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Cannot subtract number from iterators #1920

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evantypanski opened this issue Nov 8, 2024 · 1 comment · May be fixed by #1921
Open

Cannot subtract number from iterators #1920

evantypanski opened this issue Nov 8, 2024 · 1 comment · May be fixed by #1921

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@evantypanski
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In the "Changing input" section in the docs, there is a reference that seems like it implies you can subtract some number from an iterator:

For random access, you’d typically get the current position through input(), subtract from it the desired number of bytes you want to back, and then use set_input to establish that new position.

So I tried self.set_input(self.input() - 1); and got:

[error] /Users/etyp/src/spicy-redis/analyzer/resp.spicy:25:28-25:43: unsupported operator: <const iterator<stream>> - <uint<64>

which seems exactly like the docs were telling me to do, so I got a little upset.

I don't see any iterator - uint ish operators anywhere - was this ever supported, or intended to be supported? I'd hardly call it "typical" if it's not supported :) so either that should be added or the docs changed.

But, I don't have the history of this to know which one it should be. I'd just prefer if you could subtract the number of bytes, that seems nice.
evantypanski added a commit to evantypanski/spicy-redis that referenced this issue Nov 8, 2024
@evantypanski
Prefer reparsing to prepending byte
29c044d 
I must've missed the point of this section before, oops. I'd prefer not
to have the `start` var, but it's necessary because I can't subtract a
number from the iterator right now:

zeek/spicy#1920
@rsmmr
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rsmmr commented Nov 11, 2024

Hmm ... I think the docs are odd/wrong here. We do have a + operator for these iterators but indeed no -. Iirc, that's on purpose because going back is often not going to work; there's a good chance that the data will have been discarded already. It's not always wrong, and in fact we'd also catch it on access and throw an InvalidIterator, but not providing - in the first place makes it harder to get surprised later at runtime.

So with that in mind, I don't know what the docs are trying to say there. :-) We should probably replace that with some version of "use input() to remember a position, then set_input() to go back there".

evantypanski added a commit that referenced this issue Nov 11, 2024
@evantypanski
Fix some inconsistencies in random access docs.
7fbb11a 
Closes #1920
@evantypanski evantypanski linked a pull request Nov 11, 2024 that will close this issue
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evantypanski added a commit that referenced this issue Nov 12, 2024
@evantypanski
Fix some inconsistencies in random access docs.
31687e8 
Closes #1920
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Fix some inconsistencies in random access docs. zeek/spicy
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@rsmmr @evantypanski