interface-tricks

Interface Tricks

I try to use the following example to demonstrate this interface trick that is commonly seen in the Golang's source code, For examples, see the http.Handler interface and the http.HandlerFunc type.

This trick is intended to convert a function whose signature is the same as the method signature of an interface into a type that implements that interface.

As shown in main.go, an interface called Ingester is defined, having a method called Ingest that accepts a slice of float64 as input and returns a slice of float64.

type Ingester interface {
	Ingest([]float64) []float64
}

Then a type IngesterFunc is immediately defined. The type definition is just the Ingest's method signature of the Ingester interface.

type IngesterFunc func([]float64) []float64

In order to let the IngesterFunc type implement the Ingester interface, the Ingest method needs to be provided. And as you can see its implementation is straightforward: the argument data that the Ingest method accepts is directly passed to the IngesterFunc type and returned.

func (i IngesterFunc) Ingest(data []float64) []float64 {
	return i(data)
}

What is the benefit of doing this?

Next time you have a function, say addOne, whose signature is the same as the Ingest method of Ingester:

func addOne(data []float64) []float64

You can pass this function to IngesterFunc to derive the following:

IngesterFunc(addOne)

which can then be passed to a function (e.g., ScaledTransform) that accepts the Ingester interface as part of its arguments:

func ScaledTransform(i Ingester, scale float64) Ingester

The argument scale and return type Ingester do not matter a lot in this example. The point here is that now the ScaledTransform function can accepts an interface as its input argument, rather than other primitive or struct types.

Think about what if we don't define the IngesterFunc type and let it implement the Ingester interface, we would have to write the ScaledTransform function as:

func ScaledTransform(f func([]float64) []float64, scale float64) func([]float64) []float64

which is less readable compared to the version that adopts this interface trick.

It completes this demonstration.