By nonportable, I assume you mean "between implementations", because that's where the nonportability is. And yes, there is. It has to do with scheme's origin.

By nonportable, I assume you mean "between implementations", because that's where the nonportability is. And yes, there is. It has to do with scheme's origin. scheme originated in PL research (at the MIT AI lab) and was quickly pressed into service for education. As such, it has a strong userbase in academia. Those users need a language that was small, easy to implement, and powerful enough to express a wide variety of paradigms and concepts compactly. Meanwhile, Scheme users in the Real World, who want to Get Stuff Done, want that, but they also want things like OS interfaces, networking, module systems, FFIs, large standard libraries, etc. For many years, the approach the scheme standard took was to lay out the bare minimum required to be a scheme, and depend upon maintainers of various implementations to flesh it out. Furthermore, if the comitee couldn't agree on something, it went unspecified. This resulted in something akin to the unix situation, but worse: we had a family of languages named Scheme, but all but the simplest programs would inevitably depend on implementation-specific language extensions, and all the implementations extended the language in different directions. Furthermore, the academics need Scheme to be easy to implement, so it was: it was so simple any easy that anyone could implement it, and anyone did, resulting in a bevy of Scheme implementations of varying levels of quality, from "industrial strength" to "weekend hack". There are admirable efforts to write cross implementation packages (most famously, SLIB), but it's generally not recomended, because it's a PITA to do. R6RS tried to fix this, but... well, imagine that POSIX said, "we're trying to improve cross-system compatability, so you all have to implement Solaris Event Ports, Netlink, Streams, and the AIX sysadmin utilities to be considered POSIX. Just throw away whatever you were using instead of that." Needless to say, this did not go over well. R7RS tried to improve things by spliting the language into the traditional small core, and the larger stdlib, which you didn't have to implement to be scheme, but would hopefully improve interoperability between implementations. This seems to be working okay thus far, although the stdlib isn't done yet. We shall see...