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This SRFI describes, for sufficiently POSIX-compatible systems, a portable interface for compiling Scheme programs conforming to the R7RS to binaries that can be directly executed on the host system.
There are currently no issues.
While the R7RS describes portable Scheme programs, it does not describe how to actually store and run such a Scheme program together with its libraries on a host system. This makes it impossible to write portable scripts (including Makefiles) that involve compiling and running Scheme programs.
This situation of Scheme can be compared to the C language. While ISO C does define the syntax and semantics of C programs, it leaves out questions how C programs are compiled and run on the host system. Contrary to Scheme programs, however, there also exists a portable interface for compiling and running C programs (at least on POSIX-conforming systems), namely the description of the c99 utility in the POSIX standard.
It is the purpose of this SRFI to describe an analogous interface for Scheme programs conforming to the R7RS. The description assumes a sufficiently POSIX-compatible system, not because it needs any very POSIX-specific features but because POSIX is a host system interface that has been standardized. Mutatis mutandis, this specification also applies to host systems with a similar file and command line interface as described by POSIX, like Windows.
While this SRFI speaks of compiling Scheme programs, the interface described by this SRFI applies also to Scheme interpreters. The only requirement for the compiler's output is that it is a directly executable program on the host machine. In particular, the resulting binary is allowed to call an interpreter to execute the actual Scheme program.
The following description of the utility described by this SRFI follows in form roughly the description of c99 of the POSIX standard.
compile-r7rs [options...] [pathname]
compile-r7rs utility is an interface to a compiler
for Scheme programs conforming to the R7RS. The
program pathname (if present) shall be compiled to produce an
executable file. The resulting executable file shall be written to the
file specified by the -o option (if present) or to
the file file if pathname is of the
form file.scm. (Implementations on Windows
would choose file.exe instead
of file.)
The following options shall be supported:
Multiple instances of the -D, -A, and -I options can be specified.
If exactly one pathname operand of the form file.scm is specified, the application shall behave as specified. If the operand is not of this form, the behaviour is unspecified.
Rationale: This allows an implementation to support additional file types, e.g. top-level programs written for the R6RS, by assigning different extensions to these file types.
If no such operand is given, the application may do nothing or compile the library definitions contained in text files in the directories (and its subdirectories) specified by the -A, and -I options into an implementation-defined format.
Rationale: In implementations supporting precompilation of libraries, libraries are usually precompiled when they are installed as this may need administrator privileges depending on the file system location of the libraries. Thus the above description ensures that compilation of a program can be split into two phases, one happening at library installation time, the other happening when the final executable is build from the program source.
The input file shall be a text file containing a R7RS program.
The following exit values shall be returned:
As the R7RS does not describe a mapping between library names and text files
containing library
definitions, this SRFI defines such a mapping as follows: First of all, a library name
(<library name element> ... <library name element>) is mapped to the
(relative) pathname <library name element>/.../<library name
element>.sld.
If the input file processed by compile-r7rs directly or indirectly imports a library,
the application searches an implementation-defined list of directories (which is amended by the
-A and -I options) for the pathname corresponding to the
library until a valid text file containing a library definition for the imported library is
found.
This SRFI does not specify whether this algorithm applies to the standard libraries described by the R7RS.
The algorithm is unspecified if the mapping from library names to pathnames produces invalid pathnames on the host system (e.g. if a library name element contains a null character).
As the mapping from library names to pathnames is not injective (a library name element may contain a slash, for example), implementations are encouraged to allow more than one library definition in a text file that is located by this algorithm.
The application may use compiled versions of the library definitions that are produced by running the utility without a pathname argument instead of the library definitions itself.
In order for a host system to implement this specification, firstly, it has to provide a command named compile-r7rs, and secondly, this command has to fulfill the command-line arguments interface as described above.
For a Scheme system to implement this specification, it is enough to provide the second layer, that is a command, say compile-fantastic-scheme, that conforms to the command-line arguments interface as described above, and to ask the user to create a symlink from compile-fantastic-scheme to a compile-r7rs pathname. (A Scheme system may alternatively provide an option at install time specifying whether the command compile-r7rs should be installed automatically.)
Rationale: If a conforming Scheme implementation was forced to install compile-r7rs automatically, several such Scheme implementations in one host system would likely step on each other's toes. So in the case of more than one conforming Scheme implementation, it is really the system administrator's or the distribution tools' matters to provide compile-r7rs as the command name.
There exists no portable implementation (otherwise this SRFI would be moot), so each Scheme implementation supporting this SRFI should ship its own implementation. There are Scheme systems that already implement most of this SRFI: Chibi and Larceny implement the library location algorithm described above. Chibi implements the -D, -A, and -I options. Larceny implements compiling library definitions into an implementation-defined format.
A not very sophisticated sample implementation for Chibi Scheme is provided as a shell script for readers to read and experiment with. (In order for the -D flag to be supported, a post-0.7.3-release version of Chibi is needed.)
This SRFI is related to, but not overlapping with SRFI 22. While SRFI 22 describes how Scheme script executables can be written without invoking a compiler, this SRFI does not assume that the compiled file is in any human-readable format. Furthermore, this SRFI assumes an underlying implementation of R7RS and specifies how to locate libraries supplementary to the standard libraries, which is not possible with SRFI 22. Thus, neither SRFI makes the other obsolete.
The specification of c99 by the POSIX standard and the Chibi and Larceny implementations of R7RS were influential while this SRFI was written.
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