When I help people learn web development, a super-common question is, "Why can't I pass this function into ' http." Handle ' method? It looks with ' http. Handlerfunc ' is exactly the same! "'" Gofunc Demo (H http. Handler) {}func Handler (w http. Responsewriter, R *http. Request) {}func main () {//This line of code will be compiled with an error of demo (handler)} ' > Run the sample code in Go Playground →https://play.golang.org/p/ Jay4rdyqqn3 I can definitely understand their confusion. For one accept ' http. Handlerfunc ' type arguments, we can define an explicit as ' http '. Handlerfunc ' type of variable is passed into it, we can also put a ' func (http. Responsewriter, *http. Request) ' This signature function is passed in as a parameter, so why can't the above code run? To answer this question, we need to delve into the ' HTTP. Handlerfunc ' and ' http. Handler ' type, and we use some sample code to explore their features. us from ' http. Handlerfunc ' Start, it's defined as: ' ' Gotype handlerfunc func (w responsewriter, R *request) ' So we can write this code: "' Gofunc Demo (fn http. Handlerfunc) {}func handler (w http. Responsewriter, R *http. Request) {}func main () {demo (handler)} ' > Run the sample code in Go Playground →HTTPS://PLAY.GOLANG.ORG/P/NDDBOHQPFQH Note that This code is completely different from the top problem code-our ' demo ' function receives the parameter type ' HTTP '. Handlerfunc ', not ' http. Handler '. There is a fact that you may not have found that the ' handler ' in the code is actually not an ' http '. Handlerfunc ', which is the actual type of ' func ' (http. Responsewriter, *http. Request) ', we can reveal this fact by running the following test code. "' Gopackage mainimport (" FMT "" Net/http ") func main () {FMT. Printf ("%T", Handler)}func handler (w http. Responsewriter, R *http. Request) {//handler func implementation content} ' > Run the sample code in Go Playground →https://play.golang.org/p/ep8xlykuodx so, Why would we expect to pass ' handler ' as a parameter to the ' Dmeo ' function in the top problem code? It's obviously different in type! Of course, in our example, our ' handler ' function is fully compliant with ' HTTP '. Handlerfunc ' is defined, so the Go compiler can infer that we want to convert our variables to that type. In other words, the compiler can pretend that our code is written like this: "' Godemo (http. Handlerfunc (Demo)) ' Plus go ' http. The Handlerfunc ' section is derived from the ' demo ' function. Well, now we can take the second step--' http. Handlerfunc ' also implements ' HTTP. Handler ' interface. Why is this, we just need to look at both the source code can be. "' Gotype handlerfunc func (w responsewriter, R *request) func (f Handlerfunc) servehttp (w responsewriter, R *request) {f (W, R)}type Handler interface {servehttp (Responsewriter, *request)} "Please note that ' handlerfunc ' has a ' servehttp ' method that matches the ' Handler ' interface sound This means that ' handlerfunc ' implements the ' Handler ' interface. This may seem strange, but in the Go language it is perfectly valid. After all, ' Handlerfunc ' is another type, and we can add methods to any type at any time. Because ' Handlerfunc ' has achieved ' Handler '' Interface, we can pass ' handlerfunc ' to the function that accepts the ' Handler ' argument, as long as the parameter type is explicitly defined as ' Handlerfunc '. The sample code is as follows: ' ' Gopackage mainimport ' Net/http ' func demo (fn http. Handler) {}func Handler (w http. Responsewriter, R *http. Request) {}func main () {demo (http. Handlerfunc (Handler)} "> Run the Sample code in Go Playground →https://play.golang.org/p/k7sd51wnbl9 now we know ' http. Handlerfunc ' implemented ' HTTP. Handler ' interface, and we know we can type ' func (http. Responsewriter, *http. Request) ' object is passed to a need ' http. Handlerfunc ' type parameter in the function, but why we cannot pass ' func (http. Responsewriter, *http. Request) ' type to a need ' http. Handlerfunc ' type parameters in the function? Or, why is this code not compiled? "' Gofunc Demo (H http. Handler) {}func Handler (w http. Responsewriter, R *http. Request) {}func main () {///This line of code will be compiled with an error of demo (handler)} ' The simple answer is that the compiler does not know how to ' func ' (http. Responsewriter, *http. Request) ' type is converted to ' HTTP '. Handler ' type. To implement such a conversion, the compiler needs to know that we want to convert ' handler ' to ' HTTP '. Handlerfunc ', but in the code above, we don't see ' http at all. Handlerfunc ' this type. If you use the inference rule that we said before, the Go compiler can only assume that the code actually wants to write this: "' Godemo (http. Handler (Handler) "But this is obviously wrong. ' Handler ' does not implement ' HTTP '. HandlEr ' interface, unless it is converted into an ' HTTP ' that implements the interface. Handlerfunc ' type. There is also a way that we do a two-step conversion that will ' Func ' (http. Responsewriter, *http. Request) ' converted to HTTP. Handler: "1 2func, Handlerfunc, Handler" If we explicitly convert the ' Handler ' function to ' http '. Handlerfunc ', then this code is ready to run. "' Gofunc Demo (H http. Handler) {}func Handler (w http. Responsewriter, R *http. Request) {}func main () {//code can be compiled! Demo (http. Handlerfunc (Handler)} "> Run the sample code in Go Playground →https://play.golang.org/p/dm33tpgvonh again, if we assume Go The compiler can pretend that our code is automatically converting incoming parameters into the actual desired type, and the code looks like this: "' Godemo (http. Handler (http. Handlerfunc (handler)) "Well, why does the compiler not learn to automatically do this for us in this two-step, straightforward type conversion? The beginner's explanation is that the compiler doesn't know that the two-step conversion is what you really want to do. Let's assume you have another type, assuming it's called ' Cowboyfunc ', which is defined as follows: ' ' Gotype cowboyfunc func (w http. Responsewriter, R *http. Request) func (f Cowboyfunc) servehttp (w http. Responsewriter, R *http. Request) {s: = time. Now () F (W, R) fmt. Println ("Cowboy function Duration:", time. Now (). Sub (s))} ' Now, if we call ' demo (handler) ' As we did before, then the compiler automatically converts the ' handler ' type to ' http '. Handlerfunc ' type, and then compile successfully, but is this what developers really want to do? In case the developer wants to turn ' handler 'Instead of ' Cowboyfunc '? It also implements the ' HTTP. Handler ' interface. "' Godemo (Cowboyfunc (handler)) ' > Run the sample code in Go Playground →https://play.golang.org/p/tjelwdethyx you can see that this is also a valid code, This means that the compiler does not know exactly which of the two we want, unless we can tell it clearly. All in all, you cannot direct the ' func ' (http. Responsewriter, *http. Request) ' Pass to need ' http. Handler ' type parameter of the function, which may make you feel confused or cumbersome, but can not do so for a reasonable reason, and once you understand these reasons, you may be more grateful to those who appear occasionally, annoying mistakes.
via:https://www.calhoun.io/why-cant-i-pass-this-function-as-an-http-handler/
Author: Jon Calhoun Translator: Moodwu proofreading: polaris1119
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