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< ol class = "chapter" > < li class = "chapter-item expanded affix " > < a href = "../index.html" > Go语言圣经< / a > < / li > < li class = "chapter-item expanded affix " > < a href = "../preface.html" > 前言< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1.html" > < strong aria-hidden = "true" > 1.< / strong > 入门< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-01.html" > < strong aria-hidden = "true" > 1.1.< / strong > Hello, World< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-02.html" > < strong aria-hidden = "true" > 1.2.< / strong > 命令行参数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-03.html" > < strong aria-hidden = "true" > 1.3.< / strong > 查找重复的行< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-04.html" > < strong aria-hidden = "true" > 1.4.< / strong > GIF动画< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-05.html" > < strong aria-hidden = "true" > 1.5.< / strong > 获取URL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-06.html" > < strong aria-hidden = "true" > 1.6.< / strong > 并发获取多个URL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-07.html" > < strong aria-hidden = "true" > 1.7.< / strong > Web服务< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-08.html" > < strong aria-hidden = "true" > 1.8.< / strong > 本章要点< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2.html" > < strong aria-hidden = "true" > 2.< / strong > 程序结构< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-01.html" > < strong aria-hidden = "true" > 2.1.< / strong > 命名< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-02.html" > < strong aria-hidden = "true" > 2.2.< / strong > 声明< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-03.html" > < strong aria-hidden = "true" > 2.3.< / strong > 变量< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-04.html" > < strong aria-hidden = "true" > 2.4.< / strong > 赋值< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-05.html" > < strong aria-hidden = "true" > 2.5.< / strong > 类型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-06.html" > < strong aria-hidden = "true" > 2.6.< / strong > 包和文件< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-07.html" > < strong aria-hidden = "true" > 2.7.< / strong > 作用域< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3.html" > < strong aria-hidden = "true" > 3.< / strong > 基础数据类型< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-01.html" > < strong aria-hidden = "true" > 3.1.< / strong > 整型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-02.html" > < strong aria-hidden = "true" > 3.2.< / strong > 浮点数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-03.html" > < strong aria-hidden = "true" > 3.3.< / strong > 复数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-04.html" > < strong aria-hidden = "true" > 3.4.< / strong > 布尔型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-05.html" > < strong aria-hidden = "true" > 3.5.< / strong > 字符串< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-06.html" > < strong aria-hidden = "true" > 3.6.< / strong > 常量< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4.html" > < strong aria-hidden = "true" > 4.< / strong > 复合数据类型< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-01.html" > < strong aria-hidden = "true" > 4.1.< / strong > 数组< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-02.html" > < strong aria-hidden = "true" > 4.2.< / strong > Slice< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-03.html" > < strong aria-hidden = "true" > 4.3.< / strong > Map< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-04.html" > < strong aria-hidden = "true" > 4.4.< / strong > 结构体< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-05.html" > < strong aria-hidden = "true" > 4.5.< / strong > JSON< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-06.html" > < strong aria-hidd
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2022-08-24 10:48:15 +00:00
< li > < em > KusonStack一站式可编程配置技术栈(Go): < a href = "https://github.com/KusionStack/kusion" > https://github.com/KusionStack/kusion< / a > < / em > < / li >
< li > < em > KCL 配置编程语言(Rust): < a href = "https://github.com/KusionStack/KCLVM" > https://github.com/KusionStack/KCLVM< / a > < / em > < / li >
< li > < em > 凹语言™: < a href = "https://github.com/wa-lang/wa" > https://github.com/wa-lang/wa< / a > < / em > < / li >
2022-08-04 06:59:33 +00:00
< / ul >
< hr >
< h2 id = "79-示例-表达式求值" > < a class = "header" href = "#79-示例-表达式求值" > 7.9. 示例: 表达式求值< / a > < / h2 >
< p > 在本节中， ， < / p >
< pre > < code class = "language-go" > // An Expr is an arithmetic expression.
type Expr interface{}
< / code > < / pre >
< p > 我们的表达式语言包括浮点数符号（小数点）；二元操作符+， ， ， ； ， ， ； ； ： < / p >
< pre > < code class = "language-go" > sqrt(A / pi)
pow(x, 3) + pow(y, 3)
(F - 32) * 5 / 9
< / code > < / pre >
< p > 下面的五个具体类型表示了具体的表达式类型。Var类型表示对一个变量的引用。（ ） ， ； ， < / p >
< p > < u > < i > gopl.io/ch7/eval< / i > < / u > < / p >
< pre > < code class = "language-go" > // A Var identifies a variable, e.g., x.
type Var string
// A literal is a numeric constant, e.g., 3.141.
type literal float64
// A unary represents a unary operator expression, e.g., -x.
type unary struct {
op rune // one of '+', '-'
x Expr
}
// A binary represents a binary operator expression, e.g., x+y.
type binary struct {
op rune // one of '+', '-', '*', '/'
x, y Expr
}
// A call represents a function call expression, e.g., sin(x).
type call struct {
fn string // one of " pow" , " sin" , " sqrt"
args []Expr
}
< / code > < / pre >
< p > 为了计算一个包含变量的表达式， ： < / p >
< pre > < code class = "language-go" > type Env map[Var]float64
< / code > < / pre >
< p > 我们也需要每个表达式去定义一个Eval方法， ， ， ， < / p >
< pre > < code class = "language-go" > type Expr interface {
// Eval returns the value of this Expr in the environment env.
Eval(env Env) float64
}
< / code > < / pre >
< p > 下面给大家展示一个具体的Eval方法。Var类型的这个方法对一个environment变量进行查找， ， < / p >
< pre > < code class = "language-go" > func (v Var) Eval(env Env) float64 {
return env[v]
}
func (l literal) Eval(_ Env) float64 {
return float64(l)
}
< / code > < / pre >
< p > unary和binary的Eval方法会递归的计算它的运算对象， ， ， ， ， ， < / p >
< pre > < code class = "language-go" > func (u unary) Eval(env Env) float64 {
switch u.op {
case '+':
return +u.x.Eval(env)
case '-':
return -u.x.Eval(env)
}
panic(fmt.Sprintf(" unsupported unary operator: %q" , u.op))
}
func (b binary) Eval(env Env) float64 {
switch b.op {
case '+':
return b.x.Eval(env) + b.y.Eval(env)
case '-':
return b.x.Eval(env) - b.y.Eval(env)
case '*':
return b.x.Eval(env) * b.y.Eval(env)
case '/':
return b.x.Eval(env) / b.y.Eval(env)
}
panic(fmt.Sprintf(" unsupported binary operator: %q" , b.op))
}
func (c call) Eval(env Env) float64 {
switch c.fn {
case " pow" :
return math.Pow(c.args[0].Eval(env), c.args[1].Eval(env))
case " sin" :
return math.Sin(c.args[0].Eval(env))
case " sqrt" :
return math.Sqrt(c.args[0].Eval(env))
}
panic(fmt.Sprintf(" unsupported function call: %s" , c.fn))
}
< / code > < / pre >
< p > 一些方法会失败。例如， < 去构建一个unary或者binary表达式也是可能会发生的（ ） ， ， ， < / p >
< p > 下面的TestEval函数是对evaluator的一个测试。它使用了我们会在第11章讲解的testing包， ， ， ， < / p >
< pre > < code class = "language-go" > func TestEval(t *testing.T) {
tests := []struct {
expr string
env Env
want string
}{
{" sqrt(A / pi)" , Env{" A" : 87616, " pi" : math.Pi}, " 167" },
{" pow(x, 3) + pow(y, 3)" , Env{" x" : 12, " y" : 1}, " 1729" },
{" pow(x, 3) + pow(y, 3)" , Env{" x" : 9, " y" : 10}, " 1729" },
{" 5 / 9 * (F - 32)" , Env{" F" : -40}, " -40" },
{" 5 / 9 * (F - 32)" , Env{" F" : 32}, " 0" },
{" 5 / 9 * (F - 32)" , Env{" F" : 212}, " 100" },
}
var prevExpr string
for _, test := range tests {
// Print expr only when it changes.
if test.expr != prevExpr {
fmt.Printf(" \n%s\n" , test.expr)
prevExpr = test.expr
}
expr, err := Parse(test.expr)
if err != nil {
t.Error(err) // parse error
continue
}
got := fmt.Sprintf(" %.6g" , expr.Eval(test.env))
fmt.Printf(" \t%v => %s\n" , test.env, got)
if got != test.want {
t.Errorf(" %s.Eval() in %v = %q, want %q\n" ,
test.expr, test.env, got, test.want)
}
}
}
< / code > < / pre >
< p > 对于表格中的每一条记录， ， ， < / p >
< p > go test(§11.1) 命令会运行一个包的测试用例：< / p >
< pre > < code > $ go test -v gopl.io/ch7/eval
< / code > < / pre >
< p > 这个-v标识可以让我们看到测试用例打印的输出； ： < / p >
< pre > < code > sqrt(A / pi)
map[A:87616 pi:3.141592653589793] => 167
pow(x, 3) + pow(y, 3)
map[x:12 y:1] => 1729
map[x:9 y:10] => 1729
5 / 9 * (F - 32)
map[F:-40] => -40
map[F:32] => 0
map[F:212] => 100
< / code > < / pre >
< p > 幸运的是目前为止所有的输入都是适合的格式，但是我们的运气不可能一直都有。甚至在解释型语言中，为了静态错误检查语法是非常常见的；静态错误就是不用运行程序就可以检测出来的错误。通过将静态检查和动态的部分分开，我们可以快速的检查错误并且对于多次检查只执行一次而不是每次表达式计算的时候都进行检查。< / p >
< p > 让我们往Expr接口中增加另一个方法。Check方法对一个表达式语义树检查出静态错误。我们马上会说明它的vars参数。< / p >
< pre > < code class = "language-go" > type Expr interface {
Eval(env Env) float64
// Check reports errors in this Expr and adds its Vars to the set.
Check(vars map[Var]bool) error
}
< / code > < / pre >
< p > 具体的Check方法展示在下面。literal和Var类型的计算不可能失败， ， ， < / p >
< pre > < code class = "language-go" > func (v Var) Check(vars map[Var]bool) error {
vars[v] = true
return nil
}
func (literal) Check(vars map[Var]bool) error {
return nil
}
func (u unary) Check(vars map[Var]bool) error {
if !strings.ContainsRune(" +-" , u.op) {
return fmt.Errorf(" unexpected unary op %q" , u.op)
}
return u.x.Check(vars)
}
func (b binary) Check(vars map[Var]bool) error {
if !strings.ContainsRune(" +-*/" , b.op) {
return fmt.Errorf(" unexpected binary op %q" , b.op)
}
if err := b.x.Check(vars); err != nil {
return err
}
return b.y.Check(vars)
}
func (c call) Check(vars map[Var]bool) error {
arity, ok := numParams[c.fn]
if !ok {
return fmt.Errorf(" unknown function %q" , c.fn)
}
if len(c.args) != arity {
return fmt.Errorf(" call to %s has %d args, want %d" ,
c.fn, len(c.args), arity)
}
for _, arg := range c.args {
if err := arg.Check(vars); err != nil {
return err
}
}
return nil
}
var numParams = map[string]int{" pow" : 2, " sin" : 1, " sqrt" : 1}
< / code > < / pre >
< p > 我们在两个组中有选择地列出有问题的输入和它们得出的错误。Parse函数（ ） < / p >
< pre > < code > x % 2 unexpected '%'
math.Pi unexpected '.'
!true unexpected '!'
" hello" unexpected '" '
log(10) unknown function " log"
sqrt(1, 2) call to sqrt has 2 args, want 1
< / code > < / pre >
< p > Check方法的参数是一个Var类型的集合， ， ， ， ， ， < / p >
< p > 在第3.2节中， ， ， ， ， （ ， ） < / p >
< p > 这个ParseAndCheck函数混合了解析和检查步骤的过程： < / p >
< p > < u > < i > gopl.io/ch7/surface< / i > < / u > < / p >
< pre > < code class = "language-go" > import " gopl.io/ch7/eval"
func parseAndCheck(s string) (eval.Expr, error) {
if s == " " {
return nil, fmt.Errorf(" empty expression" )
}
expr, err := eval.Parse(s)
if err != nil {
return nil, err
}
vars := make(map[eval.Var]bool)
if err := expr.Check(vars); err != nil {
return nil, err
}
for v := range vars {
if v != " x" & & v != " y" & & v != " r" {
return nil, fmt.Errorf(" undefined variable: %s" , v)
}
}
return expr, nil
}
< / code > < / pre >
< p > 为了编写这个web应用， ， ： < / p >
< pre > < code class = "language-go" > func plot(w http.ResponseWriter, r *http.Request) {
r.ParseForm()
expr, err := parseAndCheck(r.Form.Get(" expr" ))
if err != nil {
http.Error(w, " bad expr: " +err.Error(), http.StatusBadRequest)
return
}
w.Header().Set(" Content-Type" , " image/svg+xml" )
surface(w, func(x, y float64) float64 {
r := math.Hypot(x, y) // distance from (0,0)
return expr.Eval(eval.Env{" x" : x, " y" : y, " r" : r})
})
}
< / code > < / pre >
< p > < img src = "../images/ch7-07.png" alt = "" / > < / p >
< p > 这个plot函数解析和检查在HTTP请求中指定的表达式并且用它来创建一个两个变量的匿名函数。这个匿名函数和来自原来surface-plotting程序中的固定函数f有相同的签名， ， ， ， ， < / p >
< p > < strong > 练习 7.13： < / strong > 为Expr增加一个String方法来打印美观的语法树。当再一次解析的时候， < / p >
< p > < strong > 练习 7.14： < / strong > 定义一个新的满足Expr接口的具体类型并且提供一个新的操作例如对它运算单元中的最小值的计算。因为Parse函数不会创建这个新类型的实例， （ ） < / p >
< p > < strong > 练习 7.15： < / strong > 编写一个从标准输入中读取一个单一表达式的程序，用户及时地提供对于任意变量的值，然后在结果环境变量中计算表达式的值。优雅的处理所有遇到的错误。< / p >
< p > < strong > 练习 7.16： < / strong > 编写一个基于web的计算器程序。< / p >
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