examples: Add compile-async task to the examples/dynamic-tasks.ts…

… example Drakefile to asynchronous execute dynamic file tasks.

examples/dynamic-tasks.ts

@@ -1,20 +1,50 @@
-import * as path from "https://deno.land/std@0.67.0/path/mod.ts";
-import { env } from "../lib/env.ts";
-import { desc, glob, run, sh, task } from "../mod.ts";
+import * as path from "https://deno.land/std@0.68.0/path/mod.ts";
+import { desc, env, execute, glob, run, sh, task } from "../mod.ts";
 
 const tasks: string[] = [];
+const mdfiles = env("mdfiles") || "*.md";
+const outdir = env("outdir") || path.dirname(mdfiles);
 
-for (const prereq of glob("*.md")) {
-  const target = `${path.basename(prereq, ".md")}.html`;
-  desc(`compile "${target}"`);
+for (const prereq of glob(mdfiles)) {
+  let target = `${prereq.replace(/\.[^/.]+$/, "")}.html`;
+  target = path.join(outdir, path.basename(target));
+  // Create a file task to compile the markdown source file.
   task(target, [prereq], async function () {
     await sh(`marked "${prereq}" > "${target}"`);
   });
   tasks.push(target);
 }
 
-desc("compile markdown");
-task("compile-md", tasks);
-env("--default-task", "compile-md");
+desc("command-line usage");
+task("help", [], function () {
+  console.log(`
+Example Drakefile illustrating dynamic task creation.
 
-run();
+Dynamically create file tasks to compile markdown source files to HTML.
+Execute them synchronously and asynchronously with 'compile-sync' and
+'compile-async' tasks respectively.
+
+Example usage:
+
+   deno run -A dynamic-tasks.ts compile-async mdfiles=~/doc/test/*.md outdir=/tmp
+
+Environment variables:
+
+mdfiles=<markdown source files glob>                (default: "*.md")
+outdir=<output directory for compiled HTML files>   (default: mdfiles directory)
+
+- The 'compile-async' task runs up to 5 times faster than 'compile-sync'.
+- Increase the maximum number of open file descriptors with ulimit(1) if
+  there are 'Too many open files (os error 24)'.
+`);
+});
+
+desc("compile markdown files synchronously");
+task("compile-sync", tasks);
+
+desc("compile markdown files asynchronously");
+task("compile-async", [], async function () {
+  await execute(...tasks);
+});
+
+await run();