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Version 0.14 "Jean Baptiste Point DuSable Lake Shore Drive"

This is the reference distribution of Falderal, a file format for literate test suites. What sets Falderal apart from most other test frameworks is that it recognizes that your tests are for a functionality, not a particular implementation of that functionality.

Falderal is particularly suited for:

  • documenting programming languages with prose and examples
  • testing multiple implementations of a programming language
  • performing Test-Driven Language Design (TDLD)
  • being embedded in Markdown documents

Falderal in three words: "Doctests for DSLs".

If you're more interested in running the tools than learning about the format itself, skip down to Implementation.


Say you have a programming language. You probably have a document describing the language. That document probably includes examples. Those examples are probably intended to produce some kind of output. That output is probably expected to be a certain, predicted thing, and if it's not that thing, the implementation is probably considered incorrect.

So why not write those examples in a format that can be run and tested?

You could write a bunch of standalone test sources, and store the output you expect from them in a bunch of other files, and write a shell script that runs each program and diffs the output with the expected output. But this is a lot of clutter — finding a particular example might not be so easy. Each test source exists in a void, not necessarily logically grouped with other, similar tests. And any text you write describing a test needs to be in the comment syntax of your programming language (if your programming language supports comments) and is also detached from all the other test descriptions.

You could write unit tests in the unit test framework of your choice, but if your programming language has more than one implementation one day (and you should really consider that possibility) then you might not be able to re-use it so easily for other implementations in other languages.

In a language like Python, you could write doctests, but that also ties your tests to one implementation of your language. There can be awkward quoting issues with how you embed your test sources inside those embedded strings that comprise your doctests, as well.

Or... you could write a Markdown document with beautiful yet precise prose describing your wonderful language, alternating with example code (in the form of embedded Falderal tests) clarifying each of the points you are making; then you could use a Falderal-comprehending tool to run each of these tests against any implementation of your language which exists or will exist in the future.

And you could even write this document before you even start implementing your language; then when it is all clear "on paper", you have a target at which you can aim while writing your language. As you implement more and more of it, more and more tests in your test suite will pass. This is simply the idea behind Test-Driven Development (TDD) applied to language design, which we will call Test-Driven Language Design (TDLD).

Features of the Format

Falderal is just a file format; it does not specify exactly what tools must do with the tests they extract. However, it is generally assumed that most tools will want to, at the very least,

  • Run tests from one or more documents.
  • Report the results, with some given level of detail.

There is, of course, a reference implementation which does both of these things. It is called falderal and it is written in Python 3.x. More about it can be read in the falderal document.

Each Falderal test is for some abstract functionality, and each functionality may have multiple concrete implementations. Thus the same tests can be run multiple times, once for each implementation of the functionality they test.

Directives in the Falderal document may assign functionalities to tests, and may define implementations for given functionalities. Implementations may be defined outside of any document, as well. Falderal defines one general kind of implementation, implementation by Bourne shell command, but is not inherently restricted from supporting other kinds of implementations.

Inherent Limitations

Being tests, rather than proofs, Falderal tests cannot tell you that your implementation of a language is correct. If one or more tests fail, that's an indication that your implementation is not correct; but even if all tests pass, you have no guarantee that the implementation doesn't do something contrary to the spec in one of the infinite number of cases that you have not enumerated.

There is also no way to test that certain programs represent infinite loops, for reasons that should be obvious.

Contents of this Distribution

This distribution contains:

  • doc — contains documents about Falderal. For the specification of the file format, see doc/Falderal Literate Test (Note that this specification should not be expected to remain stable through the 0.x version series.) There are other documents in there too.
  • bin/falderal — the reference implementation of Falderal. See "Implementation", below, for details.
  • script — miscellaneous small tools intended to be used in tests.
  • src — source code for the falderal tool.
  • tests — a set of tests for Falderal itself. (Note that these are not written in Falderal, as that would just be too confusing.)
  • — changelog for releases of Falderal.
  • — areas where Falderal and its implementations could be improved.


This distribution contains falderal, which is the reference implementation of Falderal, written in Python and sometimes referred to as "py-falderal".

To use it, you can clone this repository and run it as bin/falderal from the directory of your clone, or you can put the bin directory on your executable search path, and run it as falderal anywhere.

Or you can install it using pip:

pip install -e Falderal==0.14

(Depending on your needs, you may wish to establish a virtual environment first. Describing how to do so is outside the scope of this document.)

The implementation is (somewhat) documented in doc/

The implementation has its own test suite (not written in Falderal, as that might be too confusing). It can be run by running ./


The git repository for the Falderal distribution can be found on Codeberg at

Official release distfiles are available via the Falderal project page at Cat's Eye Technologies.

Projects using Falderal

Xoomonk, Madison, Velo, and Exanoke are good examples of how a literate test suite can be useful in both describing a programming language through examples and testing that an implementation of the language does not violate the language specification. They are, in fact, exercises in Test-Driven Language Design (TDLD), where the tests were written as part of designing the language, before any attempt at implementation; the others are more like traditional test suites, written after-the-fact.