Hi Edwin, > came across these macros in pico.h: > > #define isSym(x) (num(x)&WORD) > #define isSymb(x) ((num(x)&(WORD+2))==WORD) > > what is the intent of isSymb()? as compared to isSym()?
as ever, the files "doc64/structures", "doc/structures" and "mini/doc/structures" are the absolute key to the understanding of the working of the different PicoLisp interpreters. They are the "bible" for the internals of PicoLisp. The 64-bit version has separate tag bits for each data type: cnt xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxS010 big xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxS100 sym xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx1000 cell xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx0000 That is, if you AND a pointer with 2, you can test for a short number, with 4 for a bignum, with 6 for a number in general (cnt or big), and with 8 for a symbol. A similar case for the 32-bit version: xxxxxxxxxxxxxxxxxxxxxxxxxxxxx010 Number xxxxxxxxxxxxxxxxxxxxxxxxxxxxx100 Symbol xxxxxxxxxxxxxxxxxxxxxxxxxxxxx000 Cell It has no short numbers, so 2 checks for a number, and 4 for a symbol. In MiniPicoLisp, however, things are a little different. It has no bignums, so the patterns num xxxxxx10 sym xxxxx100 cell xxxxx000 indicate that 2 also tests for a number, but because of the lack of bignums, we want a number range as big as possible. For that reason, all remaining bits (the 'x'es) can be used for a number (e.g. 30 bits if you compile Mini on a 32-bit architecture). However, just testing for 4 does not guarantee that you indeed have a symbol. It might also be an odd number (which happens to have a least significant bit set to one). Now look at #define isSym(x) (num(x)&WORD) It is the "naive" form. WORD is 4 on a 32-bit system, so this macro just tests for 4. You can use it when you already know that you _don't_ have a number. This is quite often the case, when the arguments of a function are analyzed, and you first test for a number. If you want to immediately test for a symbol (without first checking for a number), you shoule use #define isSymb(x) ((num(x)&(WORD+2))==WORD) This effectively does an AND with 6, then compares to 4. This is more expensive that just the AND with 4 above, because for an odd number the AND with 4 will return 6 (and not 4). In summary, having these two macros is a matter of efficiency. BTW, this potential conflict also exists in the 64-bit version. The two number types have their sign bit (the 'S' in the patterns above) in the position of the symbol tag. A simple 'AND 4' shows the same problem, in that a symbol cannot be distingished from a negative number. Therefore, the 64-bit code always checks for a number before it checks for a symbol. The 32-bit version is clean in that regard. You can test for any type at any time, without worrying for conflicts. Cheers, - Alex -- UNSUBSCRIBE: mailto:firstname.lastname@example.org?subject=Unsubscribe