temp); free(rest);
      return;
    }

    /* If string */
    if((convert= sscanf(in_line, "\"%[^\"\n\r]\" %[^\n\r]", temp, rest))) {
      push_cstring(&(env->head), temp);
      break;
    }
    /* If integer */
    if((convert= sscanf(in_line, "%d %[^\n\r]", &itemp, rest))) {
      push_int(&(env->head), itemp);
      break;
    }
    /* Escape ';' with '\' */
    if((convert= sscanf(in_line, "\\%c%[^\n\r]", temp, rest))) {
      temp[1]= '\0';
      push_sym(env, temp);
      break;
    } 
    /* If symbol */
    if((convert= sscanf(in_line, "%[^][ ;\n\r]%[^\n\r]", temp, rest))) {
        push_sym(env, temp);
        break;
    }
    /* If single char */
    if((convert= sscanf(in_line, "%c%[^\n\r]", temp, rest))) {
      if(*temp==';') {
        if(!env->non_eval_flag) {
          eval(env);            /* Evaluate top element */
          break;
        }
        
        push_sym(env, ";");
        break;
      }

      if(*temp==']') {
        push_sym(env, "[");
        pack(env);
        if(env->non_eval_flag)
          env->non_eval_flag--;
        break;
      }

      if(*temp=='[') {
        push_sym(env, "[");
        env->non_eval_flag++;
        break;
      }
    }
  } while(0);

  free(temp);

  if(convert<2) {
    free(rest);
    return;
  }
   
  stack_read(env, rest);
  
  free(rest);
}

/* Relocate elements of the list on the stack. */
extern void expand(environment *env)
{
  stackitem *temp, *new_head;

  /* Is top element a list? */
  if(env->head==NULL) {
    printerr("Too Few Arguments");
    env->err=1;
    return;
  }
  if(env->head->item->type!=list) {
    printerr("Bad Argument Type");
    env->err=2;
    return;
  }

  rev(env);

  if(env->err)
    return;

  /* The first list element is the new stack head */
  new_head= temp= env->head->item->content.ptr;

  env->head->item->refcount++;
  toss(env);

  /* Find the end of the list */
  while(temp->next!=NULL)
    temp= temp->next;

  /* Connect the tail of the list with the old stack head */
  temp->next= env->head;
  env->head= new_head;          /* ...and voila! */

}

/* Compares two elements by reference. */
extern void eq(environment *env)
{
  void *left, *right;
  int result;

  if((env->head)==NULL || env->head->next==NULL) {
    printerr("Too Few Arguments");
    env->err=1;
    return;
  }

  left= env->head->item->content.ptr;
  swap(env);
  right= env->head->item->content.ptr;
  result= (left==right);
  
  toss(env); toss(env);
  push_int(&(env->head), result);
}

/* Negates the top element on the stack. */
extern void not(environment *env)
{
  int val;

  if((env->head)==NULL) {
    printerr("Too Few Arguments");
    env->err=1;
    return;
  }

  if(env->head->item->type!=integer) {
    printerr("Bad Argument Type");
    env->err=2;
    return;
  }

  val= env->head->item->content.val;
  toss(env);
  push_int(&(env->head), !val);
}

/* Compares the two top elements on the stack and return 0 if they're the
   same. */
extern void neq(environment *env)
{
  eq(env);
  not(env);
}

/* Give a symbol some content. */
extern void def(environment *env)
{
  symbol *sym;

  /* Needs two values on the stack, the top one must be a symbol */
  if(env->head==NULL || env->head->next==NULL) {
    printerr("Too Few Arguments");
    env->err=1;
    return;
  }

  if(env->head->item->type!=symb) {
    printerr("Bad Argument Type");
    env->err=2;
    return;
  }

  /* long names are a pain */
  sym=env->head->item->content.ptr;

  /* if the symbol was bound to something else, throw it away */
  if(sym->val != NULL)
    free_val(sym->val);

  /* Bind the symbol to the value */
  sym->val= env->head->next->item;
  sym->val->refcount++;         /* Increase the reference counter */

  toss(env); toss(env);
} 

/* Quit stack. */
extern void quit(environment *env)
{
  exit(EXIT_SUCCESS);
}

/* Clear stack */
extern void clear(environment *env)
{
  while(env->head!=NULL)
    toss(env);
}

/* List all defined words */
extern void words(environment *env)
{
  symbol *temp;
  int i;
  
  for(i= 0; i<HASHTBLSIZE; i++) {
    temp= env->symbols[i];
    while(temp!=NULL) {
      printf("%s\n", temp->id);
      temp= temp->next;
    }
  }
}

/* Forgets a symbol (remove it from the hash table) */
extern void forget(environment *env)
{
  char* sym_id;
  stackitem *stack_head= env->head;
  symbol **hash_entry, *temp;

  if(stack_head==NULL) {
    printerr("Too Few Arguments");
    env->err=1;
    return;
  }
  
  if(stack_head->item->type!=symb) {
    printerr("Bad Argument Type");
    env->err=2;
    return;
  }

  sym_id= ((symbol*)(stack_head->item->content.ptr))->id;
  toss(env);

  hash_entry= hash(env->symbols, sym_id);
  temp= *hash_entry;
  *hash_entry= (*hash_entry)->next;
  
  if(temp->val!=NULL) {
    free_val(temp->val);
  }
  free(temp->id);
  free(temp);
}

/* Returns the current error number to the stack */
extern void errn(environment *env){
  push_int(&(env->head), env->err);
}

int main()
{
  environment myenv;
  char in_string[100];

  init_env(&myenv);

  printf("okidok\n ");

  while(fgets(in_string, 100, stdin) != NULL) {
    stack_read(&myenv, in_string);
    if(myenv.err) {
      printf("(error %d) ", myenv.err);
      myenv.err=0;
    }
    printf("okidok\n ");
  }
  quit(&myenv);
  return EXIT_FAILURE;
}

/* + */
extern void sx_2b(environment *env) {
  int a, b;
  size_t len;
  char* new_string;
  value *a_val, *b_val;

  if((env->head)==NULL || env->head->next==NULL) {
    printerr("Too Few Arguments");
    env->err=1;
    return;
  }

  if(env->head->item->type==string
     && env->head->next->item->type==string) {
    a_val= env->head->item;
    b_val= env->head->next->item;
    a_val->refcount++;
    b_val->refcount++;
    toss(env); if(env->err) return;
    toss(env); if(env->err) return;
    len= strlen(a_val->content.ptr)+strlen(b_val->content.ptr)+1;
    new_string= malloc(len);
    strcpy(new_string, b_val->content.ptr);
    strcat(new_string, a_val->content.ptr);
    free_val(a_val); free_val(b_val);
    push_cstring(&(env->head), new_string);
    free(new_string);
    return;
  }
  
  if(env->head->item->type!=integer
     || env->head->next->item->type!=integer) {
    printerr("Bad Argument Type");
    env->err=2;
    return;
  }
  a=env->head->item->content.val;
  toss(env);
  if(env->err) return;
  b=env->head->item->content.val;
  toss(env);
  if(env->err) return;
  push_int(&(env->head), a+b);
}

/* Return copy of a value */
value *copy_val(value *old_value){
  stackitem *old_item, *new_item, *prev_item;

  value *new_value=malloc(sizeof(value));

  new_value->type=old_value->type;
  new_value->refcount=0;        /* This is increased if/when this
                                   value is referenced somewhere, like
                                   in a stack item or a variable */
  switch(old_value->type){
  case integer:
    new_value->content.val=old_value->content.val;
    break;
  case string:
    (char *)(new_value->content.ptr)
      = strdup((char *)(old_value->content.ptr));
    break;
  case func:
  case symb:
    new_value->content.ptr=old_value->content.ptr;
    break;
  case list:
    new_value->content.ptr=NULL;

    prev_item=NULL;
    old_item=(stackitem *)(old_value->content.ptr);

    while(old_item != NULL) {   /* While list is not empty */
      new_item= malloc(sizeof(stackitem));
      new_item->item=copy_val(old_item->item); /* recurse */
      new_item->next=NULL;
      if(prev_item != NULL)     /* If this wasn't the first item */
        prev_item->next=new_item; /* point the previous item to the
                                     new item */
      else
        new_value->content.ptr=new_item;
      old_item=old_item->next;
      prev_item=new_item;
    }     
    break;
  }
  return new_value;
}

/* duplicates an item on the stack */
extern void dup(environment *env) {
  if((env->head)==NULL) {
    printerr("Too Few Arguments");
    env->err=1;
    return;
  }
  push_val(&(env->head), copy_val(env->head->item));
}

/* If-Then */
extern void ift(environment *env) {

  int truth;

  if((env->head)==NULL || env->head->next==NULL) {
    printerr("Too Few Arguments");
    env->err=1;
    return;
  }

  if(env->head->next->item->type != integer) {
    printerr("Bad Argument Type");
    env->err=2;
    return;
  }
  
  swap(env);
  if(env->err) return;
  
  truth=env->head->item->content.val;

  toss(env);
  if(env->err) return;

  if(truth)
    eval(env);
  else
    toss(env);
}


/* If-Then-Else */
extern void ifte(environment *env) {

  int truth;

  if((env->head)==NULL || env->head->next==NULL
     || env->head->next->next==NULL) {
    printerr("Too Few Arguments");
    env->err=1;
    return;
  }

  if(env->head->next->next->item->type != integer) {
    printerr("Bad Argument Type");
    env->err=2;
    return;
  }
  
  rot(env);
  if(env->err) return;
  
  truth=env->head->item->content.val;

  toss(env);
  if(env->err) return;

  if(!truth)
    swap(env);
  if(env->err) return;

  toss(env);
  if(env->err) return;

  eval(env);
}
