eucalypt (the language)

eucalypt, the language, is unorthodox in many respects - probably more than you might realise on first acquaintance.

People tend to have deep-seated and inflexible opinions about programming languages and language design and will quite possibly find something in here that they have a kneejerk reaction against.

However, the design is not unprincipled and, while it is experimental in some respects, I believe it's internally consistent. Several aspects of the design and the aesthetic are driven by the primary use case, templating and generating YAML. Maybe by exploring some of the inspiration and philosophy behind the language itself, I can pre-empt potential criticism.

Accept crypticality for minimal intrusion

eucalypt is first and foremost a tool, rather than a language. It is intended to replace generation and transformation processes on semi-structured data formats. Many or most uses of eucalypt the language should just be simple one-liner tags in YAML files, or maybe eucalypt files that are predominantly data rather than manipulation.

The eucalypt language is the depth behind these one-liners that allows eucalypt to accommodate increasingly ambitious use cases without breaking the paradigm and reaching for a general purpose imperative scripting language or the lowest common denominator or text-based templating languages.

The pre-eminence of one-liners and small annotations and "logic mark-up", means that eucalypt often favours concise and cryptic over wordy and transparent. This is a controversial approach.

  • eucalypt logic should "get out of the way" of the data. Templating is attractive precisely because the generating source looks very like the result. Template tags are often short (with "cryptic" delimiters - {{}}, <%= %>, [| ]...) because these are "marking up" the data which is the main event. At the same time, the tags are often "noisy" or visually disruptive to ensure they cannot be ignored. eucalypt via operator and bracket definitions, picks and chooseS from a similar palette of expressive effects to try and be a sympathetic cohabitee with its accompanying data.

  • There are many cases where it makes sense to resist offering an incomplete understanding in favour of demanding full understanding. For example, it is spurious to say that bind(x, f) gives more understanding of what is going on than x >>= f - unless you understand the monad abstraction and the role of bind in it, you gain nothing useful from the ideas that the word bind connotes when you are trying to understand program text.

  • eucalypt just plain ignores the notion that program text should be readable as English text. This (well motivated) idea has made a resurgence in recent years through the back door of internal DSLs and "fluent" Java interfaces. There is much merit in languages supple enough to allow the APIs to approach the natural means of expression of the problem domain. However, problem domains frequently have their own technical jargon and notation which suit their purpose better than natural language so it cuts both ways. Program text should be approachable by its target audience but that does not mean it should make no demands of its target audience.

These stances lead directly to several slightly esoteric aspects of eucalypt that may be obnoxious to some:

  • eucalypt tends to be operator-heavy. Operators are concise (if cryptic) and the full range of unicode is available to call upon. Using operators keeps custom logic visually out of the way of the data whilst also signposting it to attract closer attention.

  • eucalypt lets you define your own operators and specify their precedence and associativity (which are applied at a relatively late stage in the evaluation pipeline - operator soup persists through the initial parse). There are no ternary operators.

  • For absolute minimal intrusion, merely the act of placing elements next to each other ("catenation"), x f, is meaningful in eucalypt. By default this is pipeline-order function application, but blocks can be applied as functions to make common transformations, like block merge, very succinct.

  • For even more power, eucalypt might soon let you alter the meaning of concatenation via overloaded idiot brackets 1. («x y»: ...). This is inspired by the idiom brackets that can be used to express applicative styles in functional programming 2. These may also provide an acceptable proxy for ternary and other operators too.

  • An equivalent generalisation of eucalypt block syntax to provide a capability similar to Haskell's do notation could conceivably follow.

Cohabitation of code and data

Just like templates, eucalypt source (or eucalypt-tagged YAML) should be almost entirely data.

The idea behind eucalypt is to adopt the basic maps-and-arrays organisation philosophy of these data formats but make the data active - allowing lambdas to live in and amongst it and operate on it and allowing the data to express dispositions towards its environment by addition of metadata that controls import, export, and execution preferences.

eucalypt therefore collapses the separation of code and data to some degree. You can run eu against a mixture of YAML, JSON and eucalypt files and all the data and logic appears there together in the same namespace hierarchy. The namespace hierarchy just is the data.

However, code and data aren't unified in the sense of Lisp for instance. eucalypt is not homoiconic. The relationship is more like cohabitation; code lives in amongst the data it operates on but is stripped out before export.

Nevertheless eucalypt is heavily inspired by Lisp and aims for a similar fluidity through:

  • lazy evaluation (going some way towards matching uses of Lisp macros which control evaluation order - in eucalypt, if is just a function)
  • economical syntax to facilitate (future) manipulation of code as data


  • eucalypt values simplicity in the sense of fewer moving parts (and therefore, hopefully, fewer things to go wrong). It values ease of use in the sense of offering a rich and powerful toolkit. You may not think it achieves either.

  • eucalypt values familiarity mostly in the "shallower" parts of the language where it only requires a couple of mental leaps for the average programmer in these areas - the (ab)use of catenation being the key one.

  • However, eucalypt isn't ashamed of its dusty corners. Dusty corners are areas where novices and experts alike can get trapped and lose time but they're also rich seams for experimentation, innovation and discovery. If you have to venture too far off-piste to find what you need, we'll find a way to bring it onto the nursery slopes but we won't close off the mountain.


  1. If I didn't call them that, someone else would. 

  2. Applicative Programming with Effects, Conor McBride and Ross Paterson. (2008)