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.
First of all, we need to assign a typeid to our new type. Typeids
in TinyScheme are small integers declared in an enum, very close to
the top; it begins with T_STRING. Add a new one at the end, say
T_MEMBLOCK. There can be at most 31 types, but you don't have to
worry about that limit yet.
| ...
| T_PORT,
| T_VECTOR, /* remember to add a comma to the preceding item! */
| T_MEMBLOCK
} };
Then, some helper macros would be useful. Go to where isstring() and
the rest are defined and define:
> int ismemblock(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 ismemblock(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 procede 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(isstring(a)) {
| sc->free(strvalue(a));
| }
> else if(ismemblock(a)) {
> sc->free(strvalue(x));
> }
| }
There are no MEMBLOCK literals, so we don't concern ourselfs with
the READER part (yet!). We must cater to the PRINTER, though. We
add one case more in printatom().
| } else if (iscontinuation(l)) {
| p = "#<CONTINUATION>";
> } else if (ismemblock(l)) {
> p = "#<MEMORY BLOCK>";
| }
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. There is a huge enum with OP_XXX values.
That's where the op-codes are declared. For reasons of cohesion, we add
the new op-codes right after those for vectors:
| OP_VECSET,
> OP_MKBLOCK,
> OP_MEMBLOCKP,
> OP_BLOCKLEN,
> OP_BLOCKREF,
> OP_BLOCKSET,
| OP_NOT,
We add the predicate along the other predicates:
| OP_VECTORP,
> OP_BLOCKP,
| OP_EQ,
Op-codes are really just tags for a huge C switch, only this switch
is broke up in 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. INF_ARG as a maximum arity means "unlimited".
| {opexe_2, "vector-set!", 3, 3}, /* OP_VECSET */
> {opexe_2, "make-block", 1, 2}, /* OP_MKBLOCK */
> {opexe_2, "block-length", 1, 1}, /* OP_BLOCKLEN */
> {opexe_2, "block-ref", 2, 2}, /* OP_BLOCKREF */
> {opexe_2, "block-set!",3 ,3}, /* OP_BLOCKSET */
The predicate goes with the other predicates, in opexe_3.
| {opexe_3, "vector?", 1, 1}, /* OP_VECTORP, */
> {opexe_3, "block?", 1, 1}, /* OP_BLOCKP, */
All that remains is to write the actual processing in opexe_2, right
after OP_VECSET.
> 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));
> }
Same for the predicate in opexe_3.
| case OP_VECTORP: /* vector? */
| s_retbool(isvector(car(sc->args)));
> case OP_BLOCKP: /* block? */
> s_retbool(ismemblock(car(sc->args)));