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A scripting language.

To learn about the language, visit

To read an introduction of the language, visit the tour.

To read the documentation, visit

There are three main ways to use Pyret:

  1. If all you want to do is program in Pyret, there is a web-based environment at that lets you run and save programs that should be all you need. If you're a student using Pyret, this is probably where you will do your assignments, for example.

  2. If all you want to do is program in Pyret at the command line, you should install pyret-npm from, using an install command like:

    npm install -g pyret-npm

    (If you're new to node and npm, you might find and useful background).

  3. If you want to contribute to Pyret, or try out experimental features, the README starting below is for you.

The use of vocabulary from is recommended when commenting and reporting issues.


First, make sure you've installed Node >= 6. Then run:

$ npm install
$ make
$ make test

It'll build the Pyret compiler and run the tests.

Running Pyret

If you just want to run Pyret, visit the online environment and work from there. If you're interested in Pyret development, read on:

The easiest way to run a Pyret program from the command-line is:

$ ./src/scripts/phaseX <path-to-pyret-program-here> [command-line-args...]

Where X is 0, A, B, or C, indicating a phase (described below). For example:

$ ./src/scripts/phaseA src/scripts/show-compilation.arr examples/ahoy-world.arr

Alternatively, you can compile and run a standalone JavaScript file via:

$ node build/phaseX/pyret.jarr \
    --build-runnable <path-to-pyret-program-here> \
    --outfile <path-for-standalone-js> \
    --builtin-js-dir src/js/trove/ \
    --builtin-arr-dir src/arr/trove \
    --require-config src/scripts/standalone-configA.json
$ node <path-for-standalone-js>


Pyret is a self-hosted compiler, which means that building requires some thought. If you're going to get into the guts of the compiler, a brief overview is worth reading. The build directory is separated into four distinct phases.

  1. Phase 0 is a standalone JavaScript file that contains an entire compiled Pyret compiler and runtime. This large blob gets committed into version control whenever we add a nontrivial feature to the language. It is large and somewhat slow to load, but whatever is in build/phase0/pyret.jarr is currently the canonical new-world implementation of Pyret.

  2. Phase A is set up to be populated with built versions of all the files for the compiler, built by the phase 0 compiler. Phase A is what most of the testing scripts run against, since it is the easiest to create, and after it is built it is your development snapshot of the compiler you're working on. However, just building phase1 is not enough to fully re-boostrap the compiler, because it contains a mix of old-compiler output and any new changes that were made to runtime files.

  3. Phase B is set up to be populated with built versions of all the files for the compiler, built by the phase A compiler. This version does represent a fully-bootstrapped compiler. If you run make phaseB, you get a new standalone compiler in the same format as build/phase0/pyret.js.

  4. Phase C builds from phase B. One major use of phase C is to check the bootstrapped compiler from phase B. Before committing a new standalone in phase 0, build both phaseB and phaseC, and check:

    $ diff build/phaseB/pyret.jarr build/phaseC/pyret.jarr

    And it should be empty, which indicates that the bootstrapped compiler is at least correct enough to recompile itself without error.

    To rebuild the compiler and get a new phase0, run

    $ make new-bootstrap

    which will build the phaseB and phaseC standalones, check the diff, and copy to phase0 if the diff is empty.