Parser caching: transformers would need to accept const lvalue references.

[ku...@mareimbrium.org]

I'm experimenting with caching lexer output, and have devised a `cache()` combinator

that takes a parser outputting `Foo` and returns a parser outputting `const Foo &`. The subparser

is queried only once for a given position in the input.

This works a treat until you get to transforms: the functors all take rvalue references. The quick

fix is to let the cache copy results. Another is to make the functors take `auto` arguments.

Yet another would be to change the transformer argument types to take const lvalue references

where the lower level parsing is cached.

Is there anything more generic or appropriate that you'd suggest to address this problem?

The cache might be an unnecessary addition, but at least it can be benchmarked, and the hit

ratio looked at, and then removed should it be redundant. I've ported our crusty in-house DSL

compiler to kj framework and the cache really helps there, because of how the grammar is

written. Of course, the grammar could be fixed, but the cache is a decent stop-gap.

An aside: the parsing framework is a joy to work with, as long as one doesn't need to decode

error messages. I'll have to eventually look at more obvious places to add compile-time error

catching to. Usually, it's some low level function applicator in the framework that bombs out,

with the real problem far away. Visual Studio's approach to error reporting doesn't help either.

Cheers, Kuba

[Kenton Varda]

Hi Kuba,

As a quick fix, you could always pass a pointer instead of a reference. Or you could wrap the reference in a struct.

I agree that lvalue references "should" work, but it might be tricky to design the parser code such that it doesn't accidentally decide to use a reference where inappropriate, and end up with a dangling reference. Or maybe it would be easy... I haven't looked much at that code in about 5 years, TBH, so I don't quite remember.

-Kenton

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