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C types with void explicitly in the parameter list (e.g. void (*)(void)) are assigned the wrong Haskell type #155

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lehmacdj opened this issue Feb 5, 2025 · 1 comment · May be fixed by #156
Open

C types with void explicitly in the parameter list (e.g. void (*)(void)) are assigned the wrong Haskell type #155

lehmacdj opened this issue Feb 5, 2025 · 1 comment · May be fixed by #156

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@lehmacdj
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lehmacdj commented Feb 5, 2025

When attempting to invoke a function with a void argument like in this snippet:

-- | - (void)prepareForReadingItemsAtURLs:(NSArray<NSURL *> *)readingURLs
--        options:(NSFileCoordinatorReadingOptions)readingOptions
--        writingItemsAtURLs:(NSArray<NSURL *> *)writingURLs
--        options:(NSFileCoordinatorWritingOptions)writingOptions
--        error:(NSError **)outError
--        byAccessor:(void (^)(void (^completionHandler)(void)))batchAccessor;
m_NSFileCoordinator_prepareForReadingAndWritingItems ::
  Ptr NSFileCoordinator ->
  Ptr NSArray ->  -- readingURLs
  NSFileCoordinatorReadingOptions ->
  Ptr NSArray ->  -- writingURLs
  NSFileCoordinatorWritingOptions ->
  Ptr (Ptr NSError) ->
  IO () ->
  IO ()
m_NSFileCoordinator_prepareForReadingAndWritingItems fileCoordinator readingURLs readingOptions writingURLs writingOptions errorPtr accessor = do
  -- let accessor' :: FunPtr (CInt -> IO ()) -> IO () = \completionHandler -> accessor ($(C.peekFunPtr [t| CInt -> IO () |]) completionHandler 0)
  [C.block| void {
    [$(NSFileCoordinator *fileCoordinator)
      prepareForReadingItemsAtURLs: $(NSArray *readingURLs)
      options: $(NSFileCoordinatorReadingOptions readingOptions)
      writingItemsAtURLs: $(NSArray *writingURLs)
      options: $(NSFileCoordinatorWritingOptions writingOptions)
      error: $(NSError **errorPtr)
      byAccessor: ^(void (^completionHandler)(void)) {
        $fun:(void (*accessor)(void))();
        completionHandler()
      }
    ];
  }
  |]

I get an error like so:

• Unacceptable argument type in foreign declaration:
    ‘()’ cannot be marshalled in a foreign call
• When checking declaration:
    foreign import ccall safe "wrapper" inline_c_ffi_6989586621681841573
      :: (() -> IO ()) -> IO (FunPtr (() -> IO ()))

Inspecting the FunPtr docs, I see that this is not the expected type for a function pointer of type void (*)(void) instead it should have type IO () or (). The bug is in buildArr in the antiquoter for $fun:. I'm working on a fix and will make a PR shortly.
@lehmacdj lehmacdj changed the title Handle void arguments correctly C types with void explicitly in the parameter list (e.g. void (*)(void)) are assigned the wrong Haskell type Feb 5, 2025
@lehmacdj lehmacdj linked a pull request Feb 5, 2025 that will close this issue
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@lehmacdj
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lehmacdj commented Feb 5, 2025

As a workaround you can declare the type without a parameter list, e.g. void (*)() for a void (*)(void) function. This is the correct C++ syntax but technically incorrect for C.

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Fix handling of void parameters passed to functions lehmacdj/inline-c
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@lehmacdj