The concept you are looking for is "phantom type": a type that is used solely as a "tag" of sorts, with no associated data. The Ptr type constructor uses a phantom type so you can differentiate between machine addresses pointing at different kinds of data and gain some small measure of safety.

On Wed, Nov 7, 2018 at 8:20 AM Guru Devanla <gurudev.devanla@gmail.com> wrote:

Thank you all for the responses, After reading through the responses, it became clear that this is a commonly pattern used in Haskell FFI programming.

I went back and reviewed the chapter in RWH. There is a discussion on this pattern, under 'Typed Pointers'. 

http://book.realworldhaskell.org/read/interfacing-with-c-the-ffi.html

Which means similar to the type listed in the chapter

newtype PCRE = PCRE (Ptr PCRE)

The example I provided reduces to:
newtype XEvent = XEvent (Ptr XEvent). --  a recursive newtype that cannot be dereferenced

In summary:

1. This pattern can be use to specify the types just for type-checking within Haskell. They do not contain any data. Therefore, not data can be cerated for XEvent.
2.  Data can be created for type `Ptr XEvent` using the `alloca` interface, and only way to de-reference these values would be through `peek`. But, while derefencing the resulting values will have other types.

Next, I plan to spend some time working on these examples to get a better sense.

Thank you all!

On Wed, Nov 7, 2018 at 2:29 AM Theodore Lief Gannon <tanuki@gmail.com> wrote:

The meaning of XEvent is different on the left and right sides of the newtype declaration. On the left, it's a type; on the right, it's a data constructor. The constructor wraps a value which refers to the type, but not recursively back to the constructor itself.

On Tue, Nov 6, 2018, 8:36 AM Guru Devanla <gurudev.devanla@gmail.com wrote:

Hello Haskell-Cafe,

I have been recently studying the XMonad code and some related types available in the X11 bindings library.

I came across this type:

newtype XEvent = XEvent XEventPtr

type XEventPtr = Ptr XEvent

Available here: 

http://hackage.haskell.org/package/X11-1.9/docs/Graphics-X11-Xlib-Event.html#t:XEvent

It seems that this type is circular here. how does one use this type? Is it possible to create a value out of this type? What is the use of this type?

Please could someone help me wrap my head around this?

Thanks

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