Implementing interface type to function type in Golang

Question

I have created few types including interface as:

// GetProfileHandlerFunc turns a function with the right signature into a get profile handler
type GetProfileHandlerFunc func(GetProfileParams, interface{}) middleware.Responder

// Handle executing the request and returning a response
func (fn GetProfileHandlerFunc) Handle(params GetProfileParams, principal interface{}) middleware.Responder {
    return fn(params, principal)
}

// GetProfileHandler interface for that can handle valid get profile params
type GetProfileHandler interface {
    Handle(GetProfileParams, interface{}) middleware.Responder
}

Now in my api implementation package. I am using a logic to handle the request parameters. I am trying to assign GetProfileHandlerFunc to another type since it implements GetProfileHandler interface as you can see above.

api.ProfileGetProfileHandler = profile.GetProfileHandlerFunc(func(params profile.GetProfileParams, principal *models.User) middleware.Responder {
     // contains logic to handle the request
}

Now I think I can do above logic. But I am getting type mismatch error.

cannot convert func literal (type func(profile.GetProfileParams, *"userproj/models".User) middleware.Responder) to type profile.GetProfileHandlerFuncgo

"Now I think I can do above logic" No you can not. func(params profile.GetProfileParams, principal *models.User) middleware.Responder is not the same as func(GetProfileParams, interface{}) middleware.Responder. That is: *models.User is not the same type as interface{}. — mkopriva
– mkopriva, Commented Jan 14, 2019 at 8:02
It doesn't matter what interface the GetProfileHandlerFunc type implements, the error is not saying anything about interfaces, it tells you that you cannot convert A to B, where A is the func literal func(params profile.GetProfileParams, principal *models.User) middleware.Responder and B is the func type GetProfileHandlerFunc. — mkopriva
– mkopriva, Commented Jan 14, 2019 at 9:21
... It's the same as trying to do int("hello world"), you are doing conversion between two incompatible types. — mkopriva
– mkopriva, Commented Jan 14, 2019 at 9:24
@Himanshu. NO, you cannot have seen this because it is impossible in Go. See also golang.org/doc/faq#convert_slice_of_interface. Go's type system does not work the way you think. interface{} does not mean "whatever" it means interface{} and only interface{}. You can assign anything to interface{} but as a type it is interface{} and nothing else. — Volker
– Volker, Commented Jan 14, 2019 at 9:34
You've probably misunderstood what you've seen or the code was as well broken and would not compile just like yours. — mkopriva
– mkopriva, Commented Jan 14, 2019 at 9:35

Billy Yuan · Accepted Answer · 2019-01-16 12:20:06Z

the point is: if you have a function like

func A(param interface{}) {}

you can pass anything to param when you make a call to function A. like

A(10)
A(true)
A(nil)

Because interface{} means everything. So your handle func definition:

type GetProfileHandlerFunc func(GetProfileParams, interface{}) middleware.Responder

means a function GetProfileHandlerFunc which takes two params, the first is of type GetProfileParams, the second is of type interface{}. That means second param canbe anything.

But

func(params profile.GetProfileParams, principal *models.User) middleware.Responder

means a function which takes two params, the first is of type GetProfileParams, the second is of type *models.User. So, do you think they are the same? No.

I need a function can take anything as second param,not a function who can only take User as second function.

When you call your handle

GetProfileHandlerFunc(params, 10) // this is ok

So is that ok for

func(params profile.GetProfileParams, principal *models.User) middleware.Responder

No.

The right way to do this is :

api.ProfileGetProfileHandler = profile.GetProfileHandlerFunc(func(params profile.GetProfileParams, principal interface) middleware.Responder {

     user:=principal.(*model.User) // watch this.
}

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