Using clojure.spec.alpha with babashka


If you want to use clojure.spec.alpha with babashka, upgrade to 0.7.0 or newer and use babashka/spec.alpha as a library.

The issue

Spec is a library for validating and conforming (destructuring) data that comes with Clojure. For over a year there was an open issue in the babashka Github repo about whether to include clojure.spec into babashka, a fast starting native Clojure scripting environment. Clojure.spec is used in many libraries and adding it would add compatibility with more of them. I already knew that from a technical perspective SCI (the interpreter used by babashka) and clojure.spec can work together well. I've put this into practice in grasp, a tool to "grep" Clojure source using clojure specs. It provides a binary which includes clojure spec in the same way that babashka would. The question is: should we include clojure.spec.alpha in babashka?

What does built-in mean?

Adding a library as a built-in in babashka means that it is compiled into babashka's native image. The functions are pre-compiled to native machine code and aren't run through the SCI interpreter. This is much better for performance, but also for startup time. Loading libraries from source in babashka involved parsing and analyzing code before using it. Adding a library as a built-in means that this work is already done at compile time, by the Clojure compiler instead of SCI.


So far all built-in third party libraries in babashka are well established projects that have been around for years and whose APIs are unlikely to change. Boring in the good sense. Clojure spec, as of now, is alpha. That doesn't mean that clojure.spec.alpha is unstable, but what does it mean? Will it ever disappear from Clojure once it's not alpha anymore? In 2019 Alex Miller gave an inspiring talk at ClojuTRE in which he announced that spec 2 was soon coming. I'm still hoping it will soon come, but I understand that a lot of good things happened at Cognitect and they had other things to work on. In the open babashka issue I've asked for feedback on including clojure.spec.alpha as it is and most people were in favor of waiting for clojure spec 2. I decided to be patient and investigate alternatives. If we aren't going to include clojure.spec.alpha as a built-in, what alternatives do we have? We could try to run clojure.spec.alpha from source instead of a built-in so people can use it as an optional library.


The test.check library is used by clojure.spec to generate random data from specs. Libraries operating in the same space as spec, like Malli, are using this library for the same purpose. I decided to go ahead and add this library in babashka as a preparation for the soon coming spec 2. This was done in 0.2.8, released in January 2021. Since then you could already use namespaces from test.check in babashka, without using it with spec.


When babashka was only a few months old, it didn't cover as many clojure features as it does today. Running clojure.spec.alpha from source wasn't something within reach, mostly because babashka didn't support protocols. Protocols were introduced in version 0.1.1 in June of 2020, but by then I already made a rewrite of clojure.spec.alpha that used plain hashmaps instead of protocols. This version was called spartan.spec: spartan, because it supported the basic features of spec, but not all. It did not support generators, fdef and instrumentation. Although I could have made that work, just validating and conforming data is what most people were doing with spec in the libraries that I've been trying to run with babashka. Upon requiring spartan.spec it would create the namespace clojure.spec.alpha for you, so libraries loaded after that point would think they were using the original. This approach proved out to work pretty well. Even tools like expound worked with spartan.spec. But spartan.spec was a rewrite of spec and needed more work if we wanted to cover the other features. An effort that could be spared if babashka could just load the original spec from source.

Clojure.spec.alpha from source

When introducing protocols, I hadn't thought of revisiting running clojure.spec.alpha from source again. For some reason this only occurred to me in the past few days. I discovered that the remaining incompatibilities were minimal. Here are some things I found and what I did in a fork to solve the compatibility with babashka:

With these minimal changes and some functions I needed to expose in babashka which weren't there before (inst-ms being one of them), babashka is now able to run clojure.spec. All tests are passing:

$ bb test

Testing clojure.test-clojure.spec

Testing clojure.test-clojure.instr

Testing clojure.test-clojure.multi-spec

Ran 13 tests containing 168 assertions.
0 failures, 0 errors.

In addition to spec's own tests, I've added tests from the following libraries that are using spec to babashka's CI: better-cond, coax, orchestra and integrant.

The benefit of maintaining a fork with minimal changes is that I can easily pull in changes from upstream. The fork is available here and works with the newly released babashka 0.7.0.

A demo of data generation using spec:

(require '[babashka.deps :as deps])

(deps/add-deps '{:deps {org.babashka/spec.alpha {:git/url ""
:git/sha "644a7fc216e43d5da87b07471b0f87d874107d1a"}

(require '[clojure.spec.alpha :as s]
'[clojure.spec.gen.alpha :as gen]
'[clojure.string :as str])

(prn (gen/sample (s/gen int?)))

(s/def ::street (s/and string? (complement str/blank?)))
(s/def ::street-number int?)
(s/def ::address (s/keys :req-un [::street ::street-number]))
(s/def ::name (s/and string? (complement str/blank?)))
(s/def ::contact (s/keys :req-un [::name ::address]))
(prn (gen/sample (s/gen ::contact)))

(0 -1 0 -1 -6 -1 -2 0 9 1)
({:name "r", :address {:street "Y", :street-number -1}} {:name "m", :address {:street "k8", :street-number -1}} {:name "5", :address {:street "s1E", :street-number 0}} {:name "o4H", :address {:street "4", :street-number -1}} {:name "nkhf", :address {:street "4Fh92", :street-number 1}} {:name "e", :address {:street "X", :street-number 4}} {:name "5v76a9B", :address {:street "bEf7e", :street-number 23}} {:name "213V", :address {:street "fNX3wr", :street-number 5}} {:name "8336lvbb9", :address {:street "fVP", :street-number -1}} {:name "x8X", :address {:street "utPQA", :street-number -7}})

The above example executes in about 120ms on my machine.

Discuss this post here.

Published: 2021-12-10