234 lines
9.3 KiB
Plaintext
234 lines
9.3 KiB
Plaintext
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How to hack TinyScheme
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----------------------
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TinyScheme is easy to learn and modify. It is structured like a
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meta-interpreter, only it is written in C. All data are Scheme
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objects, which facilitates both understanding/modifying the
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code and reifying the interpreter workings.
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In place of a dry description, we will pace through the addition
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of a useful new datatype: garbage-collected memory blocks.
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The interface will be:
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(make-block <n> [<fill>]) makes a new block of the specified size
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optionally filling it with a specified byte
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(block? <obj>)
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(block-length <block>)
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(block-ref <block> <index>) retrieves byte at location
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(block-set! <block> <index> <byte>) modifies byte at location
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In the sequel, lines that begin with '>' denote lines to add to the
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code. Lines that begin with '|' are just citations of existing code.
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Lines that begin with X are deleted.
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First of all, we need to assign a typeid to our new type. Typeids
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in TinyScheme are small integers declared in an enum, very close to
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the top of scheme.c; it begins with T_STRING. Add a new one before the
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end, call it T_MEMBLOCK. Adjust T_LAST_SYSTEM_TYPE.
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| T_ENVIRONMENT=14,
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X T_LAST_SYSTEM_TYPE=14
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> T_MEMBLOCK=15,
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> T_LAST_SYSTEM_TYPE=15
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| };
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Then, some helper macros would be useful. Go to where is_string() and
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the rest are defined and define:
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> int is_memblock(pointer p) { return (type(p)==T_MEMBLOCK); }
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This actually is a function, because it is meant to be exported by
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scheme.h. If no foreign function will ever manipulate a memory block,
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you can instead define it as a macro
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> #define is_memblock(p) (type(p)==T_MEMBLOCK)
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Then we make space for the new type in the main data structure:
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struct cell. As it happens, the _string part of the union _object
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(that is used to hold character strings) has two fields that suit us:
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| struct {
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| char *_svalue;
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| int _keynum;
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| } _string;
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We can use _svalue to hold the actual pointer and _keynum to hold its
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length. If we couldn't reuse existing fields, we could always add other
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alternatives in union _object.
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We then proceed to write the function that actually makes a new block.
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For conformance reasons, we name it mk_memblock
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> static pointer mk_memblock(scheme *sc, int len, char fill) {
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> pointer x;
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> char *p=(char*)sc->malloc(len);
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>
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> if(p==0) {
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> return sc->NIL;
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> }
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> x = get_cell(sc, sc->NIL, sc->NIL);
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>
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> typeflag(x) = T_MEMBLOCK|T_ATOM;
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> strvalue(x)=p;
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> keynum(x)=len;
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> memset(p,fill,len);
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> return (x);
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> }
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The memory used by the MEMBLOCK will have to be freed when the cell
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is reclaimed during garbage collection. There is a placeholder for
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that staff, function finalize_cell(), currently handling strings only.
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| static void finalize_cell(scheme *sc, pointer a) {
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| if(is_string(a)) {
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| sc->free(strvalue(a));
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> else if(is_memblock(a)) {
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> sc->free(strvalue(a));
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| } else if(is_port(a)) {
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There are no MEMBLOCK literals, so we don't concern ourselves with
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the READER part (yet!). We must cater to the PRINTER, though. We
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add one case more in atom2str().
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| } else if (is_foreign(l)) {
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| p = sc->strbuff;
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| snprintf(p,STRBUFFSIZE,"#<FOREIGN PROCEDURE %ld>", procnum(l));
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> } else if (ismemblock(l)) {
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> p = "#<MEMBLOCK>";
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| } else if (is_continuation(l)) {
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| p = "#<CONTINUATION>";
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| } else {
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Whenever a MEMBLOCK is displayed, it will look like that.
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Now, we must add the interface functions: constructor, predicate,
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accessor, modifier. We must in fact create new op-codes for the
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virtual machine underlying TinyScheme. Since version 1.30, TinyScheme
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uses macros and a single source text to keep the enums and the
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dispatch table in sync. That's where the op-codes are declared. Note
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that the opdefines.h file uses unusually long lines to accommodate
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all the information; adjust your editor to handle this. The file has
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six columns: A to Z. they contain:
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- Column A is the name of a routine to handle the scheme function.
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- Column B is the name the scheme function.
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- Columns C and D are the minimum and maximum number of arguments
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that are accepted by the scheme function.
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- Column E is a set of flags that are used when the interpreter
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verifies that the passed parameters are of the correct type.
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- Column F is used to create a set of enums. The enum is used in a
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switch in the routine listed in column A to get to the code that
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does the work needed for the scheme function.
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For reasons of cohesion, we add the new op-codes right after those
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for vectors:
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| _OP_DEF(opexe_2, "vector-set!", 3, 3, TST_VECTOR TST_NATURAL TST_ANY, OP_VECSET )
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> _OP_DEF(opexe_2, "make-block", 1, 2, TST_NATURAL TST_CHAR, OP_MKBLOCK )
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> _OP_DEF(opexe_2, "block-length", 1, 1, T_MEMBLOCK, OP_BLOCKLEN )
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> _OP_DEF(opexe_2, "block-ref", 2, 2, T_MEMBLOCK TST_NATURAL, OP_BLOCKREF )
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> _OP_DEF(opexe_2, "block-set!", 1, 1, T_MEMBLOCK TST_NATURAL TST_CHAR, OP_BLOCKSET )
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| _OP_DEF(opexe_3, "not", 1, 1, TST_NONE, OP_NOT )
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We add the predicate along the other predicates:
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| _OP_DEF(opexe_3, "vector?", 1, 1, TST_ANY, OP_VECTORP )
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> _OP_DEF(opexe_3, "block?", 1, 1, TST_ANY, OP_BLOCKP )
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| _OP_DEF(opexe_3, "eq?", 2, 2, TST_ANY, OP_EQ )
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All that remains is to write the actual processing in opexe_2, right
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after OP_VECSET.
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> case OP_MKBLOCK: { /* make-block */
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> int fill=0;
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> int len;
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>
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> if(!isnumber(car(sc->args))) {
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> Error_1(sc,"make-block: not a number:",car(sc->args));
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> }
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> len=ivalue(car(sc->args));
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> if(len<=0) {
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> Error_1(sc,"make-block: not positive:",car(sc->args));
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> }
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>
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> if(cdr(sc->args)!=sc->NIL) {
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> if(!isnumber(cadr(sc->args)) || ivalue(cadr(sc->args))<0) {
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> Error_1(sc,"make-block: not a positive number:",cadr(sc->args));
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> }
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> fill=charvalue(cadr(sc->args))%255;
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> }
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> s_return(sc,mk_memblock(sc,len,(char)fill));
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> }
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>
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> case OP_BLOCKLEN: /* block-length */
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> if(!ismemblock(car(sc->args))) {
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> Error_1(sc,"block-length: not a memory block:",car(sc->args));
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> }
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> s_return(sc,mk_integer(sc,keynum(car(sc->args))));
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>
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> case OP_BLOCKREF: { /* block-ref */
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> char *str;
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> int index;
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>
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> if(!ismemblock(car(sc->args))) {
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> Error_1(sc,"block-ref: not a memory block:",car(sc->args));
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> }
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> str=strvalue(car(sc->args));
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>
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> if(cdr(sc->args)==sc->NIL) {
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> Error_0(sc,"block-ref: needs two arguments");
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> }
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> if(!isnumber(cadr(sc->args))) {
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> Error_1(sc,"block-ref: not a number:",cadr(sc->args));
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> }
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> index=ivalue(cadr(sc->args));
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>
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> if(index<0 || index>=keynum(car(sc->args))) {
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> Error_1(sc,"block-ref: out of bounds:",cadr(sc->args));
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> }
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>
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> s_return(sc,mk_integer(sc,str[index]));
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> }
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>
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> case OP_BLOCKSET: { /* block-set! */
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> char *str;
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> int index;
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> int c;
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>
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> if(!ismemblock(car(sc->args))) {
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> Error_1(sc,"block-set!: not a memory block:",car(sc->args));
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> }
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> if(isimmutable(car(sc->args))) {
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> Error_1(sc,"block-set!: unable to alter immutable memory block:",car(sc->args));
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> }
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> str=strvalue(car(sc->args));
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>
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> if(cdr(sc->args)==sc->NIL) {
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> Error_0(sc,"block-set!: needs three arguments");
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> }
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> if(!isnumber(cadr(sc->args))) {
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> Error_1(sc,"block-set!: not a number:",cadr(sc->args));
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> }
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> index=ivalue(cadr(sc->args));
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> if(index<0 || index>=keynum(car(sc->args))) {
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> Error_1(sc,"block-set!: out of bounds:",cadr(sc->args));
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> }
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>
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> if(cddr(sc->args)==sc->NIL) {
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> Error_0(sc,"block-set!: needs three arguments");
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> }
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> if(!isinteger(caddr(sc->args))) {
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> Error_1(sc,"block-set!: not an integer:",caddr(sc->args));
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> }
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> c=ivalue(caddr(sc->args))%255;
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>
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> str[index]=(char)c;
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> s_return(sc,car(sc->args));
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> }
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Same for the predicate in opexe_3.
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| case OP_VECTORP: /* vector? */
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| s_retbool(isvector(car(sc->args)));
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> case OP_BLOCKP: /* block? */
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> s_retbool(ismemblock(car(sc->args)));
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