Cirriform

A little scheme

3 July, 2019

Brett Milford

## TLDR

  • A little scheme goes a long way.
  • Software is the art of abstraction. Maintain an intellectual curiosity about the layer below your periphery.

## How hard could it be?

This particular journey starts with my choice of editor and software to manage tasks – Emacs and Org-mode. Having taken to organising my to-dos with org-mode and wanting to track these whilst on my iPhone I found the fantastic Beorg app 1 for iOS. Eventually I found myself wanting to extend the templating system for one reason or another and upon investigation found Beorg’s configuration system, similar to Emacs, also used lisp! My first thought “How cool!”, the second, How? Apple’s app store review guidelines and have notoriously disallowed apps that emulate software to be listed on the app store 2. As I came to find, Beorg makes use of a JavaScript based lisp like interpreter called Biwascheme. This is useful as because its allows code to be executed in WebView contexts and utilised by the encompassing app. Working backwards I could see, if this feature were present in Biwascheme, it could be used Beorg and its templates.

## Biwascheme

Biwascheme 3, as the name suggests is a scheme implementation in JavaScript. But what is Scheme? Well its a minimalist dialect of Lisp developed in the 1970s with the goal of focusing on clarity and simplicity. Because of this it has a smaller core language that is easier to learn and implement. It also requires a few functional improvements, namely Lexical scoping and tail-call optimisation, rather then permitting less optimal solutions. In common with Lisp is its property of Homoiconicity, that is the representation of data as code and code as data.

So we’ve deduced that Scheme is a principled specification of a Lisp. And Biwascheme is an implementation of a Scheme specification. But who decides on the specification?

## Scheme Requests for Implementation

Scheme Requests for Implementation 4, or SRFI, indexes RFIs for Scheme and facilitates the discussion of new scheme features and their acceptance by the community. Biwascheme implements SRFIs faithfully as practically as possible, but owing to development effort, priorities and nuances of the platform (in this case, browser implementations of JavaScript), and in my case interpretation of the schema it might deviate from the baseline.

## Solution

So to create my Beorg template I need a feature providing week numbers:

biwascheme> (date->string (current-date) "~V")
[BUG] not implemented: weeknum(1~, Sun?) [date->string]
biwascheme> (date->string (current-date) "~U")
[BUG] not implemented: weeknum(0~, Sun) [date->string]

Ok, its not implemented, lets check with the project to see if there are any plans to implement this.

Being a Github project that appears to be actively developed there I opened a Github issue 5. The developers suggested that I could contribute the implementation. Perusing the code I noted the library functions I wanted to implement belonged to srfi.js and researching, it and the surrounding code lead me to Scheme Requests for Implementation. Reading the SRFI spec (SRFI-19) 6, I was left super confused by the meaning of the spec.

~U | week number of year with Sunday as first day of week (00...53) # should be 00..51 ?
~V | week number of year with Monday as first day of week (01...52) # should be 'Sunday'
...
~W | week number of year with Monday as first day of week (01...52)
~x | week number of year with Monday as first day of week (00...53) # should also be 51?

Enter ISO standards for dates and C specs. Reading through the standards in this area gives the impression that defining time is really hard. I contact the SRFI mailing list for some clarification and are met with an equally enthusiastic reply 7. But at least we have some direction on the intention of the spec. To ensure I’ve not missed the point I update the spec to reflect the discussion 8.

Now, with this understanding in place I can implement the weeknum function faithfully 9. Finally, I contact the Beorg developer to pull in this update and make my weeknum function available in the app.

## Thoughts

I’m a massive advocate for open-source software for variety of reasons, least of which are, it enables you learn, and it enables you to do. I read everything from ISO standards, C specs and Scheme RFIs to JavaScript library code, biwascheme and elisp code. This article is a testament to just how many layers I had to peal back to solve my problem, and the intellectual curiosity required to find and implement the solution. Having some sense of what its like to ‘do the thing’ both personally and professionally, I’ve developed a huge respect for colleagues and community members who put aside time to contribute open-source. Some handy resources on Scheme I’ve found along the way include

If you enjoy interesting discussions about the design and implementation of programming languages I highly recommend subscribing to SRFI mailing lists and perusing the specifications of Scheme Requests for Implementation, and the mailing list archives of Specifications your interested in.

## References

  1. https://beorgapp.com ↩︎

  2. https://developer.apple.com/app-store/review/guidelines/ ↩︎

  3. https://github.com/biwascheme/biwascheme/ ↩︎

  4. https://srfi.schemers.org/ ↩︎

  5. https://github.com/biwascheme/biwascheme/issues/130 ↩︎

  6. https://srfi.schemers.org/srfi-19/srfi-19.html ↩︎

  7. https://srfi-email.schemers.org/srfi-19/msg/10875627/ ↩︎

  8. https://github.com/scheme-requests-for-implementation/srfi-19/pull/3 ↩︎

  9. https://github.com/biwascheme/biwascheme/pull/134 ↩︎

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