Overlord is an experimental build/module system for Common Lisp, inspired by Redo and Racket.

Overlord addresses three problems which might seem unrelated, but which, on closer examination, turn out to the same problem:

  1. It provides a module system for implementing languages as libraries (similar to Racket).

  2. It provides a powerful general-purpose build system (a superset of Make, similar to Redo).

  3. It lets you reproducibly specify the desired state of a Lisp system which is to be saved as an image.

Advice for users

Overlord is experimental. For the most part, trying to document the API at this stage would be futile. Instead, this README discusses the concepts behind Overlord. If you?re looking for the current syntax, consult the test suite and the files it uses.

(If you are interested in reading the code, the substance is in impl.lisp; the rest is support.)

Before loading Overlord, it would be a good idea to make sure you are running the latest version of ASDF.

Note that, to run the test suite, you will need to download Core Lisp, and, if not on Windows, you must have the touch program in your search path. (On Windows, Powershell is used instead).

Overlord stores its persistent data structures in a cache directory. On Linux, this is ~/.cache/overlord/. The data structures stored there are versioned. Since this version number is increasing rapidly, it might worth checking the cache directory from time to time to delete obsolete files.

When I say ?experimental?, I mean it. Anything may change at any time. This code is not ready for use. It may not even be ready for release.

Embedding languages

Overlord enables languages as libraries. Overlord languages have several important properties:

  1. Languages are first-class. Modules live in their own files, just like Lisp code, and are compiled into FASLs, just like Lisp code.

  2. Languages can use any syntax. Unlike embedded DSLs, which are limited by what can be done with reader macros, full languages can use any parser they like.

  3. Languages are interoperable. Lisp code can import modules written in embedded languages, and modules written in embedded languages can import other modules ? even modules written in other languages.

  4. Languages are reusable. Support for meta-languages allows different languages to share the same parser or for the same language to be written in more than one syntax.

One goal of the language implementation is to provide a reasonable level of integration with Emacs. A prototype Emacs minor mode is included in <elisp/overlord.el>. Using overlord-minor-mode lets you recompile a module using C-c C-k, and lets you view the Lisp code that the file expands into using C-c RET ? just like an ordinary macroexpansion.

Language examples

Here are some example language embeddings:

  1. overlord/demo/js. A simple demo language built on CL-JavaScript. Shows how to convert a pre-existing CL language implementation to work with Overlord.

  2. Bosom Serpent. Shows how to wrap a foreign runtime (Python, using burgled-batteries) as an Overlord module.

  3. cl-yesql. Lisp port of Clojure?s yesql. Includes a parser, and shows how (and why) to load the same file in different languages.

  4. cl-https-everywhere. In-process HTTPS Everywhere rulesets. Shows how to combine Overlord?s support for languages with direct use of the build system.

  5. Core Lisp. A hygiene-compatible implementation of the Lisp dialect ISLISP (itself a conceptual subset of Common Lisp). Shows how to use Overlord to build ?language towers.?

Overlord vs. ASDF

Overlord expects to be used alongside ASDF, with ASDF responsible for compiling and loading Lisp code, and Overlord doing everything else.

Overlord is mostly independent of ASDF. It still, however, needs ASDF to resolve relative pathnames in Lisp code. Depending on ASDF in this respect is probably not unreasonable, since making it easy to locate a system?s files is what ASDF was designed for.

(Note that, while Overlord is mostly independent of ASDF the program, it still depends on ASDF the project, because it builds on the UIOP portability layer.)

(Note also that relying on ASDF means there is a discrepancy in the handling of relative pathnames in Overlord modules vs. Lisp files. Relative pathnames in Overlord modules are relative to the file that contains the module, but relative pathnames in Lisp files are relative to the base of the system they are loaded in. This is, unfortunately, unlikely to change.)

Overlord vs. Redo

Redo is a build system. Actually, Redo is a family of build systems, sharing a design due to Daniel J. Bernstein (although he has never released an implementation). In the space of build systems, Redo is a remarkable local optimum: it is both very powerful and very simple. It is not the most powerful build system possible, or the very simplest, but it certainly provides the most leverage.

Overlord generalizes the idea of a build system so that its dependency graph can include both files on disk and state in the Lisp system. It manages the state stored by Lisp itself, in symbols, and provides ways to manage other kinds of state. This generalized build system is modeled on Redo.

(Build systems manage state in the file system. In Common Lisp, we have a bundle of state which, in many ways, resembles a file system: a set of persistent, mutable locations with first-class, hierarchical addresses. Symbols have value cells and function cells (mutable locations); they have property lists; they are used as keys in arbitrary namespaces (more locations); they can be persisted, in fasl files and in saved images; and they are each uniquely addressed with a path (package name, symbol name).)

The most obvious, but least important, difference between Overlord and Redo in practice is that Redo uses shell scripts, while Overlord?s ?scripts? are written in Lisp. (It is unimportant because, after all, you can run shell commands from Lisp, or somehow call Lisp from the shell.) On the one hand, embedding shell syntax in Lisp is clumsy; on the other hand, Lisp special variables are much superior to any shell-based means for passing information between parent and child scripts. (See ?5.4.2 in Grosskurth 2007.)

The important difference is that Overlord uses two scripts per target: one for building the target, and another for computing its dependencies. This (mostly) replaces the need to maintain a database of dependencies.


Overlord has basic integration with the command line.

It is possible to run Overlord using either cl-launch or Roswell. The cl-launch script is in cl-launch/overlord. The Roswell script is in roswell/overlord.ros. They each use the same syntax.

Building with signals

One other potentially interesting aspect of Overlord?s implementation of the Redo model is that walking the dependencies is done using signals and handlers, rather than by recursing directly. That is, when a script calls depends-on, all it does is signal that a dependency exists. It is the default handler which is responsible for actually building the dependency.

While the dependency graph of Overlord remains fundamentally imperative, because of this separation, and because of the distinction between build and dependency scripts, it should be possible to write handlers that simply traverse the dependency graph (once it has been built at least once), without necessarily having to actually build it.

A word of warning

One thing that might not be obvious about Redo-style build systems is that they afford unusually good opportunities for parallelism. Although Overlord does not (yet) support parallelism, it tries to discourage reliance on side effects by, whenever possible, randomizing the order in which targets are built.

Overlord and Lisp images

During development, as targets are defined and re-defined, and rebuilt or not rebuilt, the actual state of the Lisp world will drift away from the one specified by Overlord?s dependency graph. Before dumping an image such discrepancies must be resolved. It is obviously undesirable, for example, for an image built on one machine to try to lazy-load a module on another machine where the source of that module is unavailable. (Actually it would be a disaster, since that source file might be provided maliciously.)

Thus, before an image is saved, Overlord needs to do two things:

  1. Finalize the state of the image by making sure that all defined targets have been built.

  2. Disable itself.

If you use uiop:dump-image to save the image, you don?t need to do anything; Overlord will finalize the state of the image, and disable itself, automatically.

If you are using implementation-specific means to save an image, however, you will need to arrange to call overlord:freeze before the image is saved.

The default policy is to allow the restored image to be unfrozen, and development to be resumed, by calling overlord:unfreeze. This is probably what you want when, say, saving an image on a server. In other scenarios, however ? like delivering a binary ? you may want to strip the build system from the image entirely. This is possible by changing Overlord?s ?freeze policy?, using the freeze-policy accessor.

;;; The default: can be reversed after
;;; loading the image by calling
;;; `overlord:unfreeze`.
(setf (overlord:freeze-policy) t)

;;; Irreversible: before saving the
;;; image, Overlord should destroy its
;;; internal state.
(setf (overlord:freeze-policy) :hard)

Overlord vs. Racket

Overlord?s approach to making languages is inspired by Racket. Racket is a language in the Lisp family, descended from Scheme. Its distinction is its focus on making languages. An impressive amount of thought, effort, and research has gone into making the whole Racket environment act as a toolkit for making languages. Racket users are both encouraged and expected to solve their problems by making special-purpose languages; and Racket itself is implemented, from a simple, Scheme-like core, as a tower of languages.

(If you want to investigate for yourself, you can find a gentle introduction to making languages with Racket in Beautiful Racket.)

You might be wondering what making languages has to do with a build system. The answer is that, once we have a build system that allows Lisp bindings as targets, the idea of a language falls out naturally. The most abstraction a file-to-file build system can support is a pattern ? an abstract relationship between two files. But what is an abstract relationship between a file and a Lisp binding? The relationship is a language; the Lisp object is a module.

Overlord models its support for making languages on Racket. It is not a direct imitation of Racket, however. It also draws on subsequent and related work in Scheme module systems to create a module system that is fundamentally dynamic.


A Overlord module is a file in a language. Overlord supports a Racket-like hash-lang (#lang) syntax, but in Overlord the language of a module can also be specified as part of the import syntax. Since the language is not an inherent part of the file, the same file can be loaded as a module in more than one language. And each language-file combination gets its own, completely independent module.

Overlord is very liberal about what can be a module. In Overlord, any value can be a module ? a string, a function, a hash table, anything ? and any module can provide exports as long as it specializes certain generic functions, like module-ref.

(Most of the time, however, what you want is simple-module.)


In Racket, languages are defined in two steps. In Racket, a language is a module. This module defines a reader, which returns syntax, and an expander, which gives that syntax meaning.

That can?t work in Common Lisp, where meaning is assigned at read time, when a symbol is interned in one or another package.

In Overlord, a language is just a package. The package exports a reader and an expander. The symbol named read-module is the package reader. The symbol named module-progn is the package expander.

The important thing: when the package?s reader is called, that same package is also bound as the current package. It is then the responsibility of the reader to make sure any symbols it reads in, or inserts into the expansion, are interned in the correct package. (There is a shortcut for this, overlord:reintern.)

(There is one exception to the rule of language=package. If another package exists, having the same name, but ending in -user, and this other package inherits from the original package, then this user package is the package that is made current while reading (and expanding). E.g. a file beginning with #lang cl would actually be read in using the cl-user package, not the cl package itself.)

In Racket the expander is entirely responsible for giving the code meaning. In Overlord the expander has less to do, because the code has been assigned meaning at read time, but it is still important for lexical bindings and whole-program transformations.

Where in Racket you would write

(module my-module MY-LANG forms ...)

to wrap FORMS with the binding of #%module-begin from MY-LANG, in Overlord you just write:

(MY-LANG:module-progn forms....)

Although for convenience you can write

(overlord:module-progn-in :MY-LANG forms...)

But this just delays resolving module-progn from read time to macro-expansion time.

Note that the reader is responsible for returning a single form, which is the module. That is, the form returned by the package reader should already be wrapped in the appropriate module-progn. The exported binding for module-progn is only looked up when the language is being used as the expander for a meta-language.

Meta-languages are for language authors who want to reuse an existing syntax. E.g., if you want to define a language that uses s-expressions, in Racket you can write:

#lang s-exp my-lang

In Overlord you can do something similar:

#lang overlord/s-exp my-lang

What happens here is simply that the overlord/s-exp language finds the package my-lang and binds it to *package* before it starts reading in forms.

Defining languages

Any package can be used as a hash lang, as long as its name is limited to certain characters ([a-zA-Z0-9/_+-]). Of course this name can also be a nickname.

(Note that resolution of package names is absolute, even in a Lisp implementation that supports package-local nicknames.)

It is strongly recommended, although not required, that your language package inherit from overlord/shadows rather than from cl. The result is the same, except that overlord/shadows shadows Common Lisp?s binding and definition forms so they can, in turn, be shadowed in further language implementations.

The package must at least export a binding for one of read-module, for direct use, or module-progn, for use with a meta-language. Preferably, it would export both.

There are other special exports. For example, if the package has a function binding for module-static-exports, it is used, unsurprisingly, to statically determine the module?s exports.

Imports and exports

What Overlord imports and exports are not values, but bindings. Bindings are indirect (and immutable): they refer to the module, rather than to the value of the export. This allows for modules to be reloaded at any time. It is even possible to unload modules.

The overhead of this indirection is very low, but when necessary it can be avoided.

Note that exports in Overlord, with one exception, form a single namespace. This is in order to keep the notation for imports simple. Importing from a language with multiple namespaces into a language with multiple namespaces would create a Cartesian product problem.

The one exception is macros. A Racket-style single namespace for run-time bindings and macros would not make sense in Overlord where modules can be dynamically reloaded.

Because Overlord imports bindings rather than values, modules are always loaded lazily. A module is never actually loaded until a function imported from it is called, or a variable imported from it is looked up.

Finally, Overlord allows local imports: imports that only take effect within the body of a with-imports form.

The combination of lazy loading and local imports may mean that, in some cases, needless imports are minimized. For example, a module that is only used inside of a macro might only be loaded when the macro is expanded at compile time. However, this does not apply when saving images: all known modules are loaded before the image is saved. The real effect of pervasive lazy loading is that, since you do not know when, or in what order, modules will be loaded, you must not rely on load-time side effects.

Simple modules

Most of the time, your language?s package expander will return a simple-module form.

(overlord:simple-module (#'moo)
  (defun make-moo (o)
    (concat "M" (make-string o :initial-element #\o)))

  (defun moo (&optional (o 2))
    (print (make-moo o))))

This exports a single name, moo, bound to a function that says ?Moo? with a varying amount of ?oo?.

What makes simple modules simple is that they cannot export macros. If you do want to export macros, you need something more complex (see below).

The simple-module form is is built on the support for internal definitions in Serapeum (the local macro), and shares its limitations with regard to the precedence of macro definitions. Macro definitions must precede all function or variable definitions, and all expressions.

Macro exports

Overlord?s syntax for import and export supports macros. Unlike in Racket, macros must be imported explicitly as macros, because implicitly distinguishing functions and macros does not make sense when it is possible to reload modules.

I am not sure that support for macros in the module system is useful. In Racket, where languages and modules are the same thing, modules must export macros as language extensions. In Overlord, modules are one thing, and languages are something else ? packages.

That said, I could hardly call the module system ?Racket-inspired? with a straight face if it didn?t support exporting macros from modules.

The ability to export macros from modules is not useful in itself. It only becomes useful in the presence of certain forms of macro hygiene. After experimenting with different ways to do this, I have concluded that the correct thing to do, if you want your language to be able to export macros, is to embed a hygiene-compatible language in Lisp, and then compile your language to that.

I?m not being flippant. Embedding a hygiene-compatible language in CL is not just doable; it?s already been done. As a proof of concept, I have converted Pascal?s Costanza?s hygiene-compatible implementation of ISLISP in Common Lisp (?Core Lisp?) to work with Overlord?s module system. This version of Core Lisp lives in its own repository.

How macro exports are supported is one aspect of the Overlord module system that is very likely to change.

Future work

  • Lots of tests.
  • More portability testing.
  • Leverage UIOP more.
  • Source locations for functions in embedded languages.
  • More convenient shell command syntax.
  • Thread safety (and eventually parallelism).
  • Fewer dependencies.
  • Improve the Emacs integration (Prototype).
  • Improve the CLI. (Look in roswell/ and cl-launch/ for prototypes.)
  • Interfaces for modules?
  • Isolate file-system side effects (cf. Boot).
  • Using build directories (cf. ASDF).

Things I might or might not do, but sure would like to link to if someone else did them.

  • A EBNF parser generator like ragg or Instaparse.
  • An at-exp meta-language.
  • A working #lang scheme. (Compare Pseudoscheme).
  • A language for literate programming.
  • A language with an ML-style type system.
  • #lang shen.
  • The Snowball language.
  • A unit-aware language (compare Frink).
  • Any language or meta-language you care to implement.