Small programming language with Lisp-like syntax.
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Small language with Lisp-like syntax. Implemented in Clojure. Aka my langdev playground :)


(in no particular order)

  • add fexprs
  • add macros? (are they needed? will it be possible to implement them just with fexprs?)
  • fix garbage collector collecting closures (or get rid of it completely)
  • TCO?
  • play with amb operator :)
  • own function (or fexpr) type, don't use Clojure functions?
  • own reader (instead of using the one from Clojure :)
  • library of useful functions etc. (with fexprs, everything should be bootstrappable)
  • explore delimited continuations
  • explore first class environments
  • explore Expansion passing style macro mechanism


You can build Slang from source:

  • install leiningen (Clojure build tool)
  • clone this repository: git clone -b gc
  • in the cloned repository, run: lein uberjar. This will create a .jar file, named slang-VERSION-standalone.jar, in the target/ directory of the cloned repository.

Or you can download already built _.jar_ file: [slang-0.1.1-standalone.jar](


Invoking the Slang interpreter:

  • run java -jar slang-VERSION-standalone.jar to start the Slang repl
  • run java -jar slang-VERSION-standalone.jar FILE to interpret the FILE containing Slang source code



(quote expr) - Quotation. Prevents the evaluation of expr.

(if test then else) - Ordinary IF clause, returning the value of then or else.

(def name val) - Binds the value val to the name in the current environment. Used to define names for values/functions.

(struct) - Returns new structure.

(set structname fieldname val) - Sets the field fieldname in the structure structname to the val and returns the structure.

(get structname fieldname) - Returns the value of the given field in the specified structure.

(do exprs...) - Evaluates all the expresions and returns the value of the last one.

(for (var init end) expr) - Cycle. The var will be bound to values in range [init; end]. Creates new local environment.

(fun (args...) expr) - Function. Creates new local environment.

(funcname args...) - Function invocation.


  • numbers: 2, 2.3
  • strings: "Hello"
  • symbols: (quote x)
  • booleans: true, false
  • non-defined value: nil

Useful functions

  • Math etc.: +, -, *, /, =, <, >, <=, >=
  • Lists etc.: car, cdr, cons, list?, symbol?
  • Direct environment manipulation: new-env, lookup, bind, unbind, exists?
  • Other: print, evals, exec-file

See [core.clj]( for more information.


Variables and functions:

(def ans 42)
;=> 42
(def second (fun (x y) y))
;=> #<some-fn-describing-string>
(second "give me" ans)
;=> 42


(def person (struct))
;=> {}
(set person name "John Doe")
;=> {name John Doe}
(set person age 35)
;=> {age 35, name John Doe}
(get person age)
;=> 35

Flow control:

(if (= 1 2) "This is weird world!" "Everything's ok.")
;=> Everything's ok.
(for (i 1 5) (do (print i) "returned value"))
;=> returned value

Higher order functions:

(def compose1 (fun (f g) (fun (x) (f (g x)))))
(def partial1 (fun (f arg1) (fun (arg2) (f arg1 arg2))))
(def plus7 (partial1 + 7))
(def mul10 (partial1 * 10))
(def plus7_mul10 (compose1 plus7 mul10))
(plus7_mul10 99)
;=> 997

It is possible to manipulate environments directly from the Slang:

;; current local environment can be accessed via the &env variable:
(do (bind (quote x) 144 &env)
    (lookup (quote x) &env))
;=> 144
(exists? (quote x) &env)            ;; executed in the current local environment
;=> true
(exists? (quote x) (new-env))       ;; in a new empty local environment
;=> false
(exists? (quote x) (new-env &env))  ;; in a new local environment referencing the current one
;=> true
(do (unbind (quote x) &env)
    (lookup (quote x) &env))
;=> nil

;; interpreter itself can be accessed from the language as well, via the 'evals' function:
(evals (quote ((fun (x) (* 7 x)) 28)))
;=> 196
(evals (quote (do (def x 50) x))     ;; executed in a new local environment
       (new-env &env))
;=> 50
;=> nil
(evals (quote (do (def x 202) x)))  ;; executed in the same environment
;=> 202
;=> 202

Copyright © 2012 Mnicky ([](

Distributed under the MIT license.