[Haskell] The meaning of #{}

[Donald Bruce Stewart]

hoelz:
> I've been pouring over the Xlib bindings for Haskell, and I've come
> across the following code:
> 
> peekXButtonEvent p = do
>         root            <- #{peek XButtonEvent,root} p
>         subwindow       <- #{peek XButtonEvent,subwindow} p
>         time            <- #{peek XButtonEvent,time} p
>         x               <- #{peek XButtonEvent,x} p
>         y               <- #{peek XButtonEvent,y} p
>         x_root          <- #{peek XButtonEvent,x_root} p
>         y_root          <- #{peek XButtonEvent,y_root} p
>         state           <- #{peek XButtonEvent,state} p
>         button          <- #{peek XButtonEvent,button} p
>         same_screen     <- #{peek XButtonEvent,same_screen} p
>         return (root, subwindow, time, x, y, x_root, y_root,
>                 state, button, same_screen)
> 
> I can't seem to find a definition or an explanation for #{}.  Is this
> some kind of operator for dealing with monads or something?

Nope! But nice guess - if in doubt, its probably to do with monads :-)

Its actually hsc2hs preprocessor code,

    http://www.haskell.org/ghc/docs/latest/html/users_guide/hsc2hs.html#id3184862

#peek struct_type, field

    A function that peeks a field of a C struct will be output. It will have the type Storable b =>
    Ptr a -> IO b. The intention is that #peek and #poke can be used for implementing the operations
    of class Storable for a given C struct (see the Foreign.Storable module in the library
    documentation).


-- Done