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|
How to hack TinyScheme
----------------------
TinyScheme is easy to learn and modify. It is structured like a
meta-interpreter, only it is written in C. All data are Scheme
objects, which facilitates both understanding/modifying the
code and reifying the interpreter workings.
In place of a dry description, we will pace through the addition
of a useful new datatype: garbage-collected memory blocks.
The interface will be:
(make-block <n> [<fill>]) makes a new block of the specified size
optionally filling it with a specified byte
(block? <obj>)
(block-length <block>)
(block-ref <block> <index>) retrieves byte at location
(block-set! <block> <index> <byte>) modifies byte at location
In the sequel, lines that begin with '>' denote lines to add to the
code. Lines that begin with '|' are just citations of existing code.
Lines that begin with X denote lines to be removed from the code.
First of all, we need to assign a typeid to our new type. Typeids
in TinyScheme are small integers declared in the scheme_types enum
located near the top of the scheme.c file; it begins with T_STRING.
Add a new entry at the end, say T_MEMBLOCK. Remember to adjust the
value of T_LAST_SYTEM_TYPE when adding new entries. There can be at
most 31 types, but you don't have to worry about that limit yet.
| T_ENVIRONMENT=14,
X T_LAST_SYSTEM_TYPE=14
> T_MEMBLOCK=15,
> T_LAST_SYSTEM_TYPE=15
| };
Then, some helper macros would be useful. Go to where is_string()
and the rest are defined and add:
> INTERFACE INLINE int is_memblock(pointer p) { return (type(p)==T_MEMBLOCK); }
This actually is a function, because it is meant to be exported by
scheme.h. If no foreign function will ever manipulate a memory block,
you can instead define it as a macro:
> #define is_memblock(p) (type(p)==T_MEMBLOCK)
Then we make space for the new type in the main data structure:
struct cell. As it happens, the _string part of the union _object
(that is used to hold character strings) has two fields that suit us:
| struct {
| char *_svalue;
| int _keynum;
| } _string;
We can use _svalue to hold the actual pointer and _keynum to hold its
length. If we couln't reuse existing fields, we could always add other
alternatives in union _object.
We then proceed to write the function that actually makes a new block.
For conformance reasons, we name it mk_memblock
> static pointer mk_memblock(scheme *sc, int len, char fill) {
> pointer x;
> char *p=(char*)sc->malloc(len);
>
> if(p==0) {
> return sc->NIL;
> }
> x = get_cell(sc, sc->NIL, sc->NIL);
>
> typeflag(x) = T_MEMBLOCK|T_ATOM;
> strvalue(x)=p;
> keynum(x)=len;
> memset(p,fill,len);
> return (x);
> }
The memory used by the MEMBLOCK will have to be freed when the cell
is reclaimed during garbage collection. There is a placeholder for
that staff, function finalize_cell(), currently handling strings only.
| static void finalize_cell(scheme *sc, pointer a) {
| if(is_string(a)) {
| sc->free(strvalue(a));
> } else if(is_memblock(a)) {
> sc->free(strvalue(a));
| } else if(is_port(a)) {
There are no MEMBLOCK literals, so we don't concern ourselves with
the READER part (yet!). We must cater to the PRINTER, though. We
add one case more in atom2str().
| } else if (iscontinuation(l)) {
| p = "#<CONTINUATION>";
> } else if (is_memblock(l)) {
> p = "#<MEMORY BLOCK>";
| } else {
Whenever a MEMBLOCK is displayed, it will look like that.
Now, we must add the interface functions: constructor, predicate,
accessor, modifier. We must in fact create new op-codes for the virtual
machine underlying TinyScheme. Since version 1.30, TinyScheme uses
macros and a single source text to keep the enums and the dispatch table
in sync. The op-codes are defined in the opdefines.h file with one line
for each op-code. The lines in the file have six columns between the
starting _OPDEF( and ending ): A, B, C, D, E, and OP.
Note that this file uses unusually long lines to accomodate all the
information; adjust your editor to handle this.
The purpose of the columns is:
- Column A is the name of the subroutine that handles the op-code.
- Column B is the name of the op-code function.
- Columns C and D are the minimum and maximum number of arguments
that are accepted by the op-code.
- Column E is a set of flags that tells the interpreter the type of
each of the arguments expected by the op-code.
- Column OP is used in the scheme_opcodes enum located in the
scheme-private.h file.
Op-codes are really just tags for a huge C switch, only this switch
is broken up in to a number of different opexe_X functions. The
correspondence is made in table "dispatch_table". There, we assign
the new op-codes to opexe_2, where the equivalent ones for vectors
are situated. We also assign a name for them, and specify the minimum
and maximum arity (number of expected arguments). INF_ARG as a maximum
arity means "unlimited".
For reasons of consistency, we add the new op-codes right after those
for vectors:
| _OP_DEF(opexe_2, "vector-set!", 3, 3, TST_VECTOR TST_NATURAL TST_ANY, OP_VECSET )
> _OP_DEF(opexe_2, "make-block", 1, 2, TST_NATURAL TST_CHAR, OP_MKBLOCK )
> _OP_DEF(opexe_2, "block-length", 1, 1, T_MEMBLOCK, OP_BLOCKLEN )
> _OP_DEF(opexe_2, "block-ref", 2, 2, T_MEMBLOCK TST_NATURAL, OP_BLOCKREF )
> _OP_DEF(opexe_2, "block-set!", 1, 1, T_MEMBLOCK TST_NATURAL TST_CHAR, OP_BLOCKSET )
| _OP_DEF(opexe_3, "not", 1, 1, TST_NONE, OP_NOT )
We add the predicate along with the other predicates in opexe_3:
| _OP_DEF(opexe_3, "vector?", 1, 1, TST_ANY, OP_VECTORP )
> _OP_DEF(opexe_3, "block?", 1, 1, TST_ANY, OP_BLOCKP )
| _OP_DEF(opexe_3, "eq?", 2, 2, TST_ANY, OP_EQ )
All that remains is to write the actual code to do the processing and
add it to the switch statement in opexe_2, after the OP_VECSET case.
> case OP_MKBLOCK: { /* make-block */
> int fill=0;
> int len;
>
> if(!isnumber(car(sc->args))) {
> Error_1(sc,"make-block: not a number:",car(sc->args));
> }
> len=ivalue(car(sc->args));
> if(len<=0) {
> Error_1(sc,"make-block: not positive:",car(sc->args));
> }
>
> if(cdr(sc->args)!=sc->NIL) {
> if(!isnumber(cadr(sc->args)) || ivalue(cadr(sc->args))<0) {
> Error_1(sc,"make-block: not a positive number:",cadr(sc->args));
> }
> fill=charvalue(cadr(sc->args))%255;
> }
> s_return(sc,mk_memblock(sc,len,(char)fill));
> }
>
> case OP_BLOCKLEN: /* block-length */
> if(!ismemblock(car(sc->args))) {
> Error_1(sc,"block-length: not a memory block:",car(sc->args));
> }
> s_return(sc,mk_integer(sc,keynum(car(sc->args))));
>
> case OP_BLOCKREF: { /* block-ref */
> char *str;
> int index;
>
> if(!ismemblock(car(sc->args))) {
> Error_1(sc,"block-ref: not a memory block:",car(sc->args));
> }
> str=strvalue(car(sc->args));
>
> if(cdr(sc->args)==sc->NIL) {
> Error_0(sc,"block-ref: needs two arguments");
> }
> if(!isnumber(cadr(sc->args))) {
> Error_1(sc,"block-ref: not a number:",cadr(sc->args));
> }
> index=ivalue(cadr(sc->args));
>
> if(index<0 || index>=keynum(car(sc->args))) {
> Error_1(sc,"block-ref: out of bounds:",cadr(sc->args));
> }
>
> s_return(sc,mk_integer(sc,str[index]));
> }
>
> case OP_BLOCKSET: { /* block-set! */
> char *str;
> int index;
> int c;
>
> if(!ismemblock(car(sc->args))) {
> Error_1(sc,"block-set!: not a memory block:",car(sc->args));
> }
> if(isimmutable(car(sc->args))) {
> Error_1(sc,"block-set!: unable to alter immutable memory block:",car(sc->args));
> }
> str=strvalue(car(sc->args));
>
> if(cdr(sc->args)==sc->NIL) {
> Error_0(sc,"block-set!: needs three arguments");
> }
> if(!isnumber(cadr(sc->args))) {
> Error_1(sc,"block-set!: not a number:",cadr(sc->args));
> }
> index=ivalue(cadr(sc->args));
> if(index<0 || index>=keynum(car(sc->args))) {
> Error_1(sc,"block-set!: out of bounds:",cadr(sc->args));
> }
>
> if(cddr(sc->args)==sc->NIL) {
> Error_0(sc,"block-set!: needs three arguments");
> }
> if(!isinteger(caddr(sc->args))) {
> Error_1(sc,"block-set!: not an integer:",caddr(sc->args));
> }
> c=ivalue(caddr(sc->args))%255;
>
> str[index]=(char)c;
> s_return(sc,car(sc->args));
> }
Finally, do the same for the predicate in opexe_3.
| case OP_VECTORP: /* vector? */
| s_retbool(is_vector(car(sc->args)));
> case OP_BLOCKP: /* block? */
> s_retbool(is_memblock(car(sc->args)));
| case OP_EQ: /* eq? */
|
