Add subscript operator to Expression to access sub-expressions by name

[jteuber]

Hi there! It might be just me, but accessing certain sub-expressions of a rule by name seems very handy for me, especially with very complicated rules. So I added this. It seems to work quite well. Especially the std::optional makes parsing expressions with optional expressions much simpler.

[TheLartians]

Hey, thanks for the PR, I definitely see the advantages of the rule based query, and good call with the optional return! Could you also add a test case, so the feature remains stable in the future?

[jteuber]

Sure, there you go! :)

[codecov[bot]]

Codecov Report

Merging #60 (e1b5022) into master (dcec3a0) will increase coverage by 0.02%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #60      +/-   ##
==========================================
+ Coverage   97.67%   97.69%   +0.02%     
==========================================
  Files           9        9              
  Lines         646      652       +6     
==========================================
+ Hits          631      637       +6     
  Misses         15       15              

Impacted Files	Coverage Δ
include/peg_parser/interpreter.h	95.71% <100.00%> (+0.40%)	:arrow_up:

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[jteuber]

Yes, you're absolutely right. That's a much better test case. I had a hard time coming up with a simple test yesterday so I basically simplified the use case I had for the subscript operator. As soon as I have some time on my PC today I copy over your suggestion.

Feb 5, 2021 10:43:43 Lars Melchior notifications@github.com:

@TheLartians commented on this pull request.

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In test/source/parser.cpp[https://github.com/TheLartians/PEGParser/pull/60#discussion_r570842318]:

+  ParserGenerator program; +  program.setSeparator(program["Whitespace"] << "[\t ]"); + +  program["Word"] << "[a-z]+"; +  program["Yell"] << "[A-Z]+"; +  program["Number"] << "[0-9]+"; +  program["IntSubscript"] << "Word Yell? Number" >> [](auto e) { return e[1].string(); }; +  program["StringSubscriptOptional"] << "Number Yell? Word" >> [](auto e) { +    if (auto expr = e["Yell"]) return expr->string(); +    return std::string(); +  }; +  program["Start"] << "IntSubscript | StringSubscriptOptional"; + +  program.setStart(program["Start"]); + +  REQUIRE(program.run("ab 0") == "0"); +  REQUIRE(program.run("ab CD 1") == "CD"); +  REQUIRE(program.run("2 ef") == ""); +  REQUIRE(program.run("2 GH ij") == "GH");

Hey, thanks for adding the test case! I do find it to be a hard to follow, as it's doing a lot of things at once. What do you think about simplifying it to a single rule that checks the new [string] operator?

For example, something like

  ParserGenerator program;   program["Word"] << "[a-z]+";   program["Yell"] << "[A-Z]+";   program["Start"] << "Word | Yell" >> [](auto e){ return bool(e["Yell"]);  }   REQUIRE(!program.run("hello"));   REQUIRE(program.run("HELLO"));

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[TheLartians]

The feature is released in v2.2.0! 🎉