Writing portable scheme code. Is anything “standard” beyond R5RS itself?

Question

I\'m learning scheme and until now have been using guile. I\'m really just learning as a way to teach myself a functional programming language, but I\'d like to publish an o

Answer 1

I write a blog that uses Scheme as its implementation language. Because I don't want to alienate users of any particular implementation of Scheme, I write in a restricted dialect of Scheme that is based on R5RS plus syntax-case macros plus my Standard Prelude. I don't find that overly restrictive for the kind of algorithmic programs that I write, but your needs may be different. If you look at the various exercises on the blog, you will see that I wrote my own regular-expression matcher, that I've done a fair amount of string manipulation, and that I've snatched files from the internet by shelling out to wget (I use Chez Scheme -- users have to provide their own non-portable shell mechanism if they use anything else); I've even done some limited graphics work by writing ANSI terminal sequences.

I'll disagree just a little bit with Jens. Instead of porting afterwards, I find it easier to build in portability from the beginning. I didn't use to think that way, but my experience over the last three years shows that it works.

Answer 2

It's worth pointing out that modern Scheme implementations are themselves fairly portable; you can often port whole programs to new environments simply by bringing the appropriate Scheme along. That doesn't help library programmers much, though, and that's where R7RS-small, the latest Scheme definition, comes in. It's not widely implemented yet, but it provides a larger common core than R5RS.

Answer 3

I believe that in Scheme, portability is a fool's errand, since Scheme implementations are more different than they are similar, and there is no single implementation that other implementations try to emulate (unlike Python and Ruby, for example).

Thus, portability in Scheme is analogous to using software rendering for writing games "because it's in the common subset between OpenGL and DirectX". In other words, it's a lowest common denominator—it can be done, but you lose access to many features that the implementation offers.

For this reason, while SRFIs generally have a portable reference implementation (where practical), some of them are accompanied by notes that a quality Scheme implementation should tailor the library to use implementation-specific features in order to function optimally.

• A prime example is case-lambda (SRFI 16); it can be implemented portably, and the reference implementation demonstrates it, but it's definitely less optimal compared to a built-in case-lambda, since you're having to implement function dispatch in "user" code.
• Another example is stream-constant from SRFI 41. The reference implementation uses an O(n) simulation of circular lists for portability, but any decent implementation should adapt that function to use real circular lists so that it's O(1).^†

The list goes on. Many useful things in Scheme are not portable—SRFIs help make more features portable, but there's no way that SRFIs can cover everything. If you want to get useful work done efficiently, chances are pretty good you will have to use non-portable features. The best you can do, I think, is to write a façade to encapsulate those features that aren't already covered by SRFIs.

^† There is actually now a way to implement stream-constant in an O(1) fashion without using circular lists at all. Portable and fast for the win!

Answer 4

Difficult question.

Most people decide to be pragmatic. If portability between implementations is important, they write the bulk of the program in standard Scheme and isolate non-standard parts in (smallish) libraries. There have been various approaches of how exactly to do this. One recent effort is SnowFort.

http://snow.iro.umontreal.ca/

An older effort is SLIB.

http://people.csail.mit.edu/jaffer/SLIB

If you look - or ask for - libraries for regular expressions and lexer/parsers you'll quickly find some.

Since the philosophy of R5RS is to include only those language features that all implementors agree on, the standard is small - but also very stable.

However for "real world" programming R5RS might not be the best fit. Therefore R6RS (and R7RS?) include more "real world" libraries.

That said if you only need portability because it seems to be the Right Thing, then reconsider carefully if you really want to put the effort in. I would simply write my program on the implementation I know the best. Then if necessary port it afterwards. This often turns out to be easier than expected.