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Simplifying Interfaces with Function Types

In Go, you can define methods on type aliases, which means that we can define a type alias of a function, and then define methods on that function.

+ +

Example

+ +

Given the following interface

type DB interface {
+	Get(string) (string, error)
+}
+

You can fulfill it using a function type like this:

type GetFn func(string) (string, error)
+
+func (f GetFn) Get(a string) (string, error) {
+	return f(a)
+}
+

Now you can use GetFn whenever a DB is required:

func main() {
+	var storeFn DB = GetFn(func(s string) (string, error) {
+		return "bar", nil
+	})
+	fmt.Println(storeFn.Get("Foo")) // Outputs: bar
+}
+

You can try this example in this [Go Playground](https://go.dev/play/p/hyBNIMblafs

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How it works

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In Go, interfaces are implicitly through method sets, which means any type (including a function type) that defines the required methods satisfies the interface. By defining the Get method on the GetFn type, the compiler treats GetFn as a valid implementation of the DB interface.

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This flexibility allows you to use function types as lightweight, dynamic implementations of interfaces, without the need for full struct definitions.

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Application

+ +

One common use case for this pattern is testing. Instead of implementing a full mock, you can use an inline function to provide test specific behavior.

func TestHandler() {
+    mockDB := GetFn(func(key string) (string, error) {
+        if key == "foo" {
+            return "bar", nil
+        }
+        return "", fmt.Errorf("not found")
+    })
+
+    result, err := mockDB.Get("foo")
+    fmt.Println(result, err) // Outputs: bar, <nil>
+}
+

This approach is not limited to testing. It’s also useful for dependency injection, where you can pass in lightweight or context specific implementations of an interface.

+ +

This pattern is similar to how http.HandleFunc works. In the HTTP package, http.HandlerFunc is a function type that fulfills the http.Handler interface by implementing its ServeHTTP method. This allows functions to act as handlers, providing great flexibility in designing web servers.

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Simplifying Interfaces with Function Types

Example

How it works

Application