Initial commit

.eslintrc

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+{
+	"extends": "@joepie91/eslint-config"
+}

.gitignore

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+node_modules

example.js

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+"use strict";
+
+const Promise = require("bluebird");
+
+const derivedStream = require("./");
+const pipe = require("@promistream/pipe");
+const collect = require("@promistream/collect");
+const rangeNumbers = require("@promistream/range-numbers");
+
+return Promise.try(() => {
+	let producedStream = pipe([
+		rangeNumbers(0, 10),
+		derivedStream((source) => {
+			/* NOTE: This example doesn't actually do anything *useful*. It
+			   only demonstrates the higher-order stream mechanism.
+			   Real-world code implementing eg. load-balanced stream forks
+			   would probably do something like producing an array of multiple
+			   streams here (instead of a single pipeline), and do some
+			   internal routing of source values across them. */
+
+			return pipe([
+				source,
+				collect()
+			]);
+		})
+	]).read();
+	
+	return producedStream.read();
+}).then((result) => {
+	console.log(result); // [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 ]
+});

index.js

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+"use strict";
+
+const Promise = require("bluebird");
+const Aborted = require("@promistream/aborted");
+const EndOfStream = require("@promistream/end-of-stream");
+const errorChain = require("error-chain");
+
+const { validateArguments } = require("@validatem/core");
+const required = require("@validatem/required");
+const isFunction = require("@validatem/is-function");
+
+module.exports = function simpleSource(_deriver) {
+	let [ deriver ] = validateArguments(arguments, {
+		deriver: [ required, isFunction ]
+	});
+
+	let hasBeenDerived = false;
+	let errorReason = null;
+
+	return {
+		_promistreamVersion: 0,
+		description: `derived stream`,
+		peek: function peekValue_derivedStream(_source) {
+			// A derived-stream is guaranteed to produce a value; either a new stream, or an EndOfStream, or an Aborted marker
+			return true;
+		},
+		read: function produceValue_derivedStream(source) {
+			// TODO: Abstract out the abort handling here and in `abort`? The logic is almost entirely shared between `simple-source` and `derived-stream`.
+			if (errorReason != null) {
+				if (errorReason === true) {
+					throw new Aborted("Stream was aborted");
+				} else if (errorReason instanceof Error) {
+					throw new errorChain.chain(errorReason, Aborted, `Stream was aborted due to error: ${errorReason.message}`);
+				}
+			} else if (hasBeenDerived === true) {
+				throw new EndOfStream;
+			} else {
+				hasBeenDerived = true;
+				return deriver(source);
+			}
+		},
+		abort: function abort_simpleSource(reason, _source) {
+			// NOTE: Even though a `source` is provided, we don't do anything with it; higher-order stream producers should *not* propagate aborts upstream, as a read from such a producer would never constitute a read upstream.
+			return Promise.try(() => { // FIXME: Remove Promise.try here?
+				if (errorReason == null) {
+					if (reason === true || reason instanceof Error) {
+						errorReason = reason;
+					} else {
+						throw new Error("You must specify a reason (either `true` or an Error object) when aborting a stream");
+					}
+				} else {
+					// FIXME: Require this behaviour in the spec? Or is there a composability-related reason to permit/require quietly ignoring this, to make it idempotent? (Duplicate of simple-source)
+					throw new Error(`The stream is already aborted`);
+				}
+			});
+		}
+	};
+};

package.json

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+{
+  "name": "@promistream/derived-stream",
+  "version": "0.1.0",
+  "main": "index.js",
+  "repository": "http://git.cryto.net/promistream/derived-stream.git",
+  "author": "Sven Slootweg <admin@cryto.net>",
+  "license": "WTFPL OR CC0-1.0",
+  "dependencies": {
+    "@promistream/aborted": "^0.1.1",
+    "@promistream/end-of-stream": "^0.1.1",
+    "@validatem/core": "^0.3.12",
+    "@validatem/is-function": "^0.1.0",
+    "@validatem/required": "^0.1.1",
+    "bluebird": "^3.7.2",
+    "error-chain": "^0.1.0"
+  },
+  "devDependencies": {
+    "@joepie91/eslint-config": "^1.1.0",
+    "@promistream/collect": "^0.1.1",
+    "@promistream/pipe": "^0.1.0",
+    "@promistream/range-numbers": "^0.1.2",
+    "eslint": "^7.29.0"
+  }
+}