[flang-dev] Question on the parser code

[Mark Dewing via flang-dev]

Hi,
  I'm trying to understand the F18 parser and the parse tree code by
creating some simple examples.   I would like to create a simple parser and
also create a parse tree programmatically, but I'm running into some
problems.

For the first step, I added a constructor to ParseState that accepts a
std::string (to avoid constructing a CookedSource object).  The following
code works:

#include "token-parsers.h"
using namespace Fortran::parser;
int main()
{
    std::string test("b");
    ParseState p(test);
    auto p1 =  "a"_ch;
    auto res = p1.Parse(p);
    std::cout << " res = " << res.value_or("no result") << std::endl;
    return 0;
}

For the second step, I would like to create a parse tree programmatically
and call unparse.
The first problem is I am unable to build a MainProgram with a minimal set
of inputs.
So far, the code to create some of the elements is as follows:

    SpecificationPart spec{std::list<OpenMPDeclarativeConstruct>{},

 std::list<Statement<Fortran::common::Indirection<UseStmt>>>{} ,

 std::list<Statement<Fortran::common::Indirection<ImportStmt>>>{},
                           ImplicitPart{std::list<ImplicitPartStmt>{}},
                           std::list<DeclarationConstruct>{}};

    ExecutionPart exec{std::list<ExecutionPartConstruct>{}};

    Statement<EndProgramStmt> end{Statement<EndProgramStmt>{std::nullopt,
EndProgramStmt{std::nullopt}}};

The problem is constructing the tuple for the MainProgram  - all these
objects cause compile errors when placed in a tuple ("no matching
constructor" errors).  If anyone has better example code, or instructions
on how to do this properly, I would appreciate it.

Thanks,
Mark Dewing
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