Backtracking support through &try/backtrack() #606
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rsmmr
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bbannier
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Dec 3, 2020
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This looks like a great tool so users can build their own tooling to parse conditionally. At the same time, I could see this being used to implement error handling for which this patch doesn't seem to go all the way or at least is missing tests (e.g., can backtrack be caught by on %error handlers; guaranteed interaction of plain old parse errors and &try fields; no access to particular error when &try field fails to parse). I left a few comments along those lines below. Like discussed offline we probably still need a better idea how we want error handling to finally look like.
That tells a vector explicitly to parse until it hits the end of the input, without having to use the look-ahead machinery to figure out when to stop.
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bbannier
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Dec 9, 2020
If a field is marked with ``&try``, a later call to ``self.backtrack()``
anywhere down in the parse tree will return to that position and
continue there. (This is ported over the old prototype).
Example:
export type test = unit {
a: bytes &length=4;
foo: Foo &try;
bar: Bar;
b: bytes &length=6;
on %done { print self; }
};
type Foo = unit {
a: int8 { print "Foo.a", self; }
b: int8 { print "Backtracking"; self.backtrack(); }
c: int8 { print "Foo.c", self; }
};
type Bar = unit {
a: int8 { print "Bar.a", self; }
b: int8 { print "Bar.b", self; }
c: int8 { print "Bar.c", self; }
};
# printf '1234\001\002\003567890' | pac-driver backtrack.pac2
Foo.a <a=1, b=(not set), c=(not set)>
Backtracking
Bar.a <a=1, b=(not set), c=(not set)>
Bar.b <a=1, b=2, c=(not set)>
Bar.c <a=1, b=2, c=3>
<a=1234, foo=(Null), bar=<a=1, b=2, c=3>, b=567890>
Closes #25.
When an example contained multiple commands separated by semicolons, we'd execute them separately but still include all of them into each of the command lines emitted into the docs. (Couple of doc outputs show slightly changed content, including updates into this commit.)
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