1 files changed, 52 insertions, 0 deletions
diff --git a/handler.go b/handler.go
new file mode 100644
index 0000000..f940b77
--- /dev/null
+++ b/handler.go
@@ -0,0 +1,52 @@
+package gus
+
+import "context"
+
+// Handler is a function which can turn a request into a response.
+//
+// A Handler can return a nil response, in which case the Server is expected
+// to build the protocol-appropriate "Not Found" response.
+type Handler func(context.Context, *Request) *Response
+
+// Middleware is a handler decorator.
+//
+// It returns a handler which may call the passed-in handler or not, or may
+// transform the request or response in some way.
+type Middleware func(Handler) Handler
+
+// FallthroughHandler builds a handler which tries multiple child handlers.
+//
+// The returned handler will invoke each of the passed-in handlers in order,
+// stopping when it receives a non-nil response.
+func FallthroughHandler(handlers ...Handler) Handler {
+	return func(ctx context.Context, request *Request) *Response {
+		for _, handler := range handlers {
+			if response := handler(ctx, request); response != nil {
+				return response
+			}
+		}
+		return nil
+	}
+}
+
+// Filter builds a middleware which only calls the wrapped under a condition.
+//
+// When the condition function returns false it instead invokes the
+// test-failure handler. The failure handler may also be nil, in which case
+// the final handler will return a nil response whenever the condition fails.
+func Filter(
+	condition func(context.Context, *Request) bool,
+	failure Handler,
+) Middleware {
+	return func(success Handler) Handler {
+		return func(ctx context.Context, request *Request) *Response {
+			if condition(ctx, request) {
+				return success(ctx, request)
+			}
+			if failure == nil {
+				return nil
+			}
+			return failure(ctx, request)
+		}
+	}
+}