Why Janet?

I never thought it could happen to me. I mean, parentheses? In this day and age? But for the past couple years, my go-to programming language for fun side projects has been a little Lisp dialect called Janet. (print "hey janet") I like Janet so much that I wrote an entire book about it, and put it on The Internet for free, in the hopes of attracting more Janetors to the language. I think you should read it, but I know that you don t believe me, so I m going to try to convince you. Here s my attempt at a sales pitch: here is why you – you of all people – should give Janet a chance. Janet is an imperative language with first-class functions, a single namespace for identifiers, and lexical block scoping. The core of the language is very small, consisting of only eight instructions: do, def, var, set, if, while, break, fn. But thanks to macros, there are lots of high-level wrappers that give you more powerful or convenient control flow. There are actually five more instructions that exist to support macros: quote, unquote, quasiquote, splice, and upscope. But you don t have to write those in regular code. You can learn Janet in an afternoon, because the runtime semantics are extremely familiar: think JavaScript, plus value types, minus all the wats. And the rest of the language is small: the entire standard library fits on one page. It was this ease of getting started that got me hooked in the first place. It s easy to compile Janet programs into native executables that statically link the Janet runtime. And you can share those programs with other people, without asking them to install Janet first – or your project s dependencies, or anything else for that matter. You don t even have to tell them it s written in Janet! The way that Janet pulls this off is very elegant: Janet compiles itself to bytecode, and then writes that bytecode into a .c file that also starts up the Janet runtime. Then it compiles that C file with your system s C compiler. Since Janet is designed to be easy to embed, this makes perfect sense: it is, essentially, embedding itself into a trivial C executable. A simple Janet hello world compiled to a native binary weighs under a megabyte (784K for Janet 1.27.0 on aarch64 macOS, but your mileage may vary). This includes the full Janet runtime, garbage collector, and even the bytecode compiler – so you can write programs that evaluate Janet code at runtime, if you want to. This makes Janet an excellent choice for writing little command-line apps. Which is especially true when you consider that Instead of regular expressions, Janet s text wrangling is based around parsing expression grammars. Parsing expression grammars are simpler, more powerful, and more predictable than regular expressions. They aren t line-oriented, so they can parse multi-line text without a problem. They can also parse HTML, or JSON, or any other non-regular language. They can also parse binary file formats – they have no problems with arbitrary null bytes. They really are parsers: structured, composable, first-class parsers. And they re pretty easy to learn! There is a third-party library called sh that provides a shell scripting DSL that allows you to express pipes and redirects directly in Janet. Like this: ($ find . -name *.janet | say) It s pretty incredible. It s such a nice DSL that I dedicated a whole chapter of Janet for Mortals to it – and the things that you can do with it. It elevates Janet from a reasonable alternative to Perl to a reasonable alternative to Bash for a surprisingly large range of programs. Lua has become the de facto embedded language, which is a shame, because well, this isn t a post about Lua. You might not care about this very much, but there s a chance that it s just because you haven t tried it yet: being able to write progams that expose scripting interfaces is a pretty fun superpower. Embedding Janet is very easy: the Janet runtime is a small C library, and all you have to do is link it in and then call regular C functions to manipulate Janet values. You can even embed it into websites, and write static sites with custom programmable DSLs! Janet s collection types come in mutable and immutable flavors. Immutable collections have value semantics: the immutable vector [1 2] is indistinguishable from (take 2 [1 2 3]), despite the fact that they have different memory addresses. Mutable collections, on the other hand, have reference semantics: the hash table @{:x 1 :y 2} is only equal to itself. Another hash table with the same keys and values is a distinct object. Not every language has immutable composite values built right into the standard library! I think this is the real reason you should learn Janet, but I didn t want to lead with it because I didn t want to scare you off. You can write Janet just fine without ever learning how to write macros. But you should learn how, because writing macros is fun. It feels different than any sort of programming that I ve done befo

Why Janet?

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