Merge pull request go-kit#96 from go-kit/endpoint-chain

Adaptation of @shore's proposal from go-kit#95

Author: peterbourgon

endpoint/endpoint.go

@@ -19,3 +19,15 @@ var ErrBadCast = errors.New("bad cast")
 
 // ContextCanceled indicates the request context was canceled.
 var ErrContextCanceled = errors.New("context canceled")
+
+// Chain is a helper function for composing middlewares. Requests will
+// traverse them in the order they're declared. That is, the first middleware
+// is treated as the outermost middleware.
+func Chain(outer Middleware, others ...Middleware) Middleware {
+	return func(next Endpoint) Endpoint {
+		for i := len(others) - 1; i >= 0; i-- { // reverse
+			next = others[i](next)
+		}
+		return outer(next)
+	}
+}

endpoint/endpoint_example_test.go

@@ -0,0 +1,50 @@
+package endpoint_test
+
+import (
+	"fmt"
+
+	"golang.org/x/net/context"
+
+	"github.com/go-kit/kit/endpoint"
+)
+
+func ExampleChain() {
+	e := endpoint.Chain(
+		annotate("first"),
+		annotate("second"),
+		annotate("third"),
+	)(myEndpoint)
+
+	if _, err := e(ctx, req); err != nil {
+		panic(err)
+	}
+
+	// Output:
+	// first pre
+	// second pre
+	// third pre
+	// my endpoint!
+	// third post
+	// second post
+	// first post
+}
+
+var (
+	ctx = context.Background()
+	req = struct{}{}
+)
+
+func annotate(s string) endpoint.Middleware {
+	return func(next endpoint.Endpoint) endpoint.Endpoint {
+		return func(ctx context.Context, request interface{}) (interface{}, error) {
+			fmt.Println(s, "pre")
+			defer fmt.Println(s, "post")
+			return next(ctx, request)
+		}
+	}
+}
+
+func myEndpoint(context.Context, interface{}) (interface{}, error) {
+	fmt.Println("my endpoint!")
+	return struct{}{}, nil
+}