Because of its simplicity and simple syntax, Scheme has become a favorite of language implementors, and as a result, dozens of Scheme implementations are available for free. These may be divided into two categories: interpreters and compilers. (Well, more correctly, the compilers usually include both compilers and interpreters.)
My favorite compiler is Bigloo. Written by Manuel Serrano of France's INRIA, Bigloo compiles Scheme code to efficient C, which is then compiled by gcc into executable code. Because of this method, Bigloo is both portable and open to improvements in the system C compiler. As mentioned earlier, programs compiled by Bigloo usually run within a few percent of the C equivalent program. Manuel, currently in the throes of his doctoral dissertation, may be reached at Manuel.Serrano@inria.fr.
Other compilers include the widely used Scheme->C, produced by Joel Bartlett (who may still be reachable as firstname.lastname@example.org) at DEC's Western Research Lab. Programs compiled by Scheme->C aren't quite as fast at simple tasks as those compiled by Bigloo, but Scheme->C sports a much more advanced garbage collector. For programs with large data sets, Scheme->C may be a wiser choice.
Chez Scheme is also available for Linux. Chez is a venerable compiler written by R. Kent Dybvig (email@example.com ), one of the authors of the de facto Scheme standard (R4RS). Chez Scheme is not free.
Several Scheme interpreters have gained popularity in recent months. One of the hottest recent products is STk, an interpreter which makes John Ousterhout's Tk easy-to-use widget set available from an object-oriented dialect of Scheme. STk isn't the best performer among the interpreter crowd, but it easily bests Ousterhout's Tcl.
Aubrey Jaffer's SCM, one of the most mature Scheme interpreters, offers small size, high speed, and a growing library of extensions. These modules include POSIX interfaces, socket I/O, and a curses screen management library. SCM's author also maintains a library of helpful Scheme functions in a package called SLIB. I use SLIB in several of my code examples. The author, Aubrey Jaffer, may be reached as firstname.lastname@example.org.
The Free Software Foundation recently began an effort to provide a standard scripting and application extension language for their products. This language, called GUILE for some acronymic reason that escapes me now, will be based upon the SCM interpreter. To find out more about GUILE, send mail to email@example.com (that's not a misspelling!).
Finally, a group of Scheme enthusiasts at MIT (Scheme's birthplace) have undertaken an ambitious project to make Scheme as practical at Unix scripting as the Bourne and Korn shells. Their Scheme shell (scsh) combines the superior Scheme language with the powerful process/pipe-based data flow mechanism of the Unix shells.
Here are some program examples to give you a feel for programming in Scheme.
Program 1 is the standard fibonacci algorithm, and Program 2 is the standard recursive factorial. These should work under any Scheme implementation. Listing 3 shows an implementation of the tried-and-true Unix->DOS file conversion utility for the SCM interpreter and SLIB, its library package. It reads a file of linefeed-terminated lines and outputs a file whose lines are terminated by carriage returns and newlines. Pay special attention to the definition of the function chomp; this function splits a string into a list of characters, filters out all the unwanted characters, and collapses the list back into a string. Listing 4 shows a definition of chomp that corresponds more closely to what a C programmer would write. Note the striking difference in complexity.
In Listing 5 is a program which takes a list of files on the command line and arranges them into disk-sized groups. This process will be familiar to those of you who installed Linux from floppy disks. The program illustrates the power of Scheme's list-manipulation procedures. map and for-each both execute a function for every member of a list. find-if returns the first element of a list that matches specified condition. These and other list techniques are usually implemented in C with cumbersome, error-prone loops.
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