Scheme Request for Implementation - FAQ

The ``Scheme Request for Implementation'' process is documented elsewhere. This document is intended to provide some of the rationale about SRFIs rather than cluttering up that document.

The SRFI process grew out of the Scheme Workshop held in Baltimore, MD, on September 26, 1998, where the attendees considered a number of proposals for standardized feature sets for inclusion in Scheme implementations. Many of the proposals received overwhelming support in a series of straw votes. Along with this there was concern that the next Revised Report would not be produced for several years and this would prevent the timely implementation of standardized approaches to several important problems and needs in the Scheme community.

The SRFI process is a service being provided to the Scheme community by the editors, currently David Rush, Francisco Solsona, Mike Sperber, and David Van Horn, with a home provided by the Programming Languages and Compilers group at the University of Tübingen.

Are SRFIs standards?
Yes and no. SRFIs are not official standards. There exist organizations such as ISO, IEEE, ANSI, etc. who are set up to develop official standards. The SRFI process is designed as an attempt to maximize the quality of SRFIs within the constraint of not assigning authority to anyone.

There are official Scheme standards, ANSI-???? and IEEE-???. SRFIs are in addition to these standards.

On the other hand, the process for creating SRFIs is standardized, and each final SRFI remains frozen and publicly available, hence they have many of the properties of standards. So you might choose to think of them as unofficial standards.

Are SRFIs replacements for RnRS?
No. The editors of the SRFIs do not currently include any of the editors of any of the Revised Reports.

Each Revised Report has been written by a different body, which determines what eventually is and is not included in that report. The SRFI process is orthogonal to all of these reports. Authors of SRFIs must not expect that their SRFIs will be included, or even be considered, by the authors of reports.

Of course anyone, including authors of future Revised Reports, is welcome to employ the definition and rationale of an SRFI, the discussion surrounding its adoption, and its widespread implementation to argue that the authors of a standard or report should consider adding the contents of the SRFI to that standard.

If SRFIs had existed before R4RS, the macro appendix to that report might have made more sense to have been released as a SRFI. But it didn't. So it wasn't.

There is considerable value in reading the discussions of the RnRS authors, as they have often considered issues that are candidates for SRFIs.

Are SRFIs a discussion forum for preliminary ideas?
No. SRFIs stands for ``Scheme Request for Implementation''. Note the last word. If someone has amorphous ideas for something that would be cool, but has no idea how it might be done, they should discuss it in journals, workshops, seminars or news groups. When the discussions have coalesced to the point where an implementation strategy is apparent, then it is time to write up a SRFI proposal.

I really think that there should be a place to archive non-implementation documents.
You're not alone! There seem to be lots of people who disagree with the editors on this point. We will almost certainly address this need in the not too distant future - possibly within the SRFI process, or more likely with a separate process. If you have ideas on how this should be done, please send mail to <srfi minus editors at srfi dot schemers dot org>

Are SRFIs ``RFCs for the Scheme community''?
Not quite. As RFC1796 (Not All RFCs are Standards) says, RFCs serve a variety of purposes. The SRFI editors feel that there are sufficient other venues for discussion of ideas. The point of SRFIs is that a programmer can dependably test to see what features a Scheme implementation provides, and can therefore program in a portable way beyond the bounds of standardized Scheme.

What's with all the time periods? Why so little time?
The time periods are an attempt to drive the SRFI process as quickly as possible while maintaining sufficient time for sober second thought. To maximize the quality of SRFIs, we want all of the relevant people to be involved in the discussion of any particular SRFI proposal. Many of those people are very busy, and this mechanism constrains the time commitment they must make to stay on top of a proposal. It also prevents discussion on a proposal from dragging on in an endless repetition of the same points, long after anyone's opinion is likely to be changed.

The other issue in the timing is to prevent editors from stone-walling a proposal. It is not their job to make qualitative judgments about proposals, but rather to maintain the quality while expediting the creation of important unofficial standards.

If a proposal is still under discussion after 90 days, it means that it has been extended several times. The editors will normally extend the discussion period to maintain a minimum of 15 days after any significant change. Any proposal still under active discussion and revision after 90 days is not ready for codification. It will then be withdrawn for a (normal) minimum of 30 days after which it may be resubmitted. If it is now in good shape, it will likely become final after the 60 day discussion period. Thus, it will have been delayed a maximum of 90 days. This is likely to happen only in very exceptional cases, and is the only cost of the fixed time periods.

What kind of standards are these, anyway? There aren't any teeth in the rules!
To have enforcement, there must be authority. There is no absolute authority in the Scheme community, so there can be no absolute enforcement. The final authority is the implementors. If they believe that a particular SRFI documents a useful or important feature, they will add it to their implementations; if not, they won't. The discussion relating to any SRFI will be retained indefinitely, and implementors can refer to that when making their decision. Hence poorly worded, reasoned, or defended SRFIs will be nothing more than a waste of some time - regrettable, but necessary to retain an open process.

Why do I have to include a reference implementation?
See the discussion above about preliminary ideas. SRFIs are about implementation. If you haven't either: (a) built one, or (b) have a very clear outline of how to build one, then you aren't documenting anything useful about implementation. As the process document says: if you think the editors are wrong to withdraw your proposal because it doesn't have a sufficient outline of implementation, then prove us wrong by going and implementing it on some system. Then your SRFI will return to draft, and eventually active status.

Do SRFIs exist to describe the features of a particular Scheme implementation?
No. Every SRFI should describe a cohesive feature set that is portable across a variety of Scheme implementations. (Here we mean portable in the sense of being possible to implement, not in the sense of a portable implementation.) Rather than testing to see if the implementation is, e.g. Guile-4.3c, a program should test for the particular features that it requires.

This is a lesson learned from the Emacs world where it used to be that code would check to see what version of Emacs it was running on and make assumptions about the features that that particular version provided. Unfortunately that made the code un-portable to alternative Emacs implementations that had the required features but different series of version numbers. The same lesson can be observed in old C code that assumed that such-and-such a system had particular features. More commonly today, C programs use a configuration program that determines what libraries and functions are available, regardless of the system or compiler. The mechanism documented in SRFI 7 essentially provides a similar capability, while staying within Scheme code.

The biggest advantage of checking for features rather than implementations is that code becomes portable to systems of which the author was unaware, if they provide the features that the program requires.

The process document mentions that different SRFIs may conflict with each other. Won't that make it impossible for an implementation to support conflicting SRFIs?
Not necessarily. See SRFI 7.

Does the SRFI copyright permit using a SRFI sample implementation (or a derivative of one) in my Scheme implementation?
Yes.

Does the SRFI copyright permit using parts of a SRFI in the documentation of my Scheme implementation?
Yes.

Where did the acronym come from? It's a mouthful
Alan Bawden suggested RFI at the Scheme workshop as a humorous reference to RFCs. The S was added because the initial editors (and others) felt that ``Scheme'' should be in there somewhere. If you pronounce it ``ess-are-eff-eye'' it certainly is a mouthful, but if you pronounce it ``surfie'', as we do, it's fine. Scheme Implementation Request (SIR) was also proposed.


The SRFI Editors
Last modified: Mon Apr 27 08:37:30 MST 2009