1 files changed, 27 insertions, 2 deletions
diff --git a/gemini/handler.go b/gemini/handler.go
index ded77a5..0f48e62 100644
--- a/gemini/handler.go
+++ b/gemini/handler.go
@@ -6,7 +6,7 @@ import "context"
 //
 // A Handler MUST NOT return a nil response. Errors should be returned in the form
 // of error responses (4x, 5x, 6x response status). If the Handler should not be
-// responsible for the requested resource it can return a "51 Not Found" response.
+// responsible for the requested resource it can return a 51 response.
 type Handler func(context.Context, *Request) *Response
 
 // Middleware is a handle decorator.
@@ -15,7 +15,11 @@ type Handler func(context.Context, *Request) *Response
 // transform the request or response in some way.
 type Middleware func(Handler) Handler
 
-func Fallthrough(handlers ...Handler) Handler {
+// FallthroughHandler builds a handler which tries multiple child handlers.
+//
+// The returned handler will invoke each of the passed child handlers in order,
+// stopping when it receives a response with status other than 51.
+func FallthroughHandler(handlers ...Handler) Handler {
 	return func(ctx context.Context, req *Request) *Response {
 		for _, handler := range handlers {
 			response := handler(ctx, req)
@@ -27,3 +31,24 @@ func Fallthrough(handlers ...Handler) Handler {
 		return NotFound("Resource does not exist.")
 	}
 }
+
+// Filter wraps a handler with a predicate which determines whether to run the handler.
+//
+// When the predicate function returns false, the Filter returns the provided failure
+// response. The failure argument may be nil, in which case a "51 Resource does not exist."
+// response will be used.
+func Filter(
+	predicate func(context.Context, *Request) bool,
+	handler Handler,
+	failure *Response,
+) Handler {
+	if failure == nil {
+		failure = NotFound("Resource does not exist.")
+	}
+	return func(ctx context.Context, req *Request) *Response {
+		if !predicate(ctx, req) {
+			return failure
+		}
+		return handler(ctx, req)
+	}
+}