1. 第十三章·条件分支

1.1. 自己动手

现在我们已经在我们的Lispy做了不少事情。相信以你现有的C语言知识已经足以独立开发功能。如果你有信心的话，这一章是一个完美的机会，让你放飞思路并尝试实现你自己的一些想法。这一章篇幅较短，主要内容是添加一些处理比较和排序的内置函数。

如果你有信心，继续并尝试在你的Lispy中实现比较和排序功能。定义一些新的内置函数用于大于、小于、等于以及其余在C语言中适用的比较运算符。尝试去定义if函数，用于检验某些条件并根据结果执行不同的代码。当你完成以后接着看下面的内容，比较你与我代码实现上的不同，分析不同之处并选择你更喜欢的实现方式。

如果你没什么思路，不用担心。接下来我将会解释我的方法。

1.2. 排序函数

为了简单起见，我们将复用之前的数字数据类型来表示比较结果。 制定一个类似于C语言的规则，假设任何不是0的数字在if语句中总为true，而0则总为false。

因此，我们的排序函数有点像算术函数的简化版本。 排序函数仅仅能够处理数字。此外，我们想要函数只能适用于两个参数的情形。

如果参数符合这些错误情形，则计算上很简单。我们希望根据两个lval输入之间的相等比较返回一个数字为0或1的lval。 使用C的比较运算符可以做到这一点。 与之前的算术函数一样，我们将使用单个函数来完成所有比较。

首先我们检查错误情形，然后我们比较参数中的数字获得结果。 最后，我们将结果作为数字返回。

lval* builtin_gt(lenv* e, lval* a) {
  return builtin_ord(e, a, ">");
}

lval* builtin_lt(lenv* e, lval* a) {
  return builtin_ord(e, a, "<");
}

lval* builtin_ge(lenv* e, lval* a) {
  return builtin_ord(e, a, ">=");
}

lval* builtin_le(lenv* e, lval* a) {
  return builtin_ord(e, a, "<=");
}

lval* builtin_ord(lenv* e, lval* a, char* op) {
  LASSERT_NUM(op, a, 2);
  LASSERT_TYPE(op, a, 0, LVAL_NUM);
  LASSERT_TYPE(op, a, 1, LVAL_NUM);

  int r;
  if (strcmp(op, ">")  == 0) {
    r = (a->cell[0]->num >  a->cell[1]->num);
  }
  if (strcmp(op, "<")  == 0) {
    r = (a->cell[0]->num <  a->cell[1]->num);
  }
  if (strcmp(op, ">=") == 0) {
    r = (a->cell[0]->num >= a->cell[1]->num);
  }
  if (strcmp(op, "<=") == 0) {
    r = (a->cell[0]->num <= a->cell[1]->num);
  }
  lval_del(a);
  return lval_num(r);
}

1.3. 等于函数

等于函数将会与排序函数有所不同，因为我们想要它不仅仅适用于数字类型。除此之外，它将用于判断输入是否为空列表，或者判断两个函数是否相同。因此我们需要定义一个用来检验两个不同类型lval变量是否相等的函数。

该函数实质上检查两个构成lval类型数据的所有字段是否相等。 若所有字段都相等，则两者被认为是相等的。 否则，若存在任何差异，则两者被认为是不相等的。

int lval_eq(lval* x, lval* y) {

  /* Different Types are always unequal */
  if (x->type != y->type) { return 0; }

  /* Compare Based upon type */
  switch (x->type) {
    /* Compare Number Value */
    case LVAL_NUM: return (x->num == y->num);

    /* Compare String Values */
    case LVAL_ERR: return (strcmp(x->err, y->err) == 0);
    case LVAL_SYM: return (strcmp(x->sym, y->sym) == 0);

    /* If builtin compare, otherwise compare formals and body */
    case LVAL_FUN:
      if (x->builtin || y->builtin) {
        return x->builtin == y->builtin;
      } else {
        return lval_eq(x->formals, y->formals)
          && lval_eq(x->body, y->body);
      }

    /* If list compare every individual element */
    case LVAL_QEXPR:
    case LVAL_SEXPR:
      if (x->count != y->count) { return 0; }
      for (int i = 0; i < x->count; i++) {
        /* If any element not equal then whole list not equal */
        if (!lval_eq(x->cell[i], y->cell[i])) { return 0; }
      }
      /* Otherwise lists must be equal */
      return 1;
    break;
  }
  return 0;
}

使用该函数，可以非常简单地新增用于比较是否相等的内置函数。 我们仅需要保证输入为两个参数，检验它们是否相等，然后我们将比较结果存储到一个新的lval中并返回它。

lval* builtin_cmp(lenv* e, lval* a, char* op) {
  LASSERT_NUM(op, a, 2);
  int r;
  if (strcmp(op, "==") == 0) {
    r =  lval_eq(a->cell[0], a->cell[1]);
  }
  if (strcmp(op, "!=") == 0) {
    r = !lval_eq(a->cell[0], a->cell[1]);
  }
  lval_del(a);
  return lval_num(r);
}

lval* builtin_eq(lenv* e, lval* a) {
  return builtin_cmp(e, a, "==");
}

lval* builtin_ne(lenv* e, lval* a) {
  return builtin_cmp(e, a, "!=");
}

1.4. If函数

为了更好的使用比较运算符，我们需要新增if函数。 该函数有点像C中的三元运算。在条件为真时，它对一段代码求值，如果条件为假，则对另一段代码求值。

我们再次使用Q表达式来对计算式进行编码。 首先，我们让用户传入比较结果，然后我们让用户传入两个Q表达式，分别表示在条件结果为true或false时进行求值的代码。

lval* builtin_if(lenv* e, lval* a) {
  LASSERT_NUM("if", a, 3);
  LASSERT_TYPE("if", a, 0, LVAL_NUM);
  LASSERT_TYPE("if", a, 1, LVAL_QEXPR);
  LASSERT_TYPE("if", a, 2, LVAL_QEXPR);

  /* Mark Both Expressions as evaluable */
  lval* x;
  a->cell[1]->type = LVAL_SEXPR;
  a->cell[2]->type = LVAL_SEXPR;

  if (a->cell[0]->num) {
    /* If condition is true evaluate first expression */
    x = lval_eval(e, lval_pop(a, 1));
  } else {
    /* Otherwise evaluate second expression */
    x = lval_eval(e, lval_pop(a, 2));
  }

  /* Delete argument list and return */
  lval_del(a);
  return x;
}

剩下的就是让我们注册这些新增内置函数。

/* Comparison Functions */
lenv_add_builtin(e, "if", builtin_if);
lenv_add_builtin(e, "==", builtin_eq);
lenv_add_builtin(e, "!=", builtin_ne);
lenv_add_builtin(e, ">",  builtin_gt);
lenv_add_builtin(e, "<",  builtin_lt);
lenv_add_builtin(e, ">=", builtin_ge);
lenv_add_builtin(e, "<=", builtin_le);

测试一下新增内置函数，检查Lispy一切是否工作正常。

lispy> > 10 5
1
lispy> <= 88 5
0
lispy> == 5 6
0
lispy> == 5 {}
0
lispy> == 1 1
1
lispy> != {} 56
1
lispy> == {1 2 3 {5 6}} {1   2  3   {5 6}}
1
lispy> def {x y} 100 200
()
lispy> if (== x y) {+ x y} {- x y}
-100

1.5. 递归函数

引入条件分支，实际上让我们的语言变得更加强大。 因为条件分支可以有效地让我们实现递归函数。

递归函数是那些自己调用自己的函数。 我们已经在C中使用这些来执行表达式的读取和求值。 我们需要条件分支的原因是因为它们让我们检验我们想要终止递归的情况。

例如，我们可以使用条件分支来实现一个函数len，该函数返回列表中元素个数。 如果我们遇到空列表，我们只返回0。 否则我们返回输入列表的tail的长度加上1。 想想为什么会这样。 它重复调用len函数，直到遇到空列表。 此时它返回0并将所有其他部分结果加在一起。

(fun {len l} {
  if (== l {})
    {0}
    {+ 1 (len (tail l))}
})

就像在C中一样，递归函数具有令人愉悦的对称性。 首先，我们对空列表（基线条件）做一些处理。 然后，如果我们得到更大的列表，我们就会取出一个部分，例如列表的头部元素，并对它做一些处理，然后再将它与已经调用了该函数的其余部分进行组合。

这是另一个递归函数——反转列表。 和之前一样，函数检查列表是否空列表，但该函数中基线条件的处理是返回空列表。这是合理的，因为空列表的反转形式同样也是空列表。 但如果是更大的列表，函数会使尾部反转，并将其放在头部前面。

(fun {reverse l} {
  if (== l {})
    {{}}
    {join (reverse (tail l)) (head l)}
})

我们将使用递归来构建许多函数。 因为这将成为在我们的语言中实现循环的主要方式。

1.6. 彩蛋

  • 实现内置逻辑运算符或||，与&&以及非not并把它们添加到我们的Lisp中。
  • 定义一个Lisp递归函数，该函数返回列表的第n项。
  • 定义一个Lisp递归函数，该函数判断元素是否在列表中，是返回1，否则返回0。
  • 定义一个Lisp函数，该函数返回列表最后一项。
  • 定义一个Lisp逻辑运算符函数比如or，and和not。
  • 通过内置变量true和false为我们的Lisp添加特定的布尔类型。

1.7. 参考

conditionals.c
#include "mpc.h"

#ifdef _WIN32

static char buffer[2048];

char* readline(char* prompt) {
  fputs(prompt, stdout);
  fgets(buffer, 2048, stdin);
  char* cpy = malloc(strlen(buffer)+1);
  strcpy(cpy, buffer);
  cpy[strlen(cpy)-1] = '\0';
  return cpy;
}

void add_history(char* unused) {}

#else
#include <editline/readline.h>
#include <editline/history.h>
#endif

/* Forward Declarations */

struct lval;
struct lenv;
typedef struct lval lval;
typedef struct lenv lenv;

/* Lisp Value */

enum { LVAL_ERR, LVAL_NUM, LVAL_SYM, LVAL_FUN, LVAL_SEXPR, LVAL_QEXPR };

typedef lval*(*lbuiltin)(lenv*, lval*);

struct lval {
  int type;

  /* Basic */
  long num;
  char* err;
  char* sym;

  /* Function */
  lbuiltin builtin;
  lenv* env;
  lval* formals;
  lval* body;

  /* Expression */
  int count;
  lval** cell;
};

lval* lval_num(long x) {
  lval* v = malloc(sizeof(lval));
  v->type = LVAL_NUM;
  v->num = x;
  return v;
}

lval* lval_err(char* fmt, ...) {
  lval* v = malloc(sizeof(lval));
  v->type = LVAL_ERR;  
  va_list va;
  va_start(va, fmt);  
  v->err = malloc(512);  
  vsnprintf(v->err, 511, fmt, va);  
  v->err = realloc(v->err, strlen(v->err)+1);
  va_end(va);  
  return v;
}

lval* lval_sym(char* s) {
  lval* v = malloc(sizeof(lval));
  v->type = LVAL_SYM;
  v->sym = malloc(strlen(s) + 1);
  strcpy(v->sym, s);
  return v;
}

lval* lval_builtin(lbuiltin func) {
  lval* v = malloc(sizeof(lval));
  v->type = LVAL_FUN;
  v->builtin = func;
  return v;
}

lenv* lenv_new(void);

lval* lval_lambda(lval* formals, lval* body) {
  lval* v = malloc(sizeof(lval));
  v->type = LVAL_FUN;  
  v->builtin = NULL;  
  v->env = lenv_new();  
  v->formals = formals;
  v->body = body;
  return v;  
}

lval* lval_sexpr(void) {
  lval* v = malloc(sizeof(lval));
  v->type = LVAL_SEXPR;
  v->count = 0;
  v->cell = NULL;
  return v;
}

lval* lval_qexpr(void) {
  lval* v = malloc(sizeof(lval));
  v->type = LVAL_QEXPR;
  v->count = 0;
  v->cell = NULL;
  return v;
}

void lenv_del(lenv* e);

void lval_del(lval* v) {

  switch (v->type) {
    case LVAL_NUM: break;
    case LVAL_FUN: 
      if (!v->builtin) {
        lenv_del(v->env);
        lval_del(v->formals);
        lval_del(v->body);
      }
    break;
    case LVAL_ERR: free(v->err); break;
    case LVAL_SYM: free(v->sym); break;
    case LVAL_QEXPR:
    case LVAL_SEXPR:
      for (int i = 0; i < v->count; i++) {
        lval_del(v->cell[i]);
      }
      free(v->cell);
    break;
  }

  free(v);
}

lenv* lenv_copy(lenv* e);

lval* lval_copy(lval* v) {
  lval* x = malloc(sizeof(lval));
  x->type = v->type;
  switch (v->type) {
    case LVAL_FUN:
      if (v->builtin) {
        x->builtin = v->builtin;
      } else {
        x->builtin = NULL;
        x->env = lenv_copy(v->env);
        x->formals = lval_copy(v->formals);
        x->body = lval_copy(v->body);
      }
    break;
    case LVAL_NUM: x->num = v->num; break;
    case LVAL_ERR: x->err = malloc(strlen(v->err) + 1);
      strcpy(x->err, v->err);
    break;
    case LVAL_SYM: x->sym = malloc(strlen(v->sym) + 1);
      strcpy(x->sym, v->sym);
    break;
    case LVAL_SEXPR:
    case LVAL_QEXPR:
      x->count = v->count;
      x->cell = malloc(sizeof(lval*) * x->count);
      for (int i = 0; i < x->count; i++) {
        x->cell[i] = lval_copy(v->cell[i]);
      }
    break;
  }
  return x;
}

lval* lval_add(lval* v, lval* x) {
  v->count++;
  v->cell = realloc(v->cell, sizeof(lval*) * v->count);
  v->cell[v->count-1] = x;
  return v;
}

lval* lval_join(lval* x, lval* y) {  
  for (int i = 0; i < y->count; i++) {
    x = lval_add(x, y->cell[i]);
  }
  free(y->cell);
  free(y);  
  return x;
}

lval* lval_pop(lval* v, int i) {
  lval* x = v->cell[i];  
  memmove(&v->cell[i],
    &v->cell[i+1], sizeof(lval*) * (v->count-i-1));  
  v->count--;  
  v->cell = realloc(v->cell, sizeof(lval*) * v->count);
  return x;
}

lval* lval_take(lval* v, int i) {
  lval* x = lval_pop(v, i);
  lval_del(v);
  return x;
}

void lval_print(lval* v);

void lval_print_expr(lval* v, char open, char close) {
  putchar(open);
  for (int i = 0; i < v->count; i++) {
    lval_print(v->cell[i]);    
    if (i != (v->count-1)) {
      putchar(' ');
    }
  }
  putchar(close);
}

void lval_print(lval* v) {
  switch (v->type) {
    case LVAL_FUN:
      if (v->builtin) {
        printf("<builtin>");
      } else {
        printf("(\\ ");
        lval_print(v->formals);
        putchar(' ');
        lval_print(v->body);
        putchar(')');
      }
    break;
    case LVAL_NUM:   printf("%li", v->num); break;
    case LVAL_ERR:   printf("Error: %s", v->err); break;
    case LVAL_SYM:   printf("%s", v->sym); break;
    case LVAL_SEXPR: lval_print_expr(v, '(', ')'); break;
    case LVAL_QEXPR: lval_print_expr(v, '{', '}'); break;
  }
}

void lval_println(lval* v) { lval_print(v); putchar('\n'); }

int lval_eq(lval* x, lval* y) {

  /* Different Types are always unequal */
  if (x->type != y->type) { return 0; }

  /* Compare Based upon type */
  switch (x->type) {
    /* Compare Number Value */
    case LVAL_NUM: return (x->num == y->num);

    /* Compare String Values */
    case LVAL_ERR: return (strcmp(x->err, y->err) == 0);
    case LVAL_SYM: return (strcmp(x->sym, y->sym) == 0);

    /* If Builtin compare functions, otherwise compare formals and body */
    case LVAL_FUN:
      if (x->builtin || y->builtin) {
        return x->builtin == y->builtin;
      } else {
        return lval_eq(x->formals, y->formals) && lval_eq(x->body, y->body);
      }

    /* If list compare every individual element */
    case LVAL_QEXPR:
    case LVAL_SEXPR:
      if (x->count != y->count) { return 0; }
      for (int i = 0; i < x->count; i++) {
        /* If any element not equal then whole list not equal */
        if (!lval_eq(x->cell[i], y->cell[i])) { return 0; }
      }
      /* Otherwise lists must be equal */
      return 1;
    break;
  }
  return 0;
}

char* ltype_name(int t) {
  switch(t) {
    case LVAL_FUN: return "Function";
    case LVAL_NUM: return "Number";
    case LVAL_ERR: return "Error";
    case LVAL_SYM: return "Symbol";
    case LVAL_SEXPR: return "S-Expression";
    case LVAL_QEXPR: return "Q-Expression";
    default: return "Unknown";
  }
}

/* Lisp Environment */

struct lenv {
  lenv* par;
  int count;
  char** syms;
  lval** vals;
};

lenv* lenv_new(void) {
  lenv* e = malloc(sizeof(lenv));
  e->par = NULL;
  e->count = 0;
  e->syms = NULL;
  e->vals = NULL;
  return e;
}

void lenv_del(lenv* e) {
  for (int i = 0; i < e->count; i++) {
    free(e->syms[i]);
    lval_del(e->vals[i]);
  }  
  free(e->syms);
  free(e->vals);
  free(e);
}

lenv* lenv_copy(lenv* e) {
  lenv* n = malloc(sizeof(lenv));
  n->par = e->par;
  n->count = e->count;
  n->syms = malloc(sizeof(char*) * n->count);
  n->vals = malloc(sizeof(lval*) * n->count);
  for (int i = 0; i < e->count; i++) {
    n->syms[i] = malloc(strlen(e->syms[i]) + 1);
    strcpy(n->syms[i], e->syms[i]);
    n->vals[i] = lval_copy(e->vals[i]);
  }
  return n;
}

lval* lenv_get(lenv* e, lval* k) {

  for (int i = 0; i < e->count; i++) {
    if (strcmp(e->syms[i], k->sym) == 0) { return lval_copy(e->vals[i]); }
  }

  if (e->par) {
    return lenv_get(e->par, k);
  } else {
    return lval_err("Unbound Symbol '%s'", k->sym);
  }
}

void lenv_put(lenv* e, lval* k, lval* v) {

  for (int i = 0; i < e->count; i++) {
    if (strcmp(e->syms[i], k->sym) == 0) {
      lval_del(e->vals[i]);
      e->vals[i] = lval_copy(v);
      return;
    }
  }

  e->count++;
  e->vals = realloc(e->vals, sizeof(lval*) * e->count);
  e->syms = realloc(e->syms, sizeof(char*) * e->count);  
  e->vals[e->count-1] = lval_copy(v);
  e->syms[e->count-1] = malloc(strlen(k->sym)+1);
  strcpy(e->syms[e->count-1], k->sym);
}

void lenv_def(lenv* e, lval* k, lval* v) {
  while (e->par) { e = e->par; }
  lenv_put(e, k, v);
}

/* Builtins */

#define LASSERT(args, cond, fmt, ...) \
  if (!(cond)) { lval* err = lval_err(fmt, ##__VA_ARGS__); lval_del(args); return err; }

#define LASSERT_TYPE(func, args, index, expect) \
  LASSERT(args, args->cell[index]->type == expect, \
    "Function '%s' passed incorrect type for argument %i. Got %s, Expected %s.", \
    func, index, ltype_name(args->cell[index]->type), ltype_name(expect))

#define LASSERT_NUM(func, args, num) \
  LASSERT(args, args->count == num, \
    "Function '%s' passed incorrect number of arguments. Got %i, Expected %i.", \
    func, args->count, num)

#define LASSERT_NOT_EMPTY(func, args, index) \
  LASSERT(args, args->cell[index]->count != 0, \
    "Function '%s' passed {} for argument %i.", func, index);

lval* lval_eval(lenv* e, lval* v);

lval* builtin_lambda(lenv* e, lval* a) {
  LASSERT_NUM("\\", a, 2);
  LASSERT_TYPE("\\", a, 0, LVAL_QEXPR);
  LASSERT_TYPE("\\", a, 1, LVAL_QEXPR);

  for (int i = 0; i < a->cell[0]->count; i++) {
    LASSERT(a, (a->cell[0]->cell[i]->type == LVAL_SYM),
      "Cannot define non-symbol. Got %s, Expected %s.",
      ltype_name(a->cell[0]->cell[i]->type), ltype_name(LVAL_SYM));
  }

  lval* formals = lval_pop(a, 0);
  lval* body = lval_pop(a, 0);
  lval_del(a);

  return lval_lambda(formals, body);
}

lval* builtin_list(lenv* e, lval* a) {
  a->type = LVAL_QEXPR;
  return a;
}

lval* builtin_head(lenv* e, lval* a) {
  LASSERT_NUM("head", a, 1);
  LASSERT_TYPE("head", a, 0, LVAL_QEXPR);
  LASSERT_NOT_EMPTY("head", a, 0);

  lval* v = lval_take(a, 0);  
  while (v->count > 1) { lval_del(lval_pop(v, 1)); }
  return v;
}

lval* builtin_tail(lenv* e, lval* a) {
  LASSERT_NUM("tail", a, 1);
  LASSERT_TYPE("tail", a, 0, LVAL_QEXPR);
  LASSERT_NOT_EMPTY("tail", a, 0);

  lval* v = lval_take(a, 0);  
  lval_del(lval_pop(v, 0));
  return v;
}

lval* builtin_eval(lenv* e, lval* a) {
  LASSERT_NUM("eval", a, 1);
  LASSERT_TYPE("eval", a, 0, LVAL_QEXPR);

  lval* x = lval_take(a, 0);
  x->type = LVAL_SEXPR;
  return lval_eval(e, x);
}

lval* builtin_join(lenv* e, lval* a) {

  for (int i = 0; i < a->count; i++) {
    LASSERT_TYPE("join", a, i, LVAL_QEXPR);
  }

  lval* x = lval_pop(a, 0);

  while (a->count) {
    lval* y = lval_pop(a, 0);
    x = lval_join(x, y);
  }

  lval_del(a);
  return x;
}

lval* builtin_op(lenv* e, lval* a, char* op) {

  for (int i = 0; i < a->count; i++) {
    LASSERT_TYPE(op, a, i, LVAL_NUM);
  }

  lval* x = lval_pop(a, 0);

  if ((strcmp(op, "-") == 0) && a->count == 0) { x->num = -x->num; }

  while (a->count > 0) {  
    lval* y = lval_pop(a, 0);

    if (strcmp(op, "+") == 0) { x->num += y->num; }
    if (strcmp(op, "-") == 0) { x->num -= y->num; }
    if (strcmp(op, "*") == 0) { x->num *= y->num; }
    if (strcmp(op, "/") == 0) {
      if (y->num == 0) {
        lval_del(x); lval_del(y);
        x = lval_err("Division By Zero.");
        break;
      }
      x->num /= y->num;
    }

    lval_del(y);
  }

  lval_del(a);
  return x;
}

lval* builtin_add(lenv* e, lval* a) { return builtin_op(e, a, "+"); }
lval* builtin_sub(lenv* e, lval* a) { return builtin_op(e, a, "-"); }
lval* builtin_mul(lenv* e, lval* a) { return builtin_op(e, a, "*"); }
lval* builtin_div(lenv* e, lval* a) { return builtin_op(e, a, "/"); }

lval* builtin_var(lenv* e, lval* a, char* func) {
  LASSERT_TYPE(func, a, 0, LVAL_QEXPR);

  lval* syms = a->cell[0];
  for (int i = 0; i < syms->count; i++) {
    LASSERT(a, (syms->cell[i]->type == LVAL_SYM),
      "Function '%s' cannot define non-symbol. "
      "Got %s, Expected %s.",
      func, ltype_name(syms->cell[i]->type), ltype_name(LVAL_SYM));
  }

  LASSERT(a, (syms->count == a->count-1),
    "Function '%s' passed too many arguments for symbols. "
    "Got %i, Expected %i.",
    func, syms->count, a->count-1);

  for (int i = 0; i < syms->count; i++) {
    if (strcmp(func, "def") == 0) { lenv_def(e, syms->cell[i], a->cell[i+1]); }
    if (strcmp(func, "=")   == 0) { lenv_put(e, syms->cell[i], a->cell[i+1]); } 
  }

  lval_del(a);
  return lval_sexpr();
}

lval* builtin_def(lenv* e, lval* a) { return builtin_var(e, a, "def"); }
lval* builtin_put(lenv* e, lval* a) { return builtin_var(e, a, "="); }

lval* builtin_ord(lenv* e, lval* a, char* op) {
  LASSERT_NUM(op, a, 2);
  LASSERT_TYPE(op, a, 0, LVAL_NUM);
  LASSERT_TYPE(op, a, 1, LVAL_NUM);

  int r;
  if (strcmp(op, ">")  == 0) {
    r = (a->cell[0]->num >  a->cell[1]->num);
  }
  if (strcmp(op, "<")  == 0) {
    r = (a->cell[0]->num <  a->cell[1]->num);
  }
  if (strcmp(op, ">=") == 0) {
    r = (a->cell[0]->num >= a->cell[1]->num);
  }
  if (strcmp(op, "<=") == 0) {
    r = (a->cell[0]->num <= a->cell[1]->num);
  }
  lval_del(a);
  return lval_num(r);
}

lval* builtin_gt(lenv* e, lval* a) {
  return builtin_ord(e, a, ">");
}

lval* builtin_lt(lenv* e, lval* a) {
  return builtin_ord(e, a, "<");
}

lval* builtin_ge(lenv* e, lval* a) {
  return builtin_ord(e, a, ">=");
}

lval* builtin_le(lenv* e, lval* a) {
  return builtin_ord(e, a, "<=");
}

lval* builtin_cmp(lenv* e, lval* a, char* op) {
  LASSERT_NUM(op, a, 2);
  int r;
  if (strcmp(op, "==") == 0) {
    r =  lval_eq(a->cell[0], a->cell[1]);
  }
  if (strcmp(op, "!=") == 0) {
    r = !lval_eq(a->cell[0], a->cell[1]);
  }
  lval_del(a);
  return lval_num(r);
}

lval* builtin_eq(lenv* e, lval* a) {
  return builtin_cmp(e, a, "==");
}

lval* builtin_ne(lenv* e, lval* a) {
  return builtin_cmp(e, a, "!=");
}

lval* builtin_if(lenv* e, lval* a) {
  LASSERT_NUM("if", a, 3);
  LASSERT_TYPE("if", a, 0, LVAL_NUM);
  LASSERT_TYPE("if", a, 1, LVAL_QEXPR);
  LASSERT_TYPE("if", a, 2, LVAL_QEXPR);

  /* Mark Both Expressions as evaluable */
  lval* x;
  a->cell[1]->type = LVAL_SEXPR;
  a->cell[2]->type = LVAL_SEXPR;

  if (a->cell[0]->num) {
    /* If condition is true evaluate first expression */
    x = lval_eval(e, lval_pop(a, 1));
  } else {
    /* Otherwise evaluate second expression */
    x = lval_eval(e, lval_pop(a, 2));
  }

  /* Delete argument list and return */
  lval_del(a);
  return x;
}

void lenv_add_builtin(lenv* e, char* name, lbuiltin func) {
  lval* k = lval_sym(name);
  lval* v = lval_builtin(func);
  lenv_put(e, k, v);
  lval_del(k); lval_del(v);
}

void lenv_add_builtins(lenv* e) {
  /* Variable Functions */
  lenv_add_builtin(e, "\\",  builtin_lambda); 
  lenv_add_builtin(e, "def", builtin_def);
  lenv_add_builtin(e, "=",   builtin_put);

  /* List Functions */
  lenv_add_builtin(e, "list", builtin_list);
  lenv_add_builtin(e, "head", builtin_head);
  lenv_add_builtin(e, "tail", builtin_tail);
  lenv_add_builtin(e, "eval", builtin_eval);
  lenv_add_builtin(e, "join", builtin_join);

  /* Mathematical Functions */
  lenv_add_builtin(e, "+", builtin_add);
  lenv_add_builtin(e, "-", builtin_sub);
  lenv_add_builtin(e, "*", builtin_mul);
  lenv_add_builtin(e, "/", builtin_div);

  /* Comparison Functions */
  lenv_add_builtin(e, "if",    builtin_if);
  lenv_add_builtin(e, "==",    builtin_eq);
  lenv_add_builtin(e, "!=",    builtin_ne);
  lenv_add_builtin(e, ">",  builtin_gt);
  lenv_add_builtin(e, "<",  builtin_lt);
  lenv_add_builtin(e, ">=", builtin_ge);
  lenv_add_builtin(e, "<=", builtin_le);
}

/* Evaluation */

lval* lval_call(lenv* e, lval* f, lval* a) {

  if (f->builtin) { return f->builtin(e, a); }

  int given = a->count;
  int total = f->formals->count;

  while (a->count) {

    if (f->formals->count == 0) {
      lval_del(a);
      return lval_err("Function passed too many arguments. "
        "Got %i, Expected %i.", given, total); 
    }

    lval* sym = lval_pop(f->formals, 0);

    if (strcmp(sym->sym, "&") == 0) {

      if (f->formals->count != 1) {
        lval_del(a);
        return lval_err("Function format invalid. "
          "Symbol '&' not followed by single symbol.");
      }

      lval* nsym = lval_pop(f->formals, 0);
      lenv_put(f->env, nsym, builtin_list(e, a));
      lval_del(sym); lval_del(nsym);
      break;
    }

    lval* val = lval_pop(a, 0);    
    lenv_put(f->env, sym, val);    
    lval_del(sym); lval_del(val);
  }

  lval_del(a);

  if (f->formals->count > 0 &&
    strcmp(f->formals->cell[0]->sym, "&") == 0) {

    if (f->formals->count != 2) {
      return lval_err("Function format invalid. "
        "Symbol '&' not followed by single symbol.");
    }

    lval_del(lval_pop(f->formals, 0));

    lval* sym = lval_pop(f->formals, 0);
    lval* val = lval_qexpr();    
    lenv_put(f->env, sym, val);
    lval_del(sym); lval_del(val);
  }

  if (f->formals->count == 0) {  
    f->env->par = e;    
    return builtin_eval(f->env, lval_add(lval_sexpr(), lval_copy(f->body)));
  } else {
    return lval_copy(f);
  }

}

lval* lval_eval_sexpr(lenv* e, lval* v) {

  for (int i = 0; i < v->count; i++) { v->cell[i] = lval_eval(e, v->cell[i]); }
  for (int i = 0; i < v->count; i++) { if (v->cell[i]->type == LVAL_ERR) { return lval_take(v, i); } }

  if (v->count == 0) { return v; }  
  if (v->count == 1) { return lval_eval(e, lval_take(v, 0)); }

  lval* f = lval_pop(v, 0);
  if (f->type != LVAL_FUN) {
    lval* err = lval_err(
      "S-Expression starts with incorrect type. "
      "Got %s, Expected %s.",
      ltype_name(f->type), ltype_name(LVAL_FUN));
    lval_del(f); lval_del(v);
    return err;
  }

  lval* result = lval_call(e, f, v);
  lval_del(f);
  return result;
}

lval* lval_eval(lenv* e, lval* v) {
  if (v->type == LVAL_SYM) {
    lval* x = lenv_get(e, v);
    lval_del(v);
    return x;
  }
  if (v->type == LVAL_SEXPR) { return lval_eval_sexpr(e, v); }
  return v;
}

/* Reading */

lval* lval_read_num(mpc_ast_t* t) {
  errno = 0;
  long x = strtol(t->contents, NULL, 10);
  return errno != ERANGE ? lval_num(x) : lval_err("Invalid Number.");
}

lval* lval_read(mpc_ast_t* t) {

  if (strstr(t->tag, "number")) { return lval_read_num(t); }
  if (strstr(t->tag, "symbol")) { return lval_sym(t->contents); }

  lval* x = NULL;
  if (strcmp(t->tag, ">") == 0) { x = lval_sexpr(); } 
  if (strstr(t->tag, "sexpr"))  { x = lval_sexpr(); }
  if (strstr(t->tag, "qexpr"))  { x = lval_qexpr(); }

  for (int i = 0; i < t->children_num; i++) {
    if (strcmp(t->children[i]->contents, "(") == 0) { continue; }
    if (strcmp(t->children[i]->contents, ")") == 0) { continue; }
    if (strcmp(t->children[i]->contents, "}") == 0) { continue; }
    if (strcmp(t->children[i]->contents, "{") == 0) { continue; }
    if (strcmp(t->children[i]->tag,  "regex") == 0) { continue; }
    x = lval_add(x, lval_read(t->children[i]));
  }

  return x;
}

/* Main */

int main(int argc, char** argv) {

  mpc_parser_t* Number = mpc_new("number");
  mpc_parser_t* Symbol = mpc_new("symbol");
  mpc_parser_t* Sexpr  = mpc_new("sexpr");
  mpc_parser_t* Qexpr  = mpc_new("qexpr");
  mpc_parser_t* Expr   = mpc_new("expr");
  mpc_parser_t* Lispy  = mpc_new("lispy");

  mpca_lang(MPCA_LANG_DEFAULT,
    "                                                     \
      number : /-?[0-9]+/ ;                               \
      symbol : /[a-zA-Z0-9_+\\-*\\/\\\\=<>!&]+/ ;         \
      sexpr  : '(' <expr>* ')' ;                          \
      qexpr  : '{' <expr>* '}' ;                          \
      expr   : <number> | <symbol> | <sexpr> | <qexpr> ;  \
      lispy  : /^/ <expr>* /$/ ;                          \
    ",
    Number, Symbol, Sexpr, Qexpr, Expr, Lispy);

  puts("Lispy Version 0.0.0.0.9");
  puts("Press Ctrl+c to Exit\n");

  lenv* e = lenv_new();
  lenv_add_builtins(e);

  while (1) {

    char* input = readline("lispy> ");
    add_history(input);

    mpc_result_t r;
    if (mpc_parse("<stdin>", input, Lispy, &r)) {

      lval* x = lval_eval(e, lval_read(r.output));
      lval_println(x);
      lval_del(x);

      mpc_ast_delete(r.output);
    } else {    
      mpc_err_print(r.error);
      mpc_err_delete(r.error);
    }

    free(input);

  }

  lenv_del(e);

  mpc_cleanup(6, Number, Symbol, Sexpr, Qexpr, Expr, Lispy);

  return 0;
}

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