haskell program compile error about hClose temph

Question

The error message show below. If remove the line in code hClose temph, the program can be compiled successfully.

tempfile.hs:85:8: error:
    • Couldn't match type ‘a’ with ‘()’
      ‘a’ is a rigid type variable bound by
        the type signature for:
          withTempFile :: forall a.
                          String -> (FilePath -> Handle -> IO a) -> IO a
        at tempfile.hs:62:1-62
      Expected type: IO a
        Actual type: IO ()
    • In a stmt of a 'do' block: hClose temph
      In the expression:
        do tempdir <- getTemporaryDirectory
           (tempfile, temph) <- openTempFile tempdir pattern
           func tempfile temph
           hClose temph
      In an equation for ‘withTempFile’:
          withTempFile pattern func
            = do tempdir <- getTemporaryDirectory
                 (tempfile, temph) <- openTempFile tempdir pattern
                 func tempfile temph
                 ....
    • Relevant bindings include
        func :: FilePath -> Handle -> IO a (bound at tempfile.hs:63:22)
        withTempFile :: String -> (FilePath -> Handle -> IO a) -> IO a
          (bound at tempfile.hs:63:1)
   |
85 |        hClose temph
   |        ^^^^^^^^^^^^

The code

withTempFile :: String -> (FilePath -> Handle -> IO a) -> IO a
withTempFile pattern func =
    do -- The library ref says that getTemporaryDirectory may raise on
       -- exception on systems that have no notion of a temporary directory.
       -- So, we run getTemporaryDirectory under catch.  catch takes
       -- two functions: one to run, and a different one to run if the
       -- first raised an exception.  If getTemporaryDirectory raised an
       -- exception, just use "." (the current working directory).
       --tempdir <- catch (getTemporaryDirectory) (\_ -> return ".")
       tempdir <- getTemporaryDirectory
       (tempfile, temph) <- openTempFile tempdir pattern 

       -- Call (func tempfile temph) to perform the action on the temporary
       -- file.  finally takes two actions.  The first is the action to run.
       -- The second is an action to run after the first, regardless of
       -- whether the first action raised an exception.  This way, we ensure
       -- the temporary file is always deleted.  The return value from finally
       -- is the first action's return value.
       --finally (func tempfile temph) 
       --        (do hClose temph
       --            removeFile tempfile)
       func tempfile temph
       -- putStrLn temph
       hClose temph
       --removeFile tempfile

Answer 1

Since the hClose is last, that's what comes out of the do block. You need the result of func to come out of the do block, so do this instead:

       rv <- func tempfile temph
       hClose temph
       return rv

You still need to do more work to make it exception-safe, but that's enough to make it compile and work right on the happy path.