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
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