目录

前言

通过开发一门类 Lisp 的编程语言来理解编程语言的设计思想，本实践来自著名的《Build Your Own Lisp》。

• 代码实现：https://github.com/JmilkFan/Lispy

前文列表

《用 C 语言开发一门编程语言 — 交互式解析器》
《用 C 语言开发一门编程语言 — 语法解析器运行原理》
《用 C 语言开发一门编程语言 — 波兰表达式解析器》
《用 C 语言开发一门编程语言 — 表达式存储器》
《用 C 语言开发一门编程语言 — 符号表达式解析器》
《用 C 语言开发一门编程语言 — 引用表达式解析器》
《用 C 语言开发一门编程语言 — 变量的设计与实现》

Lambda 表达式

Lambda 表达式（Lambda Expression），命名来自数学中的 λ 运算，是一种简单而强大的函数定义方法。在编程语言中，Lambda 表达式是一种用于定义函数的函数，可以在运行时创建，并赋值给给其他函数。

例如 Python lambda：

lambda arguments: expression

在以往的文章中，我们实现了 S-Expression、Q-Expression 和 Variable，有了这些条件，我们就可以继续实现基于 Lambda 表达式的函数定义机制了。

Lambda 表达式解析器

语法规则定义

首先，我们还是要定义 Lambda Expression 的语法规则：

• 使用 / 符号来作为声明标识，这是为了向 Lambda 表达式致敬。
• 随后紧跟形式参数列表和函数定义。

例如：

\ {x y} {+ x y}

然后，将 Lambda Expression 与 S-Expresion 进行结合，以接受实际参数，并进行运算：

(\ {x y} {+ x y}) 10 20

最后，再应用前文中实现的 “变量元素“ def 将 Lambda Expression 的函数定义赋值给一个 “别名“：

def {add-together} (\ {x y} {+ x y})

如此的，开发者就可以在后续进行函数调用了：

add-together 10 20

存储器实现

从上述语法规则的设计，Lambda Expression 应该由以下 3 个部分组成：

1. 形参列表
2. Q-Expression
3. 实参列表

我们继续扩展 Lispy Values（用户输入数据存储器）的功能，为 Lambda Expression 添加 formals 和 body 字段。并且，我们也继续扩展 LAVL_FUN 类型来同时表示内建函数和自定义的函数，通过 lbuiltin 函数指针是否为 NULL 来进行区别。

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;
};
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同样的，继续完成构造函数、析构函数、复制部分、打印部分的填充：

// 构造函数
lval* lval_lambda(lval* formals, lval* body) {
  lval* v = malloc(sizeof(lval));
  v->type = LVAL_FUN;

  /* Set Builtin to Null */
  v->builtin = NULL;

  /* Build new environment */
  v->env = lenv_new();

  /* Set Formals and Body */
  v->formals = formals;
  v->body = body;
  return v;
}

// 析构函数
case LVAL_FUN:
  if (!v->builtin) {
    lenv_del(v->env);
    lval_del(v->formals);
    lval_del(v->body);
  }
break;


// 复制的部分
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;


// 打印的部分
case LVAL_FUN:
  if (v->builtin) {
    printf("<builtin>");
  } else {
    printf("(\\ "); lval_print(v->formals);
    putchar(' '); lval_print(v->body); putchar(')');
  }
break;
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Lambda Function

内建函数实现

继续实现内建的 Lambda Function，类似前文实现的 Variable Function（def），需要检查类型是否正确，接着做其他的操作：

lval* builtin_lambda(lenv* e, lval* a) {
  /* Check Two arguments, each of which are Q-Expressions */
  LASSERT_NUM("\\", a, 2);
  LASSERT_TYPE("\\", a, 0, LVAL_QEXPR);
  LASSERT_TYPE("\\", a, 1, LVAL_QEXPR);

  /* Check first Q-Expression contains only Symbols */
  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));
  }

  /* Pop first two arguments and pass them to lval_lambda */
  lval* formals = lval_pop(a, 0);
  lval* body = lval_pop(a, 0);
  lval_del(a);

  return lval_lambda(formals, body);
}
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全局变量和局部变量实现

一个理想的状态，编程语言可以为每个函数提供相关的运行时环境，例如：函数栈帧，函数可以在这个环境中完成参数的传递和运算。

但目前为止，我们还只是实现了一个全局的交互式环境。所以，现在我们需要修改 Lenv 的定义，通过引入父子关系来实现局部的函数执行环境。

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;
}
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引入父子环境变量之后，需要改造变量读取函数，增加遍历特性。

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 no symbol check in parent otherwise error */
  if (e->par) {
    return lenv_get(e->par, k);
  } else {
    return lval_err("Unbound Symbol '%s'", k->sym);
  }
}
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同时，当我们使用 lval 结构体时，还需要一个新的函数来完成 “环境” 的复制：

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;
}
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拥有父环境也改变了我们定义一个变量的方式也改变了，现在有 2 种方法可以定义一个变量：

1. 可以在本地，最内层环境中定义它，
2. 也可以在全局，最外层环境中定义它。

所以，我们将 lenv_put 函数保持不变，用于在本地环境中的变量定义。另外，在增加 lenv_def 函数用于在全局环境中的变量定义。稍后我们将使用它来将部分计算结果写入到函数内的局部变量中。

void lenv_def(lenv* e, lval* k, lval* v) {
  /* Iterate till e has no parent */
  while (e->par) { e = e->par; }
  /* Put value in e */
  lenv_put(e, k, v);
}
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最后将新函数注册到函数路由器中，以便开发者可以在交互式解析器使用对应的符号（关键字）。

lenv_add_builtin(e, "def", builtin_def);
lenv_add_builtin(e, "=",   builtin_put);

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_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 'def' define in globally. If 'put' define in locally */
    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();
}
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函数调用实现

当这个函数是一个内置函数时，我们可以像以前一样通过函数指针的方式来调用它。

但当这个函数是一个自定义函数时，我们需要将传入的每个实际参数都绑定到 formals 字段，完成后，再计算 body 字段，此时会使用到 env 字段来作为函数调用的运算环境。

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

  /* If Builtin then simply apply that */
  if (f->builtin) { return f->builtin(e, a); }

  /* Record Argument Counts */
  int given = a->count;
  int total = f->formals->count;

  /* While arguments still remain to be processed */
  while (a->count) {

    /* If we've ran out of formal arguments to bind */
    if (f->formals->count == 0) {
      lval_del(a); return lval_err(
        "Function passed too many arguments. "
        "Got %i, Expected %i.", given, total);
    }

    /* Pop the first symbol from the formals */
    lval* sym = lval_pop(f->formals, 0);

    /* Pop the next argument from the list */
    lval* val = lval_pop(a, 0);

    /* Bind a copy into the function's environment */
    lenv_put(f->env, sym, val);

    /* Delete symbol and value */
    lval_del(sym); lval_del(val);
  }

  /* Argument list is now bound so can be cleaned up */
  lval_del(a);

  /* If all formals have been bound evaluate */
  if (f->formals->count == 0) {

    /* Set environment parent to evaluation environment */
    f->env->par = e;

    /* Evaluate and return */
    return builtin_eval(
      f->env, lval_add(lval_sexpr(), lval_copy(f->body)));
  } else {
    /* Otherwise return partially evaluated function */
    return lval_copy(f);
  }

}
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更新 lval_eval_sexpr 函数来调用 lval_call：

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);
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可变长形参列表实现

我们希望内建的函数可以具有可变长形参列表的特性，用于接受可变数量的实参，例如：+ 和 join 这样的函数可以取任意数量的参数，并在逻辑上对它们进行操作。

因此，我们将使用 & 符号，让用户可以定义看起来像 {x &。xs} 这样的形式的参数列表，类似于 C 语言中可变长形参符号 ...。这意味着一个函数将接受一个参数 x，随后跟着由零个或多个其他参数连接在一起所组成的一个名为 xs 的列表。

/* 检索符号字符串中是否存在 & 可变长形参标识符。*/
if (strcmp(sym->sym, "&") == 0) {

  /* 检查 & 标识符后，是否紧跟着一个符号，如果不是，则抛出一个错误。*/
  if (f->formals->count != 1) {
    lval_del(a);
    return lval_err("Function format invalid. "
      "Symbol '&' not followed by single symbol.");
  }

  /* Next formal should be bound to remaining arguments */
  lval* nsym = lval_pop(f->formals, 0);
  lenv_put(f->env, nsym, builtin_list(e, a));
  lval_del(sym); lval_del(nsym);
  break;
}
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假设调用函数时，用户不提供任何参数，而只提供了作为函数命的第一个参数。在这种情况下，我们需要在空列表后面设置符号。并且将这个特殊情况添加到删除参数列表之后，检查所有的 formal 求值之前。

/* If '&' remains in formal list bind to empty list */
if (f->formals->count > 0 &&
  strcmp(f->formals->cell[0]->sym, "&") == 0) {

  /* Check to ensure that & is not passed invalidly. */
  if (f->formals->count != 2) {
    return lval_err("Function format invalid. "
      "Symbol '&' not followed by single symbol.");
  }

  /* Pop and delete '&' symbol */
  lval_del(lval_pop(f->formals, 0));

  /* Pop next symbol and create empty list */
  lval* sym = lval_pop(f->formals, 0);
  lval* val = lval_qexpr();

  /* Bind to environment and delete */
  lenv_put(f->env, sym, val);
  lval_del(sym); lval_del(val);
}
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源代码

#include <stdio.h>
#include <stdlib.h>
#include "mpc.h"

#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);

#ifdef _WIN32
#include <string.h>

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

#ifdef __linux__
#include <readline/readline.h>
#include <readline/history.h>
#endif

#ifdef __MACH__
#include <readline/readline.h>
#endif

#endif

/* Forward Declarations */
struct lval;
struct lenv;
typedef struct lval lval;
typedef struct lenv lenv;

/* Lisp Value Type Enumeration */
enum {
    LVAL_NUM,
    LVAL_ERR,
    LVAL_SYM,
    LVAL_FUN,
    LVAL_SEXPR,
    LVAL_QEXPR
};

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

/* Declare lisp lval Struct */
struct lval {
    int type;

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

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

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


/* Construct a pointer to a new Number lval */
lval *lval_num(long x) {
    lval *v = malloc(sizeof(lval));
    v->type = LVAL_NUM;
    v->num = x;
    return v;
}

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";
    }
}

/* Construct a pointer to a new Error lval */
lval *lval_err(char *fmt, ...) {
    lval *v = malloc(sizeof(lval));
    v->type = LVAL_ERR;
    /* Create a va list and initialize it */
    va_list va;
    va_start(va, fmt);

    /* Allocate 512 bytes of space */
    v->err = malloc(512);

    /* printf the error string with a maximum of 511 characters */
    vsnprintf(v->err, 511, fmt, va);

    /* Reallocate to number of bytes actually used */
    v->err = realloc(v->err, strlen(v->err)+1);

    /* Cleanup our va list */
    va_end(va);

    return v;
}

/* Construct a pointer to a new Symbol lval */
lval *lval_sym(char *sym) {
    lval *v = malloc(sizeof(lval));
    v->type = LVAL_SYM;
    v->sym = malloc(strlen(sym) + 1);
    strcpy(v->sym, sym);
    return v;
}

/* A pointer to a new empty Sexpr lval */
lval *lval_sexpr(void) {
    lval *v = malloc(sizeof(lval));
    v->type = LVAL_SEXPR;
    v->count = 0;
    v->cell = NULL;
    return v;
}

/* A pointer to a new empty Qexpr lval */
lval *lval_qexpr(void) {
    lval *v = malloc(sizeof(lval));
    v->type = LVAL_QEXPR;
    v->count = 0;
    v->cell = NULL;
    return v;
}

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

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;
}


lval *lval_lambda(lval *formals, lval *body) {
    lval *v = malloc(sizeof(lval));
    v->type = LVAL_FUN;

    /* Set Builtin to Null */
    v->builtin = NULL;

    /* Build new environment */
    v->env = lenv_new();

    /* Set Formals and Body */
    v->formals = formals;
    v->body = body;
    return v;
}

void lenv_del(lenv *e);

void lval_del(lval *v) {
    switch (v->type) {
        /* Do nothing special for number type */
        case LVAL_NUM:
            break;

        /* For Err or Sym free the string data */
        case LVAL_ERR:
            free(v->err);
            break;
        case LVAL_SYM:
            free(v->sym);
            break;

        case LVAL_FUN:
            if (!v->builtin) {
                lenv_del(v->env);
                lval_del(v->formals);
                lval_del(v->body);
            }
            break;

        /* If Qexpr or Sexpr then delete all elements inside */
        case LVAL_QEXPR:
        case LVAL_SEXPR:
            for (int i = 0; i < v->count; i++) {
                lval_del(v->cell[i]);
            }
            /* Also free the memory allocated to contain the pointers */
            free(v->cell);
            break;
    }
    /* Free the memory allocated for the "lval" struct itself */
    free(v);
}


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);
}


lval *lval_copy(lval *v);

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 *lval_copy(lval *v) {
    lval *x = malloc(sizeof(lval));
    x->type = v->type;

    switch (v->type) {
        /* Copy Functions and Numbers Directly */
        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;

        /* Copy Strings using malloc and strcpy */
        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;

         /* Copy Lists by copying each sub-expression */
        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 *lenv_get(lenv *e, lval *k) {
    /* Iterate over all items in environment */
    for (int i = 0; i < e->count; i++) {
        /* Check if the stored string matches the symbol string */
        /* If it does, return a copy of the value */
        if (strcmp(e->syms[i], k->sym) == 0) {
            return lval_copy(e->vals[i]);
        }
    }

    /* If no symbol check in parent otherwise error */
    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) {
    /* Iterate over all items in environment */
    /* This is to see if variable already exists */
    for (int i = 0; i < e->count; i++) {
        /* If variable is found delete item at that position */
        /* And replace with variable supplied by user */
        if (strcmp(e->syms[i], k->sym) == 0) {
            lval_del(e->vals[i]);
            e->vals[i] = lval_copy(v);
            return;
        }
    }

    /* If no existing entry found allocate space for new entry */
    e->count++;
    e->vals = realloc(e->vals, sizeof(lval*) * e->count);
    e->syms = realloc(e->syms, sizeof(char*) * e->count);

    /* Copy contents of lval and symbol string into new location */
    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) {
    /* Iterate till e has no parent */
    while (e->par) {
        e = e->par;
    }
    /* Put value in e */
    lenv_put(e, k, v);
}


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_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 Symbol or Number return conversion to that type */
    if (strstr(t->tag, "number")) {
        return lval_read_num(t);
    }
    if (strstr(t->tag, "symbol")) {
        return lval_sym(t->contents);
    }

    /* If root (>) or sexpr then create empty list */
    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();
    }

    /* Fill this list with any valid expression contained within */
    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;
}


void lval_print(lval *v);

void lval_expr_print(lval *v, char open, char close) {
    putchar(open);
    for (int i = 0; i < v->count; i++) {

        /* Print Value contained within */
        lval_print(v->cell[i]);

        /* Don't print trailing space if last element */
        if (i != (v->count-1)) {
            putchar(' ');
        }
    }
    putchar(close);

}

/* Print an "lval*" */
void lval_print(lval *v) {
    switch (v->type) {
        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_expr_print(v, '(', ')'); break;
        case LVAL_QEXPR: lval_expr_print(v, '{', '}'); break;
        case LVAL_FUN:
            if (v->builtin) {
                printf("<builtin>");
            } else {
                printf("(\\ "); lval_print(v->formals);
                putchar(' ');
                lval_print(v->body);
                putchar(')');
            }
            break;
    }
}

/* Print an "lval" followed by a newline */
void lval_println(lval *v) {
    lval_print(v);
    putchar('\n');
}


lval *lval_pop(lval *v, int i) {

    /* Find the item at "i" */
    lval *x = v->cell[i];

    /* Shift memory after the item at "i" over the top */
    memmove(&v->cell[i], &v->cell[i+1],
            sizeof(lval*) * (v->count-i-1));

    /* Decrease the count of items in the list */
    v->count--;

    /* Reallocate the memory used */
    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;
}

lval *builtin_eval(lenv* e, lval *a);
lval *builtin_list(lenv *e, lval *a);

lval *lval_call(lenv *e, lval *f, lval *a) {
    /* If Builtin then simply apply that */
    if (f->builtin) {
        return f->builtin(e, a);
    }

    /* Record Argument Counts */
    int given = a->count;
    int total = f->formals->count;

    /* While arguments still remain to be processed */
    while (a->count) {
        /* If we've ran out of formal arguments to bind */
        if (f->formals->count == 0) {
            lval_del(a);
            return lval_err("Function passed too many arguments. "
                    "Got %i, Expected %i.", given, total);
        }

        /* Pop the first symbol from the formals */
        lval *sym = lval_pop(f->formals, 0);

        /* Special Case to deal with '&' */
        if (strcmp(sym->sym, "&") == 0) {
             /* Ensure '&' is followed by another symbol */
            if (f->formals->count != 1) {
                lval_del(a);
                return lval_err("Function format invalid. "
                                "Symbol '&' not followed by single symbol.");
            }

            /* Next formal should be bound to remaining arguments */
            lval *nsym = lval_pop(f->formals, 0);
            lenv_put(f->env, nsym, builtin_list(e, a));
            lval_del(sym); lval_del(nsym);
            break;
        }

        /* Pop the next argument from the list */
        lval* val = lval_pop(a, 0);

        /* Bind a copy into the function's environment */
        lenv_put(f->env, sym, val);

        /* Delete symbol and value */
        lval_del(sym); lval_del(val);
    }

    /* Argument list is now bound so can be cleaned up */
    lval_del(a);

    /* If '&' remains in formal list bind to empty list */
    if (f->formals->count > 0 && strcmp(f->formals->cell[0]->sym, "&") == 0) {
        /* Check to ensure that & is not passed invalidly. */
        if (f->formals->count != 2) {
            return lval_err("Function format invalid. "
                            "Symbol '&' not followed by single symbol.");
        }

        /* Pop and delete '&' symbol */
        lval_del(lval_pop(f->formals, 0));

        /* Pop next symbol and create empty list */
        lval* sym = lval_pop(f->formals, 0);
        lval* val = lval_qexpr();

        /* Bind to environment and delete */
        lenv_put(f->env, sym, val);
        lval_del(sym); lval_del(val);
    }

    /* If all formals have been bound evaluate */
    if (f->formals->count == 0) {

        /* Set environment parent to evaluation environment */
        f->env->par = e;

        /* Evaluate and return */
        return builtin_eval(f->env, lval_add(lval_sexpr(),
                            lval_copy(f->body)));
    } else {
        /* Otherwise return partially evaluated function */
        return lval_copy(f);
    }
}

lval *lval_eval(lenv *e, lval *v);
lval *builtin(lval* a, char* func);

lval *lval_eval_sexpr(lenv *e, lval *v) {
    /* Evaluate Children */
    for (int i = 0; i < v->count; i++) {
        v->cell[i] = lval_eval(e, v->cell[i]);
    }

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

    /* Empty Expression */
    if (v->count == 0) { return v; }

    /* Single Expression */
    if (v->count == 1) { return lval_take(v, 0); }

    /* Ensure first element is a function after evaluation */
    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;
    }

    /* Evaluate Sexpressions */
    if (v->type == LVAL_SEXPR) {
        return lval_eval_sexpr(e, v);
    }

    /* All other lval types remain the same */
    return v;
}

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

    /* Ensure all arguments are numbers */
    for (int i = 0; i < a->count; i++) {
        LASSERT_TYPE(op, a, i, LVAL_NUM);
    }

    /* Pop the first element */
    lval *x = lval_pop(a, 0);

    /* If no arguments and sub then perform unary negation */
    if ((strcmp(op, "-") == 0) && a->count == 0) {
        x->num = -x->num;
    }

    /* While there are still elements remaining */
    while (a->count > 0) {
        /* Pop the next element */
        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_head(lenv* e, lval *a) {
    LASSERT_NUM("head", a, 1);
    LASSERT_TYPE("head", a, 0, LVAL_QEXPR);
    LASSERT_NOT_EMPTY("head", a, 0);

    /* Otherwise take first argument */
    lval *v = lval_take(a, 0);

    /* Delete all elements that are not head and return */
    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);

    /* Take first argument */
    lval *v = lval_take(a, 0);

    /* Delete first element and return */
    lval_del(lval_pop(v, 0));

    return v;

}

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

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 *lval_join(lval *x, lval *y) {

    /* For each cell in 'y' add it to 'x' */
    while (y->count) {
         x = lval_add(x, lval_pop(y, 0));
    }

    /* Delete the empty 'y' and return 'x' */
    lval_del(y);
    return 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) {
        x = lval_join(x, lval_pop(a, 0));
    }

    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, "/");
}

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);
}

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 'def' define in globally. If 'put' define in locally */
        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_lambda(lenv *e, lval *a) {
    /* Check Two arguments, each of which are Q-Expressions */
    LASSERT_NUM("\\", a, 2);
    LASSERT_TYPE("\\", a, 0, LVAL_QEXPR);
    LASSERT_TYPE("\\", a, 1, LVAL_QEXPR);

    /* Check first Q-Expression contains only Symbols */
    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));
    }

    /* Pop first two arguments and pass them to lval_lambda */
    lval *formals = lval_pop(a, 0);
    lval *body = lval_pop(a, 0);
    lval_del(a);

    return lval_lambda(formals, body);
}

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


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

    /* Create Some Parsers */
    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");

    /* Define them with the following Language */
    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 0.1");
    puts("Press Ctrl+c to Exit\n");

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

    while(1) {

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

        /* Attempt to parse the user input */
        mpc_result_t r;

        if (mpc_parse("<stdin>", input, Lispy, &r)) {
            /* On success print and delete the AST */
            lval *x = lval_eval(e, lval_read(r.output));
            lval_println(x);
            lval_del(x);
            mpc_ast_delete(r.output);
        } else {
            /* Otherwise print and delete the Error */
            mpc_err_print(r.error);
            mpc_err_delete(r.error);
        }

        free(input);

    }

    lenv_del(e);

    /* Undefine and delete our parsers */
    mpc_cleanup(6, Number, Symbol, Sexpr, Qexpr, Expr, Lispy);

    return 0;
}
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编译：

gcc -g -std=c99 -Wall main.c mpc.c -lreadline -lm -o main

运行：

Lispy Version 0.1
Press Ctrl+c to Exit

lispy> def {add-mul} (\ {x y} {+ x (* x y)})
()
lispy> add-mul 10 20
210
lispy> add-mul 10
(\ {y} {+ x (* x y)})
lispy> def {add-mul-ten} (add-mul 10)
()
lispy> add-mul-ten 50
510
lispy>
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