How to Assign a Function Returning a Struct Implementing an Interface in Go?

Using Go Function Types that Return Structs with Interfaces

Question:

In Go, I have a struct in one package that implements an interface. In another dependent package, I want to create a fake implementation of this struct for testing purposes. However, Go is complaining about type safety when I try to assign a function returning a struct that implements the interface to a variable of that interface.

Architecture:

// Package expensive
type myStruct struct{}

func (m *myStruct) DoSomething() {...}
func (m *myStruct) DoSomethingElse() {...}
func CreateInstance() *myStruct {...} // Expensive to call

// Package main
type myInterface interface {
    DoSomething()
}

type structToConstruct struct {
    factoryFunction func() myInterface
}
func (s *structToConstruct) performAction() {...}

Error:

"cannot use expensive.CreateInstance (type func() *expensive.myStruct) as type func() myInterface in field value"

Answer:

The issue arises because the CreateInstance function returns a *myStruct value, which differs from the func() myInterface type required by factoryFunction. To resolve this:

// Package main
type myInterface interface {
    DoSomething()
}

type myStruct struct{}

func (m *myStruct) DoSomething() {...}

func getInstance() myInterface {
    return &myStruct{}
}

With this modification, getInstance now returns an myInterface type, allowing it to be assigned to factoryFunction.

Additional Notes:

• Originally, the question involved a function type that returned a pointer to the struct rather than the struct itself, which caused the type mismatch.
• To implement this behavior in the original code, a wrapper function can be used:

wrapper := func() myInterface {
    return expensive.CreateInstance()
}

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