关于 appHandle自定义类型和HandleFileList的调用
来源：3-3 服务器统一出错处理_浏览器需放大
joyefish
2021-12-27 09:35:32
type appHandler func(writer http.ResponseWriter, request *http.Request) error
func errWrapper(handler appHandler) func(http.ResponseWriter, *http.Request) {这里appHandler是一个自定义的函数类型,errWrapper函数的参数是一个appHandler类型的参数
func HandleFileList(writer http.ResponseWriter, request *http.Request) error {这是在handler.go种定义实现的HandleFileList,并未显示的指定是appHandler类型，按照Go语言基础知识HandleFileList和appHandler应该是两个不同类型。
为什么在main函数中errWrapper可以直接传入HandleFileList?如下：
http.HandleFunc("/list/", errWrapper(fileListing.HandleFileList))还麻烦老师解答一下
1回答
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这是它的规定。我帮同学翻到了原文。
这个问题对应其中的这种情况：
A value x is assignable to a variable of type T ("x is assignable to T") if one of the following conditions applies:
x's type is identical to T.
x's type V and T have identical underlying types and at least one of V or T is not a defined type.
T is an interface type and x implements T.
x is a bidirectional channel value, T is a channel type, x's type V and T have identical element types, and at least one of V or T is not a defined type.
x is the predeclared identifier nil and T is a pointer, function, slice, map, channel, or interface type.
x is an untyped constant representable by a value of type T.
这条是说，赋值的话，他们"underlying"，我大致翻成实际上，的类型相同。然后等号左右两边至少有一个不是defined type。defined type是指用type xxx 定义的类型，显然appHandler是，但HandleFileList不是。
我们这里虽然是函数传参，但是遵循的是assignment（赋值）的逻辑，相当于把HandleFileList赋值给了一个appHandler。参数列表一样，而且只有一个是defined type，所以满足黑体这一条，可以赋值。
那么我们根据这个规则，试一试两个defined type还能赋值吗？
type appHandler func(writer http.ResponseWriter, request *http.Request) error
type appHandler2 func(writer http.ResponseWriter, request *http.Request) error
var ah appHandler = HandleFileList // 编译通过
var ah2 appHandler2 = ah // 编译错误，此处等号右边必须写成appHandler2(ah)
1hp>解释非常清楚，感谢老师解答，实际实验如下：
h021-12-30
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