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R5RS, R6RS, R7RS

2017-10-01 22:11 -0300. Tags: comp, prog, lisp, scheme, in-english

Over the last few days I have skimmed over R7RS, the Revised⁷ Report on [the Algorithmic Language] Scheme. I thought I'd write up some of my impressions about it, but I decided first to write a bit about the history and the context in which R7RS came about and the differing opinions in the Scheme community about R6RS and R7RS. In a future post, I intend to write up about my impressions of specific features of the standard itself.

The Scheme language was first described in a document named the "Report on the Algorithmic Language Scheme". Afterwards, a second version, called the "Revised Report on the Algorithmic Language Scheme", came out. The following version of the standard was called the "Revised Revised Report …", or "Revised² Report …" for short. Subsequent versions have kept this naming tradition, and the abbreviation RnRS (for some n) is used to refer to each version of the standard.

Up to (and including) R5RS, all versions of the standard were ratified only by unanimous approval of the Scheme Steering Committee. As a result, each iteration of the standard was a conservative extension of the previous version. R5RS defines a very small language: the whole document is just 50 pages. The defined language is powerful and elegant, but it lacks many functions that are typically expected from the standard library of a modern language and necessary for many practical applications. As a result, each Scheme implementation extended the standard in various ways to provide those features, but they did so in incompatible ways with each other, which made it difficult to write programs portable across implementations.

To amend this situation a bit, the Scheme community came up with the Scheme Requests for Implementation (SRFI) process. SRFIs are somewhat like RFCs (or vaguely like Python's PEPs): they are a way to propose new individual features that can be adopted by the various implementations, in a way orthogonal to the RnRS standardization process. A large number of SRFIs have been proposed and approved, and some are more or less widely supported by the various implementations.

R6RS attempted to address the portability problem by defining a larger language than the previous reports. As part of this effort, the Steering Committee broke up with the tradition of requiring unanimous approval for ratification, instead requring a 60% majority of approval votes. R6RS defines a much larger language than R5RS. The report was split into a 90 page report on the language plus a 71 page report on standard libraries (plus non-normative appendices and a rationale document). The report was ratified with 67 yes votes (65.7%) and 35 no votes (34.3%).

The new report caused mixed feelings in the community. Some people welcomed the new standard, which defined a larger and more useful language than the minimalistic R5RS. Others felt that the report speficied too much, reinvented features in ways incompatible with existing SRFIs, and set some things in stone too prematurely, among other issues.

In response to this divide, the Scheme Steering Committee decided to split the standard into a small language, more in line with the minimalistic R5RS tradition, and a large language, intended to provide, well, a larger language standardizing a larger number of useful features. The R7RS-small report was completed in 2013. The R7RS-large process is still ongoing, being developed in a more incremental way rather than as one big thing to be designed at once.

I think that the R6RS/R7RS divide in part reflects not only differing views on the nature of the Scheme language, but also differing views on the role of the RnRS standards, the Steering Committee, and the SRFI process. In a discussion I read these days, a person was arguing that R6RS was a more useful standard to them, because for most practical applications they needed hashtables, which R6RS standardized but R7RS did not. My first thought was "why should hashtables be included in the standard, if they are already provided by SRFI 69?". This person probably does not consider SRFIs to be enough to standardize a feature; if something is to be portable across implementations, it should go in the RnRS standard. In my (current) view, the RnRS standard should be kept small, and SRFIs are the place to propose non-essential extensions to the language. My view may be colored by the fact that I started using Scheme "for real" with CHICKEN, an implementation which not only supports a large number of SRFIs, but embraces SRFIs as the way various features are provided. For example, whereas many implementations provide SRFI 69 alongside their own hashtable functions, CHICKEN provides SRFI 69 as the one way of using hashtables. So, CHICKEN users may be more used to regard SRFIs as a natural place to get language extensions from, whereas users of some other implementations may see SRFIs as something more abstract and less directly useful.

I have already expressed my view on Scheme's minimalism here, so it's probably no surprise that I like R7RS better than R6RS. I don't necessarily think R6RS is a bad language per se (and I still have to stop and read the whole R6RS report some day), I just have a preference for the standardized RnRS language to be kept small. (I'm okay with a larger standard a la R7RS-large, as long as it remains separate from the small language standard, or at least that the components of the large language remain separate and optional.) I also don't like every feature of R7RS-small, but overall I'm pretty satisfied with it.

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