Getting the file descriptor from a handle, without closing it

[Bas van Dijk]

On 1 October 2011 08:30, Volker Wysk <pf3 at volker-wysk.de> wrote:
> 1.
>
> data FD = FD {
>  fdFD :: {-# UNPACK #-} !CInt,
>  fdIsNonBlocking :: {-# UNPACK #-} !Int
>  }
>
> What is that exclamation mark?

That's a strictness annotation and is haskell98/2010:

http://www.haskell.org/onlinereport/haskell2010/haskellch4.html#x10-680004.2

And that "{-# UNPACK #-}"?

To quote:

http://www.haskell.org/ghc/docs/latest/html/users_guide/pragmas.html#unpack-pragma

"The UNPACK indicates to the compiler that it should unpack the
contents of a constructor field into the constructor itself, removing
a level of indirection"

> 2.
>
> handleToFd' :: Handle -> Handle__ -> IO (Handle__, Fd)
> handleToFd' h h_ at Handle__{haType=_,..} = do
>  case cast haDevice of
>       -- ...
>
> haDevice should be a function. How could you cast it?

Note the .. in the record pattern. That is a language extension called
RecordWildCards. It replaces each elided field f by the pattern f = f:

http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#record-wildcards

> 3.
>
> data Handle__
>  = forall dev enc_state dec_state .
>      (IODevice dev, BufferedIO dev, Typeable dev) =>
>    Handle__ {
>      -- ...
>    }
>    deriving Typeable
>
> What's that "forall" thing?

Handle__ is an existentially quantified data constructors:

http://www.haskell.org/ghc/docs/latest/html/users_guide/data-type-extensions.html#existential-quantification

> 4.
>
> handleToFd' h h_ at Handle__{haType=_,..} = do
>  case cast haDevice of
>    Nothing -> -- ...
>    Just fd -> do
>      -- ...
>      return (Handle__{haType=ClosedHandle,..},
>                                 Fd (fromIntegral (FD.fdFD fd)))
>
> What's this ".." inside "Handle__{haType=ClosedHandle,..}"?

See answer of 2

Regards,

Bas