There is an increasing need for methods to encode algorithmic or programmatic content in digital humanities projects. For example, a comprehensive digital edition of an interactive novel must include some means to express the work's state transition logic. The predominant solution to this problem is to simply embed code written in a conventional programming language, such as JavaScript or Python, in a TEI document. However, such languages are unsuited to digital preservation because programs written in them require huge, brittle webs of transitive dependencies. Will code which relies on particular versions of dozens of Python packages, a particular version of Python itself, and a particular operating system still be executable in 5 years, yet alone 100?

In computer science, it is common practice to express algorithms in pseudocode. However, pseudocode, as the name suggests, is not actually code and has no precise computational meaning. The programming language Algol (in its various incarnations) was in fact first specified as a standard notation, with a formal semantics, for describing algorithms. But as Algol fell into disuse, so did the notion of a standard, executable notation for algorithms.

In this paper I introduce AlgolML, a new, XML-based executable notation for algorithms and computer programs, designed to function as a companion technology to TEI. Unlike nearly all programming languages, the AlgolML standard includes a formal specification of its semantics. As a programming language, AlgolML is conservative in its design, but includes unique features related to digital preservation and draws inspiration from the related notion of Permacomputing. Crucially, AlgolML is easily implementable: a conforming implementation should run no more than a few thousand lines of code in any common programming language. This property of AlgolML ensures that algorithms and programs so encoded will continue to be interpretable as long as the specification itself persists.