TinySCHEME Version 1.32

"Safe if used as prescribed"
-- Philip K. Dick, "Ubik"

This software is open source, covered by a BSD-style license.

Please read accompanying file COPYING.

This Scheme interpreter is based on MiniSCHEME version 0.85k4 (see miniscm.tar.gz in the Scheme Repository) Original credits in file MiniSCHEMETribute.txt. D. Souflis (dsouflis@acm.org)

What is TinyScheme?

TinyScheme is a lightweight Scheme interpreter that implements as large a subset of R5RS as was possible without getting very large and complicated. It is meant to be used as an embedded scripting interpreter for other programs. As such, it does not offer IDEs or extensive toolkits although it does sport a small top-level loop, included conditionally. A lot of functionality in TinyScheme is included conditionally, to allow developers freedom in balancing features and footprint.

As an embedded interpreter, it allows multiple interpreter states to coexist in the same program, without any interference between them. Programmatically, foreign functions in C can be added and values can be defined in the Scheme environment. Being a quite small program, it is easy to comprehend, get to grips with, and use.

Known bugs

SCM tests had revealed a memory allocation error in the past, but not anymore. It probably had to do with vectors, and version 1.21 probably got rid of it.

Things that keep missing, or that need fixing

There are no hygienic macros. No rational or complex numbers. No unwind-protect and call-with-values.

Maybe (a subset of) SLIB will work with TinySCHEME...

I will add a debugger... The user will be able to specify breakpoints, and a new toplevel will be entered when the breakpoint is reached. Most of the actual debugger will be in Scheme, with minimal additions to scheme.c.

Scheme Reference

If something seems to be missing, please refer to the code and "init.scm", since some are library functions. Refer to the MiniSCHEME readme as a last resort.

Environments
(interaction-environment)
See R5RS. In TinySCHEME, immutable list of association lists.

(current-environment)
The environment in effect at the time of the call. An example of its use and its utility can be found in the sample code that implements packages in "init.scm":

          (macro (package form)
               `(apply (lambda ()
                         ,@(cdr form)
                         (current-environment))))
The environment containing the (local) definitions inside the closure is returned as an immutable value.

(defined? <symbol>) (defined? <symbol> <environment>) Checks whether the given symbol is defined in the current (or given) environment.

Symbols
(gensym)
Returns a new interned symbol each time. Will probably move to the library when string->symbol is implemented.

Directives
(gc)
Performs garbage collection immediatelly.

(gcverbose) (gcverbose <bool>)
The argument (defaulting to #t) controls whether GC produces visible outcome.

(quit) (quit <num>)
Stops the interpreter and sets the 'retcode' internal field (defaults to 0). When standalone, 'retcode' is returned as exit code to the OS.

(tracing <num>)
1, turns on tracing. 0 turns it off. (Only when USE_TRACING is 1).

Mathematical functions
Since rationals and complexes are absent, the respective functions are also missing.

Supported: exp, log, sin, cos, tan, asin, acos, atan, floor, ceiling, trunc, round and also sqrt and expt when USE_MATH=1. Number-theoretical quotient, remainder and modulo, gcd, lcm. Library: exact?, inexact?, odd?, even?, zero?, positive?, negative?, exact->inexact. inexact->exact is a core function.

Type predicates
boolean?,eof-object?,symbol?,number?,string?,integer?,real?,list?,null?, char?,port?,input-port?,output-port?,procedure?,pair?,environment?', vector?. Also closure?, macro?.

Types
Types supported:

Literals
String literals can contain escaped quotes \" as usual, but also \n, \r, \t and \xDD (hex representations). Note also that it is possible to include literal newlines in string literals, e.g.

          (define s "String with newline here
          and here
          that can function like a HERE-string")
Character literals contain #\space and #\newline and are supplemented with #\return and #\tab, with obvious meanings. Hex character representations are allowed (e.g. #\x20 is #\space).

When USE_ASCII_NAMES is defined, various control characters can be refered to by their ASCII name.

     0	     #\nul	       17       #\dc1
     1	     #\soh             18       #\dc2
     2	     #\stx             19       #\dc3
     3	     #\etx             20       #\dc4
     4	     #\eot             21       #\nak
     5	     #\enq             22       #\syn
     6	     #\ack             23       #\etv
     7	     #\bel             24       #\can
     8	     #\bs              25       #\em
     9	     #\ht              26       #\sub
     10	     #\lf              27       #\esc
     11	     #\vt              28       #\fs
     12	     #\ff              29       #\gs
     13	     #\cr              30       #\rs
     14	     #\so              31       #\us
     15	     #\si
     16	     #\dle             127      #\del
Numeric literals support #x #o #b and #d. Flonums are currently read only in decimal notation. Full grammar will be supported soon.

Quote, quasiquote etc.
As usual.

Immutable values
Immutable pairs cannot be modified by set-car! and set-cdr!. Immutable strings cannot be modified via string-set!

I/O
As per R5RS, plus String Ports (see below). current-input-port, current-output-port, close-input-port, close-output-port, input-port?, output-port?, open-input-file, open-output-file. read, write, display, newline, write-char, read-char, peek-char. char-ready? returns #t only for string ports, because there is no portable way in stdio to determine if a character is available. Also open-input-output-file, set-input-port, set-output-port (not R5RS) Library: call-with-input-file, call-with-output-file, with-input-from-file, with-output-from-file and with-input-output-from-to-files, close-port and input-output-port? (not R5RS). String Ports: open-input-string, open-output-string, open-input-output-string. Strings can be used with I/O routines.

Vectors
make-vector, vector, vector-length, vector-ref, vector-set!, list->vector, vector-fill!, vector->list, vector-equal? (auxiliary function, not R5RS)

Strings
string, make-string, list->string, string-length, string-ref, string-set!, substring, string->list, string-fill!, string-append, string-copy. string=?, string<?, string>?, string>?, string<=?, string>=?. (No string-ci*? yet). string->number, number->string. Also atom->string, string->atom (not R5RS).

Symbols
symbol->string, string->symbol

Characters
integer->char, char->integer. char=?, char<?, char>?, char<=?, char>=?. (No char-ci*?)

Pairs & Lists
cons, car, cdr, list, length, map, for-each, foldr, list-tail, list-ref, last-pair, reverse, append. Also member, memq, memv, based on generic-member, assoc, assq, assv based on generic-assoc.

Streams
head, tail, cons-stream

Control features
Apart from procedure?, also macro? and closure? map, for-each, force, delay, call-with-current-continuation (or call/cc), eval, apply. 'Forcing' a value that is not a promise produces the value. There is no call-with-values, values, nor dynamic-wind. Dynamic-wind in the presence of continuations would require support from the abstract machine itself.

Property lists
TinyScheme inherited from MiniScheme property lists for symbols. put, get.

Dynamically-loaded extensions
(load-extension <filename without extension>)
Loads a DLL declaring foreign procedures.

Esoteric procedures
(oblist)
Returns the oblist, an immutable list of all the symbols.

(macro-expand <form>)
Returns the expanded form of the macro call denoted by the argument

(define-with-return (<procname> <args>...) )
Like plain 'define', but makes the continuation available as 'return' inside the procedure. Handy for imperative programs.

(new-segment <num>)
Allocates more memory segments.

defined?
See "Environments"

(get-closure-code <closure>)
Gets the code as scheme data.

(make-closure <code> <environment>)
Makes a new closure in the given environment.

Obsolete procedures
(print-width <object>)

Programmer's Reference

The interpreter state is initialized with "scheme_init". Custom memory allocation routines can be installed with an alternate initialization function: "scheme_init_custom_alloc". Files can be loaded with "scheme_load_file". Strings containing Scheme code can be loaded with "scheme_load_string". It is a good idea to "scheme_load" init.scm before anything else.

External data for keeping external state (of use to foreign functions) can be installed with "scheme_set_external_data". Foreign functions are installed with "assign_foreign". Additional definitions can be added to the interpreter state, with "scheme_define" (this is the way HTTP header data and HTML form data are passed to the Scheme script in the Altera SQL Server). If you wish to define the foreign function in a specific environment (to enhance modularity), use "assign_foreign_env".

The procedure "scheme_apply0" has been added with persistent scripts in mind. Persistent scripts are loaded once, and every time they are needed to produce HTTP output, appropriate data are passed through global definitions and function "main" is called to do the job. One could add easily "scheme_apply1" etc.

The interpreter state should be deinitialized with "scheme_deinit".

DLLs containing foreign functions should define a function named init_<base-name>. E.g. foo.dll should define init_foo, and bar.so should define init_bar. This function should assign_foreign any foreign function contained in the DLL.

The first dynamically loaded extension available for TinyScheme is a regular expression library. Although it's by no means an established standard, this library is supposed to be installed in a directory mirroring its name under the TinyScheme location.

Foreign Functions

The user can add foreign functions in C. For example, a function that squares its argument:

          pointer square(scheme *sc, pointer args) {
           if(args!=sc->NIL) {
               if(sc->isnumber(sc->pair_car(args))) {
                    double v=sc->rvalue(sc->pair_car(args));
                    return sc->mk_real(sc,v*v);
               }
           }
           return sc->NIL;
          }
Foreign functions are now defined as closures: 

   sc->interface->scheme_define( 
        sc, 
        sc->global_env, 
        sc->interface->mk_symbol(sc,"square"), 
        sc->interface->mk_foreign_func(sc, square));
Foreign functions can use the external data in the "scheme" struct to implement any kind of external state.

External data are set with the following function:
void scheme_set_external_data(scheme *sc, void *p);

As of v.1.17, the canonical way for a foreign function in a DLL to manipulate Scheme data is using the function pointers in sc->interface.

Standalone

Usage: tinyscheme -?
or: tinyscheme [<file1> <file2> ...]
followed by
-1 <file> [<arg1> <arg2> ...]
-c <Scheme commands> [<arg1> <arg2> ...]
assuming that the executable is named tinyscheme.

Use - in the place of a filename to denote stdin.
The -1 flag is meant for #! usage in shell scripts. If you specify
#! /somewhere/tinyscheme -1
then tinyscheme will be called to process the file. For example, the following script echoes the Scheme list of its arguments.

	       #! /somewhere/tinyscheme -1
	       (display *args*)
The -c flag permits execution of arbitrary Scheme code.

Error Handling

Errors are recovered from without damage. The user can install his own handler for system errors, by defining *error-hook*. Defining to '() gives the default behavior, which is equivalent to "error". USE_ERROR_HOOK must be defined.

A simple exception handling mechanism can be found in "init.scm". A new syntactic form is introduced:

          (catch <expr returned exceptionally>
               <expr1> <expr2> ... <exprN>)
"Catch" establishes a scope spanning multiple call-frames until another "catch" is encountered.

Exceptions are thrown with:

          (throw "message")
If used outside a (catch ...), reverts to (error "message").

Example of use:

          (define (foo x) (write x) (newline) (/ x 0))

          (catch (begin (display "Error!\n") 0)
               (write "Before foo ... ")
               (foo 5)
               (write "After foo"))
The exception mechanism can be used even by system errors, by

          (define *error-hook* throw)
which makes use of the error hook described above.

If necessary, the user can devise his own exception mechanism with tagged exceptions etc.

Reader extensions

When encountering an unknown character after '#', the user-specified procedure *sharp-hook* (if any), is called to read the expression. This can be used to extend the reader to handle user-defined constants or whatever. It should be a procedure without arguments, reading from the current input port (which will be the load-port).

Colon Qualifiers - Packages

When USE_COLON_HOOK=1:
The lexer now recognizes the construction <qualifier>::<symbol> and transforms it in the following manner (T is the transformation function): 
          T(<qualifier>::<symbol>) = (*colon-hook* 'T(<symbol>) <qualifier>)
where <qualifier> is a symbol not containing any double-colons.

As the definition is recursive, qualifiers can be nested. The user can define his own *colon-hook*, to handle qualified names. By default, "init.scm" defines *colon-hook* as EVAL. Consequently, the qualifier must denote a Scheme environment, such as one returned by (interaction-environment). "Init.scm" defines a new syntantic form, PACKAGE, as a simple example. It is used like this:

          (define toto
               (package
                    (define foo 1)
                    (define bar +)))

          foo                                     ==>  Error, "foo" undefined
          (eval 'foo)                             ==>  Error, "foo" undefined
          (eval 'foo toto)                        ==>  1
          toto::foo                               ==>  1
          ((eval 'bar toto) 2 (eval 'foo toto))   ==>  3
          (toto::bar 2 toto::foo)                 ==>  3
          (eval (bar 2 foo) toto)                 ==>  3
If the user installs another package infrastructure, he must define a new 'package' procedure or macro to retain compatibility with supplied code.

Note: Older versions used ':' as a qualifier. Unfortunately, the use of ':' as a pseudo-qualifier in existing code (i.e. SLIB) essentially precludes its use as a real qualifier.