How Can I Handle Go Functions Returning Structs Implementing Interfaces?

Go Function Types that Return Structs Inside Interfaces

In Go, it is common to use interfaces to define the behavior of structs. However, when working with functions that return structs that implement interfaces, you may encounter type-safety errors.

Understanding the Issue

Let's examine the code example you provided:

package expensive

type myStruct struct { ... } // Struct with time-consuming methods
func CreateInstance() *myStruct { ... } // Expensive factory function

package main

import "expensive"

type myInterface interface { DoSomething() }
type structToConstruct struct { factoryFunction func() myInterface }
func (s *structToConstruct) performAction() { instance := s.factoryFunction(); instance.DoSomething() }

Here, you defined a factory function CreateInstance that returns a *myStruct. You then created an interface myInterface that *myStruct implements. However, you assigned the factory function to a field in structToConstruct that expects a function returning myInterface, causing the compilation error.

Solving the Issue

To resolve this, you have two options:

1. Wrapper Function: You can wrap your factory function within the main package. For example:

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

2. Revised Factory Function: Alternatively, you can modify CreateInstance to return myInterface directly:

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

Why Option 1 Works

In Option 1, the wrapper function converts the result of CreateInstance to myInterface before assigning it to factoryFunction. This satisfies the type signature of structToConstruct because the wrapper function matches the expected function type.

Why Option 2 Requires Proposal 12754

In Option 2, if you attempt to directly assign CreateInstance to factoryFunction, Go will complain because CreateInstance returns a structured pointer, not an interface. Proposal 12754 suggests extending the language to support such assignments, but it was ultimately rejected.

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