Clojure(ish) interpreter in Swift
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(CI is probably broken because Xcode 8 is not working properly on Travis. Sorry.)

An interpreter for a dialect of Clojure, implemented in Swift (and formerly known as Lambdatron). The interpreter endeavors to match Clojure's behavior as closely as possible. The eventual goal is a library that can be used independently of the REPL front-end.

Bug reports, feature requests, PRs, and other contributions are welcome!

Web site: (coming soon); Twitter: @cormorantlang.


Cormorant is a framework written in Swift 3. You will need Xcode 8 beta 1 or later to build.

Cormorant comes with a REPL demonstrating its capabilities. Due to current limitations with Swift, the REPL is a Cocoa OS X application. It can either be run directly (in GUI mode), or the underlying executable can be invoked from the command line with the -c flag to run the interactive REPL as a command line application. When in the REPL, type expressions at the command prompt and press 'Enter'.

Getting started: Clone the repository. Open up the Cormorant project in Xcode. Select the CormorantREPLRunner target and press the Run button to build and run the REPL. Alternately, select CormorantTests and run that to run the test suite. See the Development section below for more details on the development environment and configuration.

Running as command line application: Use the terminal command ./path/to/ -c, and replace /path/to with whatever the absolute or relative path is to the Cocoa bundle.

Grimoire is a high-quality Clojure API reference, and can be used to reference the intended behavior of all included functions and special forms.

Need ideas? Try:

Basic arithmetic

  • (+ (* 2 4) (- 8 6) (+ (+ 1 3) 4))

Working with collections

  • (cons 1 '(2 3 4))
  • (rest '(1 2 3 4 5))
  • (concat '(1 2 3) [4 5] {"six" 6 "seven" 7})
  • (seq {1 "one" 2 true "three" [nil]})

Defining and calling a function

  • (defn myfunc [a b] (+ a b 1)), then (myfunc 10 20)
  • Functions returning functions: (defn f1 [arg1] (fn [arg2] (+ arg1 arg2))), then (let [plusone (f1 1)] (plusone 3))

Recursion and iteration

  • Basic recursion: (defn r [a] (print a " ") (if (> a 0) (r (- a 1)))), then (r 10)
  • Tail-call recursion using recur: (defn recadd [mylist sofar] (if (= (first mylist) nil) sofar (recur (rest mylist) (+ (first mylist) sofar)))), then (recadd '(1 2 3 4 5) 0)
  • Iteration using loops: (loop [a 10 b 0] (if (= a 0) b (recur (- a 1) (+ b a))))

Creating and using a macro

  • (defmacro my-when [predicate then-do] `(if ~predicate ~then-do nil)), then (my-when (= 1 1) "good") or (my-when (= 1 2) (do (print "this shouldn't show up") "bad"))

Current Limitations

Cormorant has a couple of limitations, due mostly to its work-in-progress status:

  • The REPL can only take one form at a time.

These will disappear as the feature set is filled out.


Cormorant has a number of possibly useful features. Cormorant's data structures should share identical semantics with Clojure's data structures (in that they are immutable and persistent), although some of them are implemented using Swift's native copy-on-write features rather than as persistent data structures.

Sequences, like in Clojure. Sequences are immutable, and come in several flavors. You can prepend items using cons, extract the first element using first, or get the sequence consisting of all but the first element using rest. Create a sequence or get a sequence view using seq.

  • Cons lists, the bread and butter of Lisp. Create the empty list using (). Or use the list function to create a list from zero or more arguments.
  • Lazy sequences, which store a thunk whose evaluation is deferred until absolutely necessary, after which they cache their value. Create a lazy sequence using the stdlib lazy-seq macro. Some of the stdlib and built-in functions may also return lazy sequences.
  • Lightweight views into strings, vectors, and hashmaps, which allow treating those collections like sequences without requiring expensive copying operations. These are transparent to the end user.
  • Contiguous lists backed by an Array. These are transparent to the end user.

Vectors, declared using square brackets: [1 2 true "Lisp"], or the vector function. Unlike lists, vectors can't be used to invoke functions. Vectors can be used in function position with an integer index argument.

Maps, declared using braces: {"one" 1 "two" nil "three" [1 2 3]}, or the hash-map function. Maps can be used in function position in order to get a value for a key.

Strings, declared using double quotes: "this is a string literal". Strings can be manipulated using first, rest, etc. Cormorant also supports a mutating string builder which can be used to efficiently construct strings through concatenation. Create one using sb, work with the buffer using sb-append and sb-reverse, and turn it back into a string using str.

Regular expressions, declared as such: #"[0-9]+", and backed by NSRegularExpression. Use re-pattern to create a regex at runtime. Use re-first to get the first match of a pattern in a string, and re-seq to get a list of all matches of a pattern in a string. Call re-iterate with a pattern, a string, and a function that takes two arguments; the function will be called once for each match with a vector of match tokens and a vector of ranges. The function can return true to end iteration, or any other value to allow iteration to continue.

Functions are first-class citizens which capture their environment (except for values defined using def). Create them using fn, followed by an optional name, a vector containing parameter bindings, and one or more forms comprising the function body. Or create a function bound to a global name using defn. Multiple arities can be defined by passing in one or more lists, each of which starts with a vector containing parameter bindings followed by the function body. Define varargs by passing in a parameter binding vector ending with & and the name of a vector to place the rest of the arguments (e.g. [a b & others]).

Macros are like functions, except that their arguments aren't evaluated before being passed in and the output is intended to be a form which can be further evaluated at runtime. Like functions, macros capture their (non parameter binding) context. Create them using defmacro. Macros can be defined with multiple arities and/or varargs.

Let-binding, using let, allows you to create a lexical context with new bindings available only within the scope of that context.

Vars are mutable memory cells that are interned within a namespace, and can be referred to by a qualified or unqualified symbol. Create them using def (e.g. def myVar 100). Vars are reified. Use the var special form to get the Var that a symbol corresponds to, and deref to extract the value contained within a Var.

Basic types include:

  • Booleans (true and false)
  • nil
  • integers
  • floating-point numbers (e.g. 1.234)
  • character literals (\a, \tab, \space, \newline, \return), which can be used in function position. Character literals can also be specified using the hexadecimal \uNNNN or octal \oNNN forms.
  • keywords (:else), which can be used in function position

Namespaces make organizing your code easier. Currently, all stdlib code is contained within the core namespace, and the user begins in the user namespace. Namespaces are reified and can be manipulated as normal objects using some of the built-in functions.

Syntax-quote makes defining macros slightly less tedious. Use ' to denote a normal quoted form. Use ` to denote a quote that should be syntax-quoted; within such a form ~ (unquote) can be used to force evaluation of the unquote form, while ~@ (unquote-splice) can be used to force evaluation of a form to a collection whose elements are then spliced in. Within a syntax-quoted expression, unqualified symbols are qualified to the current namespace (e.g. a might become user/a), while unqualified symbols suffixed by a # are converted into gensym'ed symbols.

Comments start with a semicolon and continue until the end of the current line: ; this is a comment

The following special forms, reader macros, and functions are built into the interpreter:

  • Special forms: quote, if, do, def, let, var, fn, defmacro, loop, recur, apply, attempt
  • Reader macros: ', `, ~, ~@, #', @
  • Namespace manipulation: ns-create, ns-set, ns-get, ns-name, ns-all, ns-find, ns-unmap, ns-alias, ns-aliases, ns-unalias, ns-refer, ns-map, ns-interns, ns-refers, ns-resolve, ns-remove
  • Collection manipulation: list, vector, hash-map, cons, first, rest, next, conj, concat, nth, seq, lazy-seq, get, assoc, dissoc, count, reduce
  • Primitive manipulation: symbol, keyword, int, double
  • String manipulation: str, subs, lower-case, upper-case, replace, replace-first
  • String building: sb, sb-append, sb-reverse
  • Regular expressions: re-pattern, re-first, re-seq, re-iterate, re-quote-replacement
  • I/O: read, print, println
  • Testing: nil?, number?, int?, float?, string?, char?, symbol?, keyword?, fn?, eval?, true?, false?, var?, seq?, vector?, map?, pos?, neg?, zero?, subnormal?, infinite?, nan?
  • Arithmetic: +, -, *, /, rem, quot
  • Comparison: =, ==, <, <= >, >=
  • Miscellaneous: deref, gensym, read-string, rand, eval, fail

Additional functions and macros are available as part of the standard library.


Some notes on Cormorant development tools follow.

Development and OS X's SIP

When debugging Cormorant, you have at least three options:

  • Run Cormorant's REPL as a Cocoa app, and debug as usual.
  • Run Cormorant's REPL in the built-in Xcode console by setting the -c command-line argument in the scheme. Unfortunately, anything you type will be echoed twice due to an ancient and still-unfixed bug.
  • Add Cormorant as a library to your own Cocoa or command-line app, and debug from there.

If, in the future, it becomes possible to properly build command-line applications that depend upon Swift frameworks, you might be able to debug using Xcode by running the REPL in, but you will have to disable SIP to do so.

Code Organization

Cormorant is divided into three components:

  • The core framework (Cormorant)
  • The REPL framework (CormorantREPL)
  • A Cocoa/command-line app and associated build settings for running the REPL (CormorantREPLRunner)


You can directly debug the Cormorant framework through the Cocoa REPL app.

If you want to profile using the command-line REPL, follow these instructions (you will need to disable SIP):

  1. Copy the app to a convenient location.
  2. Invoke the underlying binary from the command line using the -c flag to start the command-line REPL.
  3. In Xcode, go to "Debug" --> "Attach to Process" and choose the "CormorantREPLRunner" target. If you are fortunate it'll be at the top under "Likely Targets".
  4. Debug as usual. Note that modifying a breakpoint might cause libedit to redraw the prompt in Terminal. This is annoying, but merely cosmetic.


You can directly profile the Cormorant framework through the Cocoa REPL app.

If you want to profile using the command-line REPL, follow these instructions (you will need to disable SIP):

  1. Copy the app to a convenient location.
  2. Invoke the underlying binary from the command line using the -c flag to start the command-line REPL.
  3. Open Instruments and choose the Instrument you want to profile with.
  4. Choose the target manually: it will be named "CormorantREPLRunner", under "System Processes". (Use Activity Monitor to get the PID if you can't find it.)
  5. Profile as usual.


Logging is in an embryonic state. In the command-line version of the REPL, type ?logging <DOMAIN> on or ?logging <DOMAIN> off to turn logging on or off for a given domain, or omit the <DOMAIN> argument to turn logging on or off globally.

The only currently supported domain is eval. This logging domain prints out messages detailing how macros, functions, etc are evaluated, and can be useful to see exactly what the interpreter is doing when it evaluates a form.


The command-line version of the REPL includes a basic benchmarking tool. Invoke it using ?benchmark <SOME_FORM> <ITERATIONS>, where <SOME_FORM> is a valid code snippet and <ITERATIONS> is the number of times to repeat the execution. The benchmark tool will print the average, minimum, and maximum run times (in milliseconds).

The code snippet is lexed, parsed, and expanded before the benchmark, and the expanded data structure is cached between benchmark iterations, so the benchmarking tool only measures evaluation time. As well, the context is not cleared between executions, so side effects caused by one iteration are visible to all subsequent iterations.

Unit Tests

Cormorant has a comprehensive unit test suite that exercises the interpreter (not the standard library). Run the unit tests from within Xcode.

Cormorant relies on Travis CI for continuous integration. Click on the badge at the top of the README to see more information about CI testing.

Development Objectives

Development objectives can be divided into two categories.

Working On

These are objectives I am working on right now, or plan on doing in the near future.

  • Optimizing and refactoring code to take advantage of Swift 3's new features
  • Interpreter (translate source to bytecode and then execute)
  • Expanding standard library
  • Support for sets
  • Ability to type in multiple forms at the top level
  • Metacontext - allow consumer to define custom functions visible to the user
  • Performance optimization (once development stabilizes)
  • Full unit test suite (once development stabilizes)
  • Metadata

(Very) Long Term Goals

These are objectives that are either too big in scope to schedule, too technically challenging at this time, or of uncertain utility.

  • Persistent data structures
  • STM and support for multithreading
  • Destructuring via pattern matching
  • Custom types (e.g. deftype) and multimethods (may not be possible at the moment)
  • Rationals and bignums (may need to wait for a suitable Swift library handling these first)
  • Full Foundation/Cocoa bindings
  • Better Swift runtime interop (if proper reflection support ever comes to Swift)

Differences From Clojure

Aside from the (long) list of features not yet implemented (see the Working On and (Very) Long Term Goals sections above), there are a couple of intentional deviations from Clojure's API or conventions:

  • Hashmap iteration is not guaranteed to traverse the elements in the same order as in Clojure. No guarantees are made on hashmap iteration except that each key-value pair is visited exactly once. This has implications for any function that converts a map into an ordered sequence.
  • ifn? doesn't exist; use eval? instead. This is because Cormorant does not use protocols (i.e. interfaces) to define constructs that can be used in function position.
  • try doesn't exist. attempt is a (very basic) error handling facility. It takes one or more forms, executing each sequentially, and returns the first successful value (or the error from executing the final form).
  • The byte, short, long, and float functions are not implemented, as Cormorant only has an integer and a double-precision floating point numerical data type.
  • The subnormal?, infinite?, and nan? functions return false for integer arguments, and can be used to test whether floating point numbers are subnormal, infinite, or NaN (respectively).
  • keyword returns nil if given an empty string as an argument, not an invalid empty symbol.
  • read does not take an optional argument representing a reader object.
  • char-escape-string returns nil for the \formfeed and \backspace arguments, since Swift does not recognize the \f and \b escape sequences.
  • Regex support follows Cocoa conventions, since NSRegularExpression is very different from java.util.pattern.Regex and java.util.pattern.Match. re-iterate provides an idiomatic wrapper for enumerateMatchesInString:options:range:usingBlock:.
  • Once a namespace has been marked for deletion using ns-remove, all its aliases are automatically unregistered, and new aliases or refers can no longer be set for it.


Cormorant © 2015-2016 Austin Zheng, released as open-source software subject to the following terms.

The use and distribution terms for this software are covered by the Eclipse Public License 1.0 ( which can be found in the file epl-v10.html at the root of this distribution. By using this software in any fashion, you are agreeing to be bound by the terms of this license. You must not remove this notice, or any other, from this software.