[flang-dev] Regarding main program structure

[Timothy Keith via flang-dev]

SpecificationPart is made up of lists of other nodes. When the specification-part of a program is absent, it is represented by a SpecificationPart containing only empty lists. So there is no need to make it optional. The same applies to ExecutionPart.

Tim

From: flang-dev <flang-dev-bounces at lists.llvm.org> on behalf of Rajesh K Singh via flang-dev <flang-dev at lists.llvm.org>
Reply-To: Rajesh K Singh <rajeshsingh.h606 at gmail.com>
Date: Friday, October 25, 2019 at 2:05 AM
To: "flang-dev at lists.llvm.org" <flang-dev at lists.llvm.org>
Subject: [flang-dev] Regarding main program structure

Hello,

According to specification R1401, a Fortran main program *must* have an “end statement” whereas other parts are optional.
// R1401 main-program ->
//         [program-stmt] [specification-part] [execution-part]
//         [internal-subprogram-part] end-program-stmt

Thus, the below case.f90 is a legitimate fortran program:
[user at localhost ~/F18]$ cat -n case.f90
     1  end  !only end-program-stmt is specified.
[user at localhost ~/F18]$

The implementation of MainProgram structure (“lib/parser/parse-tree.h”) shows that only “ProgramStmt” and “InternalSubprogramPart” are optional.
struct MainProgram {
  TUPLE_CLASS_BOILERPLATE(MainProgram);
  std::tuple<std::optional<Statement<ProgramStmt>>, SpecificationPart,
      ExecutionPart, std::optional<InternalSubprogramPart>,
      Statement<EndProgramStmt>>
      t;
};

Why “SpecificationPart” and “ExecutionPart” are not implemented as optional (std::optional), while these are optional similar to “ProgramStmt” and “InternalSubprogramPart”?

Thanks in anticipation.

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