Command Language for Awelon

Claw (command language for awelon) is an editable view and syntactic sugar for AO (awelon object). AO is a subset of Awelon Bytecode (ABC) that limits tokens and texts. Tokens of the form {%foo} link definitions of other words in a dictionary. Dependencies within the dictionary form a directed acyclic graph. (Recursion is anonymous, via Z-combinator.)

Claw provides a Forth-like experience, suitable for simple REPLs and command shells, albeit with better support for higher order programming and weaker support for ad-hoc side-effects. AO is purely functional, but we can model effects monadically.

Claw is interesting because it’s extremely simple while remaining very effective.

Claw Semantics

Claw semantics is a trivial multi-step expansion, ultimately into bytecode. For example:

Code Expansion
2/3 2 3 ratio
4/10 4 10 ratio
3.141 3141 3 decimal
-1.20 -120 2 decimal
6.02e23 6.02 23 exp10
42 \#42 integer
-7 \#7- integer
[inc mul] \[inc mul] block
[1,2,3] [\[2,3] after 1]
inc mul \{%inc} \{%mul}
“foo” \”foo
~ literal

The lowest level of expansions are the escaped forms, indicated by prefix `\`.

  • bytecode escape, e.g. `\vrwlc`
  • token escape, e.g. `\{foo}`
  • text escape, e.g. `\”foo…\n~`
  • block escape, e.g. `\[inc mul]`

These expand almost directly into bytecode, excepting blocks which must be recursively expanded. Escaped forms shouldn’t see much use at a command line. Users should be encouraged to define words rather than issue commands at this level.

The claw expansion has two important properties.

First, the expansion is context-free. That is, the expansion of a number, text, block is completely independent of its surrounding context. Context free expansion has valuable locality properties, and guarantees that claw code preserves the same tacit concatenative composition and refactoring properties as the bytecode.

Second, this expansion is reversible. Bytecode may be ‘parsed’ back into the forms that generate them, e.g. starting with escaped forms at the bottom level, reducing some tokens back to words, and reducing sequences such as `2 3 ratio` to `2/3`. This reversibility property allows us to understand and treat claw code as a lens or editable view of the underlying bytecode.

Claw code (command histories, etc.) should always be stored in the canonical, fully expanded bytecode format. The burden of providing an editable claw ‘view’ then shifts to either the editor or an intermediate layer (e.g. a FUSE-based filesystem view, or a web server). Doing this improves claw’s portability and extensibility. Bytecode can easily be stored and processed by backends that know nothing of claw syntax. We could keep command sessions in an AO dictionary for regression testing and refactoring purposes. If an editor didn’t know about rationals it could still preserve meaning and present `2 3 ratio` to a user.

Command Sequences

Command sequences enable concise and aesthetic expression of streaming data, monadic DSLs and effects, incremental computation, and continuation passing styles. As such, they are essential to the expressiveness of Claw.

Trivially, [foo, bar, baz] expands to [\[bar, baz] after foo], with recursive expansion to [\[\[baz] after bar] after foo]. Each command separator – the comma – might informally be understood as a yield action to enable a client to perform intermediate work.

Claw Namespaces

In practice, we will develop alternative command environments (shells, REPLs, etc.) for different use cases. I imagine these developments will share metaphors and structures with existing environments, so we’ll need overlap in the command words used, albeit often with a slightly different implementation. Namespaces provide a simple means to reuse words in different contexts.

Claw has exactly one namespace for any given region of code.

The namespace is added as a prefix in the expansion of every word, including words integer, decimal, block, etc. used in code expansions. For example, word `integer` will expand to \{%bar:integer}` if we are in namespace `bar:`. Namespaces are the exception to claw’s context-free expansion constraint. The singular namespace is simple enough to avoid compromising locality and composition, though a little care is needed to preserve namespace information if refactoring at the claw code layer.

A namespace annotation may appear at any point in a Claw command stream. A namespace annotation has the form `#bar:`, meaning `switch to namespace `bar:`. Claw reserves the `#` prefix for this purpose. To guarantee reversibility, a namespace annotation expands into the annotation `{&ns:bar}` at the bytecode layer.

A change in namespace lasts until we either reach the end of the current block or we again change namespaces. Blocks become implicit regional delimiters for namespaces, e.g. for `[#bar: 42 foo]` the `#bar:` annotation only applies within the block. This supports a common use case where we apply a block to a different context than where we constructed it.

Given the restriction to a single namespace, there is never any ambiguity about which foo you’re invoking. There is no need to consult a dictionary (e.g. to search for a defined word) to determine how words link to each other. There are no boiler-plate import lists. This restriction contributes wonderfully to claw’s simplicity and concision.

OTOH, every namespace must define its own version of every common word. If performed by hand, this would be a huge investment of a developer’s time and energy just to catch up and keep up. This cost might be mitigated by developing a few tools or software agents to automate the process.

Claw Extensions

Claw is extremely extensible.

While my immediate focus is on simple numbers and small texts (the original ‘AO’ feature set), it is feasible to support matrices, tables, and other structured data. Structured programming is also viable, e.g. conventional loops and if-then-else expressions converting to use of blocks and keywords (e.g. expansion `\[cond] \[body] while_do_` could correspond to a conventional while loop). Similarly, sugar for monadic expression could simplify integration of ad-hoc sequential effects models.

Any extensions must preserve the properties that expansion is context free (hence local) and reversible. But those are not difficult constraints to work with.

Of course, there are limits on what we can effectively support in context of line-oriented command text. Some syntax extensions, such as matrices or structured programming, would require a page-oriented programming environment. Something akin to iPython-notebook environments, wikis, or structure editors, might be appropriate. With structure editors, an interesting possibility is interactive extensions – e.g. checkboxes, sliders, color pickers, canvases.

Aside: This claw-structured bytecode (based on parsing bytecode into a structured editable view) is independent of the AO structured definitions (based on evaluating bytecode into a manipulable value structure). I hope these orthogonal kinds of structure will augment each other, or at least address different niches. But, with extensions, there is some overlap in potential utility. I’ll need to see how this works in practice.

Command Shells and Effects Models

Commands and sequences thereof can manipulate an external state resource, e.g. a model of a REPL. With command sequences, we can also model a multi-step system with many intermediate outputs, which is convenient for animated renders, e.g. Turtle graphics.

See Wikilon Dictionary Applications for a more up-to-date view.

Summary

Claw is simple, transparent, composeable, and extensible. These are excellent properties for my long term goals.

Edits: Modified model for command sequences.

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One Response to Command Language for Awelon

  1. Pingback: Wikilon Dictionary Applications | Awelon Blue

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