Using Scheme libraries in a Racket program

I wrote a program in Racket (the source code is in a .rkt file with #lang racket at the top). I also wrote a library in (mostly) portable R7RS Scheme. Can I use the library in the program in a clean way?

My goal is for the library to be widely portable between Scheme implementations (at least the R7RS-compliant ones, ideally others as well). There is a third-party R7RS shim for Racket but as far as I can tell it requires me to type #lang r7rs at the top of my source file. I presume this #lang directive would confuse Schemes other than Racket.

Can I put the core of my library in one or more portable .scm source files and then have one .rkt file with the #lang r7rs directive that tells Racket to include the portable files somehow? Does Racket understand some kind of library definition file such as the .sld used on snow-fort?

I tried to look all over the Racket documentation but I can't find this discussed anywhere. Nor did I find a general Scheme portability FAQ or best practices document.

I managed to mix Racket and R7RS code for real work and made an example of the technique on GitHub.

Here's a copy of the readme from that repo:

The Racket R7RS Shim

Racket doesn't ship with R7RS support. It's in the third-party package r7rs by Alexis King: https://github.com/lexi-lambda/racket-r7rs Despite not being an official part of Racket, it worked just fine for me (I used a moderately complex library to do HTML parsing and wrote some farily involved string processing and tree walking on top of it, so this is definitely useful for real work).

You can install the shim via raco pkg install r7rs. Note also that info.rkt lists r7rs in the dependencies, which you need for Heroku and the like.

Modules

app -- a Racket application
lib -- an R7RS library used by app
sublib -- an R7RS library used by lib

What files the modules are made of

So app needs just one file, app.rkt, like any normal Racket module.

But lib and sublib need 3 files each. lib.scm is the Scheme code. lib.sld is the Scheme library definition. And lib.rkt is a Racket wrapper for it. Technically you could combine lib.sld and lib.scm into one file but it's cleaner to have them separate. You could also copy all your Scheme code directly into lib.rkt but then you can't import it into other Schemes.

Note that lib.scm doesn't have an (import ...) form at the top. The imports are inside the define-library form in lib.sld. The define-library form uses (include ...) to include the actual code in lib.scm.

The job of lib.rkt is just to say #lang r7rs to Racket and then include the Scheme stuff. It first needs to (import (scheme base)) so that we can use include and export. The included .sld files import everything else from the Scheme standard that the library needs.

Note that lib depends on sublib but sublib is not imported by the define-library form in lib.sld. Instead, lib.rkt has to load lib and all its dependencies: it contains (include "sublib.sld") in addition to the obvious (include "lib.sld").

So lib.sld imports only stuff from the Scheme standard whereas lib.rkt imports all our custom libraries. I had to resort to this hack because I couldn't get the Racket module finder to find sublib if I put it in the (define-library ...) imports. I didn't try hard at all so there may well be a way to make it work.

Mutable vs immutable lists

Racket uses immutable cons cells (made by Racket's cons, satisfies pair?) by default whereas R7RS uses mutable cons cells (made by Racket's mcons, satisfies mpair?). That is, when you call cons on the Scheme side, it actually makes something that looks to Racket as if you had called mcons on the Racket side. A mutable cons means you can use Scheme's set-car! and set-cdr! to alter it in place, whereas the car and cdr of an immutable cons can't be changed after the initial cons.

By default, Racket displays lists made out of mutable conses using {curly braces} instead of (ordinary parentheses). This will bite you when you pass lists over the R7RS--Racket boundary. You can print using ordinary parentheses by changing the print-mpair-curly-braces parameter but for many things it may be easier to convert your lists (and trees) from mutable to immutable.

I don't know whether the Racket R7RS shim allows you to make immutable conses on the Scheme side. It would be nice to have an option for Scheme cons to make immutable conses (in that case set-car! and set-cdr! would cause an error, which is fine for code using only immutable data structures).

Where to find R7RS libraries

Lots of R7RS libraries are collected by Alex Shinn at http://snow-fort.org/

Bottom line

The upshot of all this is that you can mix R7RS and Racket with a little work and your codebase stays pretty clean (at least for simple cases).