[Haskell-cafe] Teach me to cooperate with IOException

[Henning Thielemann]

On Tue, 16 Apr 2024, Folsk Pratima wrote:

> foreign import ccall unsafe "complex.c syscall_that_fails" c_syscall_that_fails
>    :: Ptr CInt -> IO CInt
>
> sysCall :: Int -> IO ()
> sysCall i =
>    allocaBytes (sizeOf (0 :: CInt)) $ \ptr -> do
>        st <- c_syscall_that_fails ptr
>        errno <- peekElemOff ptr 0
>        case st of
>            (-1) -> throwIO $ userError $ "syscall: " ++ show errno
>            _ -> return ()
>
> ```
>
> Now, I do not think it is a "user error", it is a real system call
> error that might or might not be already covered by System.IO.Error. In
> my case errno is 2, it is "No such file or directory", so it is covered
> by System.IO.Error and I may just use the library. But my C code may
> call whatever!


Generally I discourage using the exceptions implicit in IO. Exceptions 
must be properly handled by the function caller, the caller must know what 
exceptions can occur. Thus I prefer using Exception monad transformers 
like transformers:Trans.ExceptT or 
explicit-exception:Trans.Exception.Synchronous.