Small Clojure Interpreter

I want a limited dialect of Clojure for a single-purpose, scripted application. Sci will fit nicely.

— @tiagoluchini

Quickstart

Use from Clojure(Script)

(require '[sci.core :as sci])
(sci/eval-string "(inc 1)") => ;; 2
(sci/eval-string "(inc x)" {:bindings {'x 2}}) ;;=> 3

More on how to use sci from Clojure. Use from JavaScript. Use from Java.

Rationale

You want to evaluate code from user input, or use Clojure for a DSL inside configuration files, but eval isn't safe or simply doesn't work.

This library works with:

• Clojure on the JVM
• Clojure compiled with GraalVM native
• ClojureScript, even when compiled with :advanced, and (as a consequence) JavaScript

It is used in:

• Babashka. A Clojure scripting tool that plays well with Bash.
• Bootleg. An HTML templating CLI.
• Closh. Bash-like shell based on Clojure. GraalVM port is work in progress.
• Jet. CLI to convert between JSON, EDN and Transit.
• Malli. Plain data Schemas for Clojure/Script.

Status

Experimental. Breaking changes are expected to happen at this phase.

Installation

Use as a dependency:

Usage

Currently the only API function is sci.core/eval-string which takes a string to evaluate and an optional options map.

In sci, defn does not mutate the outside world, only the evaluation context inside a call to sci/eval-string.

By default sci only enables access to the pure non-side-effecting functions in Clojure. More functions can be enabled, at your own risk, using :bindings:

user=> (require '[sci.core :as sci])
user=> (sci/eval-string "(println \"hello\")" {:bindings {'println println}})
hello
nil

It is also possible to provide namespaces which can be required:

user=> (def opts {:namespaces {'foo.bar {'println println}}})
user=> (sci/eval-string "(require '[foo.bar :as lib]) (lib/println \"hello\")" opts)
hello
nil

You can provide a list of allowed symbols. Using other symbols causes an exception:

user=> (sci/eval-string "(inc 1)" {:allow '[inc]})
2
user=> (sci/eval-string "(dec 1)" {:allow '[inc]})
ExceptionInfo dec is not allowed! [at line 1, column 2]  clojure.core/ex-info (core.clj:4739)

Providing a list of disallowed symbols has the opposite effect:

user=> (sci/eval-string "(inc 1)" {:deny '[inc]})
ExceptionInfo inc is not allowed! [at line 1, column 2]  clojure.core/ex-info (core.clj:4739)

Preventing forever lasting evaluation of infinite sequences can be achieved with :realize-max:

user=> (sci/eval-string "(vec (range))" {:realize-max 10})
ExceptionInfo Maximum number of elements realized: 10 [at line 1, column 1]  clojure.core/ex-info (core.clj:4739)

The preset :termination-safe, which is currently {:deny '[loop recur trampoline] :realize-max 100}, is helpful for making expressions terminate:

user=> (sci/eval-string "(loop [] (recur))" {:preset :termination-safe})
ExceptionInfo loop is not allowed! [at line 1, column 2]  clojure.core/ex-info (core.clj:4739)

Providing a macro as a binding can be done by providing a normal function that:

• has :sci/macro on the metadata set to true
• has two extra arguments at the start for &form and &env:

user=> (def do-twice ^:sci/macro (fn [_&env _&form x] (list 'do x x)))
user=> (sci/eval-string "(do-twice (f))" {:bindings {'do-twice do-twice 'f #(println "hello")}})
hello
hello
nil

Feature parity

Currently the following special forms/macros are supported: def, fn, function literals (#(inc %)), defn, quote, do,if, if-let, if-not, when, when-let, when-not, cond, let, and, or, ->, ->>, as->, comment, loop, lazy-seq, for, doseq, case, try/catch/finally, declare, cond->, cond->>, require, import, in-ns, ns. Sci also supports user defined macros.

More examples of what is currently possible can be found at babashka.

If you miss something, feel free to post an issue.

Caveats

To make the rand-* functions behave well when compiling to a GraalVM native binary, use this setting:

--initialize-at-run-time=java.lang.Math\$RandomNumberGeneratorHolder

Use from JavaScript

> const { evalString } = require('@borkdude/sci');
> const opts = {bindings: {f: function() { console.log('hello'); }}};
> evalString("(dotimes [i 2] (f))", opts);
hello
hello

Note for JavaScript users: the JS API is similar to the Clojure one. Instead of symbols and keywords it expects strings. Instead of kebab-case, use camelCase. Read here how to use sci from Clojure.

Use from Java

import borkdude.sci.*;
import borkdude.sci.options.*;

Namespace fooBar = new Namespace("foo.bar");
fooBar.addVar("x", 1);
Options opts = new Options().addNamespace(fooBar);
Sci.evalString("foo.bar/x", opts); // returns 1

Note for Java users: the Java API for is conceptually similar to the Clojure one, but made more idiomatic for Java users. Check the generated Java documentation.

Test

Required: lein, the clojure CLI and GraalVM.

To succesfully run the GraalVM tests, you will have to compile the binary first with script/compile.

To run all tests:

script/test/all

For running individual tests, see the scripts in script/test.

Thanks

• adgoji for financial support

License

Copyright © 2019 Michiel Borkent

Distributed under the Eclipse Public License 1.0. This project contains code from Clojure and ClojureScript which are also licensed under the EPL 1.0. See LICENSE.