Language guide

The language guide covers all of Lask, one topic per page. Each topic is a small, complete project: a main.lask whose comments explain the code next to them, and a few commands to run while you read.

If you haven't yet, start with Getting started, Your first real task file and Core concepts. This guide assumes you know what a task, an environment and lask sync are.

How to try a topic

Every topic lives in the lask repository, under example/02-language/. Clone it once:

git clone https://github.com/lask-task-runner/lask.git
cd lask/example/02-language/02-functions
lask check
lask eval greet alice --prefix hi

Most topics are a single file. For those, you can also paste the source shown on the page into a main.lask in an empty directory. The topic page says when it has more than one file.

Topics marked "Lask only" in the table run no commands, so they need no Docker. For the others, run lask sync in the topic's directory before the first run or eval, to pull the images it uses.

In the commands on each page, eval prints a task's return value, so you can see what it computed. run prints only the command log, on stderr.

Topics

The table is in reading order. The first three topics are the language itself; Environments and Commands are what make it a task runner; the rest build on those.

Topic	Covers	Needs
Values and types	Bindings, annotations, type aliases, records, maps, unions, optional fields, generics, cast	Lask only
Functions	Positional, variadic and keyword parameters, lambdas, higher-order functions, |> and >>	Lask only
Control flow	do, if with and without else, c ? a : b, all three forms of case, for	Lask only
Execution environments	#local, tags, digests, run options and runnable, a Dockerfile recipe, environments as values	Docker
Commands	$, $1, $2, $*, interpolation and its escaping, raw holes #!{...}, dispatch, run	Docker
Concurrency	async / await, spawn, all, race	Docker
Errors	try / catch / finally, fail, error, recover, exit codes	Docker
Modules and imports	Named and namespace imports, internal, re-export, per-module command declarations	Docker for the last task only
External dependencies	lask.json, the committed lock file, importing a shared module by name, lask deps	Docker, and one network step
The built-in library	Numbers, strings, regular expressions, arrays, maps	Lask only
Structured data	JSON, YAML, TOML, CSV, dotenv, cast, hashes, base64	Lask only
Input, output and secrets	stdin, the stdout contract, get_env and friends, !!, log	Docker for the two secret tasks only
Files and paths	read_file / write_file / glob and the environment each one names	Docker for two tasks; the rest run on #local
Documentation and the command line	Doc comments, @param, @complete, @hidden, and how a signature becomes a command line	Lask only

The Quick reference covers the same ground on one page, and the specification has the exact rules.