jank development update - Lots of new changes
Dec 08, 2022 · Jeaye Wilkerson

I was previously giving updates only in the #jank Slack channel, but some of these are getting large enough to warrant more prose. Thus, happily, I can announce that jank has a new blog and I have a lot of new progress to report! Let's get into the details.

Over the past couple of months, jank has gained initial C++ interop, an upgrade of Cling to resolve Linux crashes, support for multiple fn arities, including variadic arities, support for if and let*, which involved a rework of the C++ codegen, and initial metadata support, and many smaller changes.

Before I get into the details, I'd like to point out that feedback is welcome on all of this. Very little of jank's design is set in stone right now, so that means your feedback can matter a lot.

Initial C++ interop

When digging deep enough into clojure.core implementations, most things eventually just turn into Java. Crucial fns like println, for example, rely on Java streams, as well as direct interop. Something like str, as well, reaches right for StringBuilder. So jank needs a way to call into C++ to get the important things done.

C++ is syntactically and semantically more complicated than Java. Its reflection story is poorer and is made worse by template usage, which requires a C++ compiler to generate the necessary instantiations ahead of time. With all of this in mind, I figured jank's interop is not going to look like Clojure JVM's, or ClojureScript's. I did dive into what Ferret and Carp are doing for this and came up with some design notes that may be of interest. The notes cover both approaches, some pros/cons, and the rationale for jank's approach.

In short, jank is introducing a special native/raw form which accepts inline C++ code. Within that code can be interpolated jank code, escaped using the #{} form.


; No macros yet, so no defn or fn.
(def str
    ; Unary case uses interop and interpolation to call
    ; the to_string fn on the object.
     (native/raw "__value = make_box(#{ o }->to_string());"))
    ; Variadic case could recurse, but we can iterate
    ; and avoid extra boxing of each intermediate.
    ([o & args]
     (native/raw "std::string ret(#{ o }->to_string().data);
                  auto const * const l(#{ args }->as_list());
                  for(auto const &elem : l->data)
                  { ret += elem->to_string().data; }
                  __value = make_box(ret);"))))

(assert (= "" (str)))
(assert (= "1" (str 1)))
(assert (= "12" (str 1 2)))
(assert (= ":foo:bar[2 3]" (str :foo :bar [2 3])))

This is clearly a low-level primitive for interop and I'm interested to see how we can build on it. For now, it unlocks all the bits

Cling upgrade to 0.9

A crash on Linux was requiring jank to use Cling 0.7, which meant using Clang/LLVM 5. After dozens of hours of compiling Cling/Clang/LLVM different ways, to troubleshoot, I found the issue! Now jank's build system has been updated to build Cling 0.9 for both macOS and Linux.

What came out of this, as well, is that jank can now use a pre-compiled header (PCH) for improving Cling startup times. That PCH is packaged with jank. This changed dropped the Cling startup time from 1.4 seconds to 265 milliseconds! More similar improvements to come.

Support for if/let*

For a while, jank's codegen (to C++) was benefiting from the fact that Clojure is such an expression-based language. Code such as:

(println (foo (bar)))

Could be generated as something like:


But that breaks altogether with something like:

(println (if (even? n)

To address this, I've completely switched the codegen approach to be entirely statement based. Every sub-expression is now pulled up into a statement with a temporary and then each outer expression just refers to those symbols. So, the corresponding C++ for the above Clojure would look something like:

// This is cleaned up a bit, to make it easier
// to read, but the shape of the code is the same.
object_ptr if_result;
object_ptr even_result{ even_QMARK->call(n) };
{ if_result = kw_foo; }
{ if_result = kw_bar; }

The way this manifests in the compiler is really clean, since every jank expression resolves to a single C++ symbol, which is its temporary. Could be a let*, if, fn call, or anything else, but it's all boiled down to a symbol for that value. I have more detailed design notes on my approach, and Clojure's approach, in case you're interested.

What's next

I'm working on macros next, since that will help clean up a lot of the jank code I'm writing. After that, there are some more special forms which would really help, such as loop. Finally, extending the runtime behaviors to add analogous abstractions for functionality like clojure.lang.Associative, as well as better clojure.lang.Seqable support within jank itself would mean I can start implementing fns like assoc, map, filter, reduce, etc.

Would you like to join in?

  1. Join the community on Slack
  2. Join the design discussions on Github
  3. Considering becoming a Sponsor