Re: [Boost-users] [proto][spirit] parsing into proto expressions

Subject: Re: [Boost-users] [proto][spirit] parsing into proto expressions
From: llonesmiz (jorge.navalon_at_[hidden])
Date: 2016-05-11 01:56:15

llonesmiz wrote
> 
> Igor R. wrote
>> Hi,
>> 
>> Long story short: I would like to create proto expressions at run-time
>> using spirit parsers. Would it be possible?
>> More in details: I'd like to parse disassembler output, creating proto
>> expression template for instruction and its operands.
>> Lets assume we define proto expressions that would allow to express
>> the following asm:
>> "lea eax, [ebx + 2 * ecx]"
>> like this:
>> BOOST_PROTO_AUTO(lea_instr, eax = &*(ebx + 2 * ecx)); // registers and
>> immediates are proto terminals
>> Of course, we can't write such an expression in C++ code, because the
>> actual operands get known after the asm string is parsed.
>> So we can express any "lea" in the code (at compile-time) in a more
>> general form, using placeholders: _1 = &_2;
>> Then, when the parser matches "lea", we need to parse and create at
>> run-time the actual operands, i.e. to substitute _1 with eax and _2
>> with *(ebx + 2 * ecx).
>> 
>> I'd appreciate any pointer!
>> 
>> Thanks.
> I'm not an expert in either Spirit or Proto but my understanding is that
> Spirit requires specific (fixed) types as the result of its parsers and a
> proto expression generated as you intend wouldn't fit in that model(unless
> you use some kind of type erasure, but I don't know if that would be even
> possible).
> 
> I think what you need to use is Boost.Metaparse (in Boost 1.61). Abel
> Sinkovics has a great tutorial (sadly not completely up-to-date with the
> most recent version, but there are very little differences) at
> https://github.com/sabel83/metaparse_tutorial (the real interesting part
> is in "lab 6", the rest is a great introduction to metaprogramming and mpl
> conventions). I think this
> answer(http://stackoverflow.com/questions/17783393/how-to-parse-text-for-a-dsl-at-compile-time/17807607#17807607)
> could also be useful. It's not as well explained but it's up-to-date and
> tries to compare Metaparse with a Spirit approach.

Sorry, I have failed to consider your runtime requirements. If you need to
input your asm code at runtime that would disqualify the Metaparse approach.
If your use case is simply to write the expressions in your code in a style
similar to your "BOOST_PROTO_AUTO(lea_instr, eax = &*(ebx + 2 * ecx))" then
I think it could work.

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