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Comment by lifthrasiir

6 years ago

How can you be sure that that } is the end of a certain defined block? This most importantly affects the scoping and in many cases it's ambiguous. IDEs do have rich metadata besides from the source code but then parsers should be aware of them.

2 comments

lifthrasiir

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throwaway_pdp09  6 years ago

You're ignoring the ; which are sync points.

> How can you be sure that that } is the end of a certain defined block

If it's not in a string, what else is it but a typo? If a typo, it fails to parse but so long as it doesn't crash, fine.

  • lifthrasiir  6 years ago

    Maybe my wording is not accurate, imagine the following (not necessarily idiomatic) C code:

        int main() {
        int x;
        {
            int x[];
        // <-- caret here
        x += 42;
    }

    This code doesn't compile, so the IDE tries to produce a partial AST. A naive approach will result in the first } matching with the second {, so `x += 42;` will cause a type error. But as noticable from the indentation, it is more believable that there was or will be } matching with the second { at the caret position and `x += 42;` refers to the outer scope.

    Yes, of course parsers can account for the indentation in this case. But more generally this kind of parsing is sensitive to a series of edit sequences, not just the current code. This makes incremental parsing a much different problem from ordinary parsing, and also is likely why ibains and folks use packrat parsing (which can be easily made incremental).