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Pycket is a Racket/Scheme implementation that is generated using the RPython framework. Given an interpreter written in RPython (in our case a CEK machine interpreter for Racket), RPython framework produces a fast binary for it. It can also add a tracing JIT.

There are currently two different modes that we refer as OLD and NEW. The NEW Pycket uses linklets and bootstraps the Racket using the expander linklet exported by Racket (version 7+). The OLD Pycket, on the other hand, uses Racket's binary to fully expand the program and generates json asts and evaluates them.

Note that both versions require an unmodified Racket installation. The OLD Pycket requires a Racket binary, and while the NEW Pycket doesn't require a Racket binary, it still requires the Racket packages and libraries to bootstrap.

See the Makefile targets section about how to build both versions.

Quick Links:


Building Pycket means translating the interpreter (written in RPython) into a binary. You can use the make targets to translate Pycket. We recommend using the PyPy for translation, since it'll be much faster than CPython. If you don't have a pypy binary in your environment, then make targets will default to CPython.

You can clone and make the pypy with make make-pypy target. It will clone the latest pypy repo in Pycket's directory and will start making it. Note that it will take more than 2 hours to build the pypy. It will result in a binary created at pypy/pypy/goal/pypy, and you need to add it to your environment for Pycket's translation to use it.

Additionally, it helps to have the build dependencies of PyPy installed. On a Debian or Ubuntu system:

$ sudo apt-get build-dep pypy

To produce a Pycket executable, use one of the provided make targets to build the binary you need.

Make Targets

PyPy Stuff:

Assumes the mercurial binary hg to be in the environment.

  • make clone-pypy : clones the latest pypy into Pycket's directory
  • make update-pypy : pulls and updates the pypy
  • make make-pypy : builds pypy, assumes that pypy directory exists in Pycket's directory

Building Pycket

  • make pycket-c : translates OLD Pycket with JIT

  • make pycket-c-linklets : translates NEW Pycket with JIT

  • make pycket-c-nojit : translates OLD Pycket without JIT (which may be a lot faster to translate but runs a lot lot slower)

  • make pycket-c-linklets-nojit : translates NEW Pycket without JIT (which may be a lot faster to translate but runs a lot lot slower)

  • make setup-old-pycket : installs the pycket-lang to Racket and runs update-pypy

  • make expander : generates the expander linklet (it assumes an unmodified Racket install and PLTHOME environment variable -- see the Environment Variables section below)

  • make setup-local-racket : if you don't have a Racket and don't want to deal with Racket related stuff, then run this to get a latest running Racket. Make sure to run export PLTHOME=`pwd`/ after it's done (see environment variables section).

Running Pycket

Pycket currently defaults to the OLD Pycket. To use the NEW version with the linklets, run it with:

$ ./pycket-c-linklets <arguments>

You can run with the -h option to see the different command line options for each versions:

$ ./pycket-c -h

$ ./pycket-c-linklets -h

You can run pycket-c like the racket binary:

$ ./pycket-c program.rkt

You can also run pycket under plain python (or pypy if its available), like this:

$ ./ program

Environment Variables

Running the interpreter on CPython or PyPy (i.e. running the requires a PYTHONPATH that includes both RPython (that should be the pypy directory cloned above) and pycket (that should be this directory).

Also there are a couple of variables need to be set for the NEW Pycket to interact with the Racket, since it bootstraps Racket by reading and evaluating Racket's main collection by loading and using the bootstrap linklets (currently only the expander.rktl.linklet) exported by Racket. So the NEW Pycket needs to be able to locate various different parts of the Racket installation. The OLD Pycket is lucky to use the Racket's own binary.

Naturally, it varies on the way in which the Racket is installed:

If Racket is installed in a single directory (non-Unix-style) :

Then all the NEW Pycket needs is a PLTHOME environment variable to point to the surrounding directory. For example it will assume the collects directory is at:


If Racket is installed in Unix-style :

Then NEW Pycket needs to know the locations of various directories. In particular, it needs:

  • PLTEXECFILE to point to the location of the racket binary * PLTCOLLECTS to point to the collects directory for the main collections
  • PLTCONFIGDIR to point to Racket's etc directory that contains config.rktd
  • (optional) PLTADDONDIR to point to a directory for user-specific Racket configuration, packages, and extensions. It defaults to .racket in USERHOME.
  • (optional) PLTUSERHOME to point to the home directory of the user. It's optional since Pycket will also look at other environment variables to figure out the home directory (e.g. $HOME).

You can also use the command line options to provide these paths, e.g. -X, -G etc.. Run it with -h to see all the commands and options.

$ ./pycket-c-linklets -h

Also, the Makefile reacts to some variables:

  • PYPYPATH for when your pypy checkout is not in this directory. Defaults to pypy.
  • PYTEST for when you don’t want to use pypy’s version of pytest. Defaults to $(PYPYPATH)/
  • RPYTHON for when you want to use something other than the default rpython script, but you probably would not want that. Defaults to $(PYPYPATH)/rpython/bin/rpython --batch.

Testing Pycket

Now that Pycket has two different modes with options, we run the unit tests on each of those settings using the following targets:

  • make test to run the unit tests on OLD Pycket.
  • make test-new-with-expander to run the unit tests on NEW Pycket using the Racket's expander linklet.
  • make test-new-no-expander to run the unit tests on NEW Pycket without using the expander.

For the NEW Pycket, using the expander linklet means that for each test expression string we use the read and eval functions in that linklet to read and evaluate the test. If we're not using the expander, on the other hand, then we manually create a linklet containing the expression and instantiate it directly (mostly with an empty target) to get the result.

Using Compiled Files

The NEW Pycket is able to generate and use its own .zo files. For now both the generation and the use are manual.

To generate a .zo file for a .rkt source file, use make compile-file:

$ make compile-file FILE=$(PLTHOME)/racket/collects/racket/private/qq-and-or.rkt

The parameter that enables Racket expander to use compiled code is use-compiled-file-paths, which defaults to pycket-compiled in Pycket. Whenever a module is required, the expander will use the compiled code if it exists, otherwise it will use the source code of the module (read, expand, etc.).

pycket-repl> (#%require racket/private/qq-and-or)

Note that pycket-compiled is a folder that make compile-file is going to generate by itself.

Currently we have two make targets for working with compiled files:

$ make compile-racket-modules

will create .zo files for the predefined list of Racket modules (see compile-file-pycket.rkt).

$ make clean-compiled-files

will remove all the .zo files under the pycket-compiled directories in Racket libraries.

This is a work in progress. We plan to have a make target that compiles all the Racket modules automatically by following the module dependencies (as opposed to using a predefined list of modules with the respective paths).


One of the beautiful perks of bootstrapping Racket is to be able to run on Pykcet some interesting programs implemented in Racket (as long as we have enough runtime support for it in Pycket -- i.e. having the runtime primitives implemented in RPython).

The NEW Pycket now features an original Racket REPL that's implemented in Racket. Try it out!

$ ./pycket-c-linklets

You can make it more verbose with the --verbose flag if you're curious about what's going on in the background.

$ ./pycket-c-linklets --verbose

Also increase the verbosity level (defaults to 0).

$ ./pycket-c-linklets --verbose 1


Getting a version error like : version mismatch expected: "" found: ""

There are a couple of things that might produce this.

  • It might be the case that your PLTCOLLECTS environment variable is set and pointing to a different place than you think.
  • If you're using the NEW Pycket (pycket-c-linklets), then it might be the case that you have compiled zo files in your Racket directory, and they need to be recompiled (you pulled a more recent Racket, maybe?).
  • The expander.rktl.linklet might not be up to date with the latest Racket binary. You can use make expander target to rebuild the expander linklet, or shoot a message on the slack channel to us to update it.

Deprecated Stuff Below -- Will be revised

Or even this, when this directory is in your PYTHONPATH:

$ python -mpycket program

You can edit the shell script to make it use pypy, if desired.


You can generate a coverage report with pytest:

$ pypy/ --cov .

or via

$ make coverage

which also generates an HTML report in pycket/test/coverage_report. You need these Python packages for that to work:

  • pytest-cov (provided with the pypy checkout)
  • cov-core and coverage


A rudimentary Racket implementation using RPython







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