Interface Conversion — Type Does Not Implement Interface

Why This Happens

In Go, interfaces are satisfied implicitly. If a type is missing even one method, or if the method signature does not match exactly (wrong receiver type, different parameters), the type does not satisfy the interface. A common mistake is defining methods on the value type but trying to use a pointer, or vice versa.

The Problem

type Writer interface {
    Write(data []byte) error
}

type MyWriter struct{}

func (w MyWriter) Write(data string) error { // wrong parameter type
    return nil
}

var w Writer = &MyWriter{} // does not implement Writer

The Fix

type MyWriter struct{}

func (w *MyWriter) Write(data []byte) error {
    return nil
}

var w Writer = &MyWriter{} // now implements Writer

Step-by-Step Fix

1. 1

   Identify the missing method

   The compiler error tells you exactly which method is missing or has the wrong signature.

2. 2

   Compare the signatures

   Check the interface definition and your method signature side by side. Look for differences in parameter types, return types, and receiver type.

3. 3

   Fix the method

   Update your method to match the exact signature required by the interface.