Only consider non-null streams

index.js

@@ -1,11 +1,14 @@
 "use strict";
 
 module.exports = function pipe(streams) {
+	// NOTE: We explicitly check for *null* here, not undefined. We still want to detect accidentally-omitted streams, but permit the user to explicitly insert nulls for streams that are not always inserted (eg. optional parallelization in a library). */
+	let existentStreams = streams.filter((stream) => stream !== null);
+
 	// FIXME: Validate input properly
-	if (streams.length < 1) {
+	if (existentStreams.length < 1) {
 		throw new Error("Must specify at least one stream when defining a pipeline");
 	} else {
-		let firstStream = streams[0];
+		let firstStream = existentStreams[0];
 		let requiresSource = (firstStream.read.length > 0 && firstStream.__ppstreams_hasSource !== true);
 
 		/* NOTE: We never clean up the cache, because it's very unlikely that this cache will ever grow big. In the vast majority of cases, it's going to contain at most one item. So instead, we let the garbage collector worry about getting rid of it once the pipeline itself becomes obsolete. */
@@ -13,7 +16,7 @@ module.exports = function pipe(streams) {
 	
 		function getPipeline(source) {
 			if (!boundPipelineCache.has(source)) {
-				let pipeline = streams.reduce((bound, stream) => {
+				let pipeline = existentStreams.reduce((bound, stream) => {
 					if (stream.read == null) {
 						throw new Error(`Stream is missing a read handler (stream description: ${stream.description})`);
 					} else if (stream.abort == null) {
@@ -47,7 +50,7 @@ module.exports = function pipe(streams) {
 		return {
 			// NOTE: This is set to convey to other `pipe` calls (as well as any other composability tools) that this is explicitly a composed stream that does not require a source, even if the `read` signature suggests otherwise. FIXME: Figure out how and whether to spec this, or whether there is a better way to deal with this.
 			__ppstreams_hasSource: !requiresSource,
-			description: `piped stream [${streams.map((stream) => stream.description).join(" => ")}]`,
+			description: `piped stream [${existentStreams.map((stream) => stream.description).join(" => ")}]`,
 			read: function (source) {
 				if(!verifyFullPipeline(source)) {
 					throw new Error("Cannot read from a partial pipeline; maybe you forgot to specify a source stream?");