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 []) makes a new block of the specified size optionally filling it with a specified byte (block? ) (block-length ) (block-ref ) retrieves byte at location (block-set! ) 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 = "#"; > } else if (ismemblock(l)) { > p = "#"; | } 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)));