feature request: a way to query the current line number during parsing about rust-peg HOT 5 CLOSED

Use position!(), which returns the current byte offset into the input string. It usually makes more sense to store these byte offsets in the AST and convert to line:col form only when needed for an error message for performance and storage reasons, but you could do that mapping within the parser if you really needed to.

The implementation in peg used for the default error messages just counts the `\n' in the slice before the position, in linear time, since it's called at most once. That would be slow to run for every AST node. In Signalspec, I build an index of line start offsets, and binary search it to convert byte position to line in logarithmic time.

from rust-peg.

Yes, the current implementation would be inefficient for multiple, successive calls.

Line numbers have been requested by other GitHub users before me. I rather enjoy PEG and would love to see this feature gain more support.

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You could build the index before parsing, pass it in as a grammar argument, and write a rule that wraps position!() to do the lookup in the index. Doing that doesn't belong in this library because most users would be better off deferring byte position to line mapping past the AST anyway.

from rust-peg.

Hmm, food for thought.

Thank you for offering (several) workarounds. I admit feeling some anxiety about managing my first AST. I'll see what I can do.

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You could build the index before parsing, pass it in as a grammar argument, and write a rule that wraps position!() to do the lookup in the index.

Alright, I think that's what I want to do.

Could you please direct me to a working example, such as a unit test? Then I can copy that in detail, into my peg application.

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