Preserve exception

[roberth]

Currently, exceptions are caught by the Cpp.Exceptions module, where the original exception is thrown away. While this can be circumvented at a small scale by catching certain exceptions in C++ code, this is not feasible at a large scale, as it goes against what exceptions are intended for: make error handling implicit and centralize it around a few high-level call sites.

This PR fixes this problem by using std::exception_ptr to represent C++ exceptions as first-class Haskell values.

As a result, the caller can write their own exception handling function, to extract valuable information from specific exceptions. For example:

renderException :: SomeException -> IO Text
renderException e | Just (C.CppStdException ex _msg _ty) <- fromException e = renderStdException ex
-- ...

renderStdException :: C.CppExceptionPtr -> IO Text
renderStdException e =
  [C.throwBlock| char * {
    std::string r;
    std::exception_ptr *e = $fptr-ptr:(std::exception_ptr *e);
    try {
      std::rethrow_exception(*e);
    } catch (const somelib::Error &e) {
      std::stringstream s;
      // obtain detailed exception info
      somelib::showErrorInfo(s, e.info());
      r = s.str();
    } catch (const std::exception &e) {
      r = e.what();
    }
    return strdup(r.c_str());
  }|] >>= unsafePackMallocCString <&> decodeUtf8With lenientDecode

Review and implementation notes

The second commit renames a file, making the combined diff unhelpful. The first commit contains all the actual changes.

I've decided to move the code from Exceptions to Exception so that

• Compatibility with the old interface is preserved, as far as is feasible without introducing a second CppException representation
• It is consistent with #include <exception> and import Control.Exception; both singular

I've decided to keep the string representations of the message and exception type, in order to avoid calling C++ code in the Show instance. This can be a little inefficient, but much better than storing these in [Char] like we did.

I've decided to make the exception module(s) depend on C++11. I believe requiring a decade old standard should not pose a problem. The alternative is to revive the old code as an independent module, but until we know that such a maintenance overhead is useful to anyone, this would be a wasted effort and a liability.

[bitonic]

Thanks!

[roberth]

Could you make a release?
This one can be breaking, so inline-c-cpp needs a bump 0.4.x.y -> 0.5.0.0.
inline-c is still equal to its released version, so it's only about inline-c-cpp.

[bitonic]

Embarassingly, my Haskell setup is currently broken somehow (looks like this https://discourse.nixos.org/t/stack-build-fails-with-startprocess-invalid-argument-bad-file-descriptor/1668 ). I'll do a release as soon as I have a second to fix it.

[roberth]

This seems to work (if you excuse my unpinned <nixpkgs>; it's the latest nixos-21.11)

{ pkgs ? import <nixpkgs> {} }:

pkgs.mkShell {
  nativeBuildInputs = [
    pkgs.stack
    pkgs.ghc
    pkgs.cabal-install
    pkgs.haskellPackages.haskell-language-server
    pkgs.haskellPackages.hie-bios
  ];
}

nix-shell -I nixpkgs=channel:nixos-21.11 shell.nix

If that doesn't solve it, I've had some success clearing the ~/.stack and $(find -name .stack-work) directories by hand. Possibly because stack produces store references but no gc roots. Not sure if it solves your error, but errno can be deceptive.

[bitonic]

Done, removing ~/.stack did it

[nh2]

@roberth Was it intended that there's this e in the message?

[nh2]

Ah, now I get it. The e is suppoed to represent the _ (first field of constructor CppStdException) which is not shown because it's a pointer.

[nh2]

Shouldn't this be "CppOtherException" before and after?

[nh2]

This would also have been pointed out by -Wall for the test suite.

Fixed in my propsed PR #154

[roberth]

Thanks @nh2 for fixing the show formatting and improving the tests!