42 lines
2.2 KiB
Plaintext
42 lines
2.2 KiB
Plaintext
- readable gensyms. have uninterned symbols, but have all same-named
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gensyms read to the same (eq) symbol within an expression.
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- fat pointers, i.e. 64 bits on 32-bit platforms. we could have full 32-bit
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integers too. the mind boggles at the possibilities.
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(it would be great if everybody decided that pointer types should forever
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be wider than address spaces, with some bits reserved for application use)
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- any way at all to provide O(1) computed lookups (i.e. indexing).
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CL uses vectors for this. once you have it, it's sufficient to get
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efficient hash tables and everything else.
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- could be done just by generalizing cons cells to have more than
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car, cdr: c2r, c3r, etc. maybe (1 . 2 . 3 . 4 . ...)
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all you need is a tag+size on the front of the object so the collector
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knows how to deal with it.
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(car x) == (ref x 0), etc.
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(rplaca x v) == (rplac x 0 v), etc.
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(size (cons 1 2)) == 2, etc.
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- one possibility: if we see a cons whose CAR is tagptr(0x10,TAG_SYM),
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then the CDR is the size and the following words are the elements.
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. this approach is especially good if vectors are separate types from
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conses
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- another: add u_int32_t size to cons_t, making them all 50% bigger.
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access is simpler and more uniform, without fully doubling the size like
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we'd get with fat pointers.
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Notice that the size is one byte more than the number of characters in
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the string. This is because femtoLisp adds a NUL terminator to make its
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strings compatible with C. No effort is made to hide this fact.
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But since femtoLisp tracks the sizes of cvalues, it doesn't need the
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terminator itself. Therefore it treats zero bytes specially as rarely
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as possible. In particular, zeros are only special in values whose type
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is exactly <tt>(array char)</tt>, and are only interpreted in the
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following cases:
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<ul>
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<li>When printing strings, a final NUL is never printed. NULs in the
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middle of a string are printed though.
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<li>String constructors NUL-terminate their output.
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<li>Explicit string functions (like <tt>strlen</tt>) treat NULs the same
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way equivalent C functions would.
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</ul>
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Arrays of uchar, int8, etc. are treated as raw data and zero bytes are
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never special.
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