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Small Clojure Interpreter


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.


You want to evaluate code from user input, or use Clojure for a DSL inside your project, 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.
  • Chlorine. Socket-REPL and nREPL package for Atom editor.
  • Bytefield-svg. NodeJS library to generate byte field diagrams.
  • Closh. Bash-like shell based on Clojure. GraalVM port is work in progress.
  • Dad. A configuration management tool.
  • Jet. CLI to convert between JSON, EDN and Transit.
  • Malli. Plain data Schemas for Clojure/Script.
  • PCP. Clojure Processor (PHP replacement).
  • Spire. Pragmatic provisioning using Clojure.


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


Use as a dependency:

Clojars Project NPM Project

API docs

For Clojure, see the generated codox documentation.


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}})

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

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

In fact {:bindings ...} is just shorthand for {:namespaces {'user ...}}.

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

user=> (sci/eval-string "(inc 1)" {:allow '[inc]})
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 [_&form _&env x] (list 'do x x)))
user=> (sci/eval-string "(do-twice (f))" {:bindings {'do-twice do-twice 'f #(println "hello")}})


Sci has a var type, distinguished from Clojure vars. In a sci program these vars are created with def and defn just like in normal Clojure:

(def x 1)
(defn foo [] x)
(foo) ;;=> 1
(def x 2)
(foo) ;;=> 2

Dynamic vars with thread-local bindings are also supported:

(def ^:dynamic *x* 1)
(binding [*x* 10] x) ;;=> 10
(binding [*x* 10] (set! x 12) x) ;;=> 12
x ;;=> 1

Pre-creating vars that can be used in a sci program can be done using sci/new-var:

(def x (sci/new-var 'x 10))
(sci/eval-string "(inc x)" {:bindings {'x x}}) ;;=> 11

To create a dynamic sci var you can set metadata or use sci/new-dynamic-var:

(require '[sci.core] :as sci)
(def x1 (sci/new-var 'x 10 {:dynamic true}))
(sci/eval-string "(binding [*x* 12] (inc *x*))" {:bindings {'*x* x1}}) ;;=> 13
(def x2 (sci/new-dynamic-var 'x 10))
(sci/eval-string "(binding [*x* 12] (inc *x*))" {:bindings {'*x* x2}}) ;;=> 13

Pre-created sci vars can also be externally rebound:

(def x (sci/new-dynamic-var 'x 10))
(sci/binding [x 11] (sci/eval-string "(inc *x*)" {:bindings {'*x* x2}})) ;;=> 11

The dynamic vars *in*, *out*, *err* in a sci program correspond to the dynamic sci vars sci.core/in, sci.core/out and sci.core/err in API. These vars can be rebound as well:

(def sw (
(sci/binding [sci/out sw] (sci/eval-string "(println \"hello\")")) ;;=> nil
(str sw) ;;=> "hello\n"

A shorthand for rebinding sci/out is sci/with-out-str:

(sci/with-out-str (sci/eval-string "(println \"hello\")")) ;;=> "hello\n"

Stdout and stdin

To enable printing to stdout and reading from stdin you can bind sci.core/out and sci.core/in to *out* and *in* respectively:

(sci/binding [sci/out *out*
              sci/in *in*]
  (sci/eval-string "(print \"Type your name!\n> \")")
  (sci/eval-string "(flush)")
  (let [name (sci/eval-string "(read-line)")]
    (sci/eval-string "(printf \"Hello %s!\" name)
                     {:bindings {'name name}})))
Type your name!
> Michiel
Hello Michiel!


Creating threads with future and pmap is disabled by default, but can be enabled by requiring sci.addons and applying the sci.addons/future function to the sci options:

   [sci.core :as sci]
   [sci.addons :as addons]))

(sci/eval-string "@(future (inc x))"
                 (-> {:bindings {'x 1}}
;;=> 2

For conveying thread-local sci bindings to an external future use sci.core/future:

   [sci.core :as sci]
   [sci.addons :as addons]))

(def x (sci/new-dynamic-var 'x 10))

@(sci/binding [x 11]
     (sci/eval-string "@(future (inc x))"
                      (-> {:bindings {'x @x}}
;;=> 12


Adding support for classes is done via the :classes option:

(sci/eval-string "(java.util.UUID/randomUUID)"
  {:classes {'java.util.UUID java.util.UUID}})
;;=> #uuid "312ba519-37e2-4109-b164-97fb140b57b0"

To make this work with GraalVM you will also need to add an entry to your reflection config for this class. Also see reflection.json.


Sci uses an atom to keep track of state changes like newly defined namespaces and vars. You can carry this state over from one call to another by providing the atom yourself as the value for the :env key:

(def env (atom {})
(sci/eval-string "(defn foo [] :foo)" {:env env})
(sci/eval-string "(foo)" {:env env}) ;;=> :foo

The contents of the the :env atom should be considered implementation detail.

Using an :env atom you are allowed to change options at each invocation of eval-string. If your use case doesn't require this, the recommendation is to use a sci context instead.

A sci context is derived once from options as documented in sci.core/eval-string and contains the runtime state of a sci session.

(def opts {:namespaces {' {'x 1}}})
(def sci-ctx (sci/init opts))

Once created, a sci context should be considered final and should not be mutated by the user. The contents of the sci context should be considered implementation detail.

The sci context can be re-used over successive invocations of sci.core/eval-string*.

The major difference between eval-string and eval-string* is that eval-string will call init on the passed options and will pass that through to eval-string*. When you create a sci context yourself, you can skip the extra work that eval-string does and work directly with eval-string*.

(sci/eval-string* sci-ctx "") ;;=> 1
(sci/eval-string* sci-ctx "(ns (def x 2) x") ;;=> 2
(sci/eval-string* sci-ctx "") ;;=> 2

Implementing require and load-file

Sci supports implementation of code loading via a function hook that is invoked by sci's internal implementation of require. The job of this function is to find and return the source code for the requested namespace. This passed-in function will be called with a single argument that is a hashmap with a key :namespace. The value for this key will be the symbol of the requested namespace.

This function can return a hashmap with the keys :file (containing the filename to be used in error messages) and :source (containing the source code text) and sci will evaluate that source code to satisfy the require. Alternatively the function can return nil which will result in sci throwing an exception that the namespace can not be found.

This custom function is passed into the sci context under the :load-fn key as shown below.

(defn load-fn [{:keys [namespace]}]
  (when (= namespace 'foo)
    {:file "foo.clj"
     :source "(ns foo) (def val :foo)"}))
(sci/eval-string "(require '[foo :as fu]) fu/val" {:load-fn load-fn})
;;=> :foo

Note that internally specified namespaces (either those within sci itself or those mounted under the :namespaces context setting) will be utilised first and load-fn will not be called in those cases, unless :reload or :reload-all are used:

  "(require '[foo :as fu])
  {:load-fn load-fn
   :namespaces {'foo {'val (sci/new-var 'val :internal)}}})
;;=> :internal

  "(require '[foo :as fu] :reload)
  {:load-fn load-fn
   :namespaces {'foo {'val (sci/new-var 'val :internal)}}})
;;=> :foo

Another option for loading code is to provide an implementation of clojure.core/load-file. An example is presented here.

    (:require [sci.core :as sci]
              [ :as io]))

(spit "example1.clj" "(defn foo [] :foo)")
(spit "example2.clj" "(load-file \"example1.clj\")")

(let [env (atom {})
      opts {:env env}
      load-file (fn [file]
                  (let [file (io/file file)
                        source (slurp file)]
                      {sci/ns @sci/ns
                       sci/file (.getCanonicalPath file)}
                      (sci/eval-string source opts))))
      opts (assoc-in opts [:namespaces 'clojure.core 'load-file] load-file)]
  (sci/eval-string "(load-file \"example2.clj\") (foo)" opts))
;;=> :foo

Feature parity

Currently the following special forms/macros are supported: def, fn, function literals (#(inc %)), defn, quote, do,if, if-let, if-not, if-some when, when-first, when-let, when-not, when-some, cond, let, letfn, and, or, ->, ->>, as->, comment, loop, lazy-seq, for, doseq, case, try/catch/finally, declare, cond->, cond->>, some->, require, import, in-ns, ns, binding, with-out-str, with-in-str, future. 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.


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


Use from JavaScript

Sci is available on NPM:

$ npm install @borkdude/sci

The JavaScript API consists of two functions, evalString to evaluate Clojure expressions and toJS to convert Clojure data structures back to JavaScript.

> const { evalString, toJS } = require('@borkdude/sci');
> x = evalString("(assoc {:a 1} :b 2)")
> toJS(x)
{ a: 1, b: 2 }

The function evalString takes an optional second argument to pass options. Read here how to use those options. Instead of symbols and keywords it expects strings. Instead of kebab-case, use camelCase.

Use as native shared library

To use sci as a native shared library from e.g. C, C++, Rust, read this tutorial.


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:


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



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.

Can you improve this documentation? These fine people already did:
Michiel Borkent, sogaiu, Nate Jones, Crispin Wellington, Tommi Reiman, Maurício Szabo, Jeroen van Dijk, Lee Read, Rahuλ Dé & JC
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