Shows how hWaitForInput eventually calls sycalls in GHC Haskell

hWaitForInput-call-chain.hs

-- Call chain of hWaitForInput:
-- 
--   hWaitForInput
--     ready from the IODevice typeclass for type FD (from haDevice of a Handle__)
--       ready from base/GHC.IO.FD
--         fdReady foreign import from base/GHC.IO.FD

-- Exact locations and code:

-- http://hackage.haskell.org/package/base-4.10.0.0/docs/src/GHC.IO.Handle.Text.html#local-6989586621679305954

hWaitForInput :: Handle -> Int -> IO Bool
hWaitForInput h msecs = do
  wantReadableHandle_ "hWaitForInput" h $ \ handle_@Handle__{..} -> do
  cbuf <- readIORef haCharBuffer

  if not (isEmptyBuffer cbuf) then return True else do

  if msecs < 0
        then do cbuf' <- readTextDevice handle_ cbuf
                writeIORef haCharBuffer cbuf'
                return True
        else do
               -- there might be bytes in the byte buffer waiting to be decoded
               cbuf' <- decodeByteBuf handle_ cbuf
               writeIORef haCharBuffer cbuf'

               if not (isEmptyBuffer cbuf') then return True else do

                r <- IODevice.ready haDevice False{-read-} msecs -------ready call HERE
                if r then do -- Call hLookAhead' to throw an EOF
                             -- exception if appropriate
                             _ <- hLookAhead_ handle_
                             return True
                     else return False


-- http://hackage.haskell.org/package/base-4.10.0.0/docs/GHC-IO-Device.html#t:IODevice

class IODevice a where
  -- | @ready dev write msecs@ returns 'True' if the device has data
  -- to read (if @write@ is 'False') or space to write new data (if
  -- @write@ is 'True').  @msecs@ specifies how long to wait, in
  -- milliseconds.
  --
  ready :: a -> Bool -> Int -> IO Bool


-- https://github.com/ghc/ghc/blob/ghc-8.2.1-release/libraries/base/GHC/IO/FD.hs#L103

instance GHC.IO.Device.IODevice FD where
  ready = ready


-- https://github.com/ghc/ghc/blob/ghc-8.2.1-release/libraries/base/GHC/IO/FD.hs#L395

ready :: FD -> Bool -> Int -> IO Bool
ready fd write msecs = do
  r <- throwErrnoIfMinus1Retry "GHC.IO.FD.ready" $
          fdReady (fdFD fd) (fromIntegral $ fromEnum $ write)
                            (fromIntegral msecs)
#if defined(mingw32_HOST_OS)
                          (fromIntegral $ fromEnum $ fdIsSocket fd)
#else
                          0
#endif
return (toEnum (fromIntegral r))



-- https://github.com/ghc/ghc/blob/ghc-8.2.1-release/libraries/base/GHC/IO/FD.hs#L407

foreign import ccall safe "fdReady"
  fdReady :: CInt -> CInt -> CInt -> CInt -> IO CInt