[Haskell-cafe] Help understanding this type

[Kim-Ee Yeoh]

Normally phantom types do add type safety.

But Ptr provides castPtr :: Ptr
<http://hackage.haskell.org/package/base-4.10.1.0/docs/Foreign-Ptr.html#t:Ptr>
a
-> Pt
<http://hackage.haskell.org/package/base-4.10.1.0/docs/Foreign-Ptr.html#t:Ptr>r
b, which throws type safety out the window.

Newtype is applied to ameliorate the situation, but I don't see how to make
type safety airtight, do you?



On Wednesday, November 7, 2018, Brandon Allbery <allbery.b at gmail.com> wrote:

> 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 at 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 at 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 at 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
>>>> <http://hackage.haskell.org/package/base-4.10.1.0/docs/Foreign-Ptr.html#t:Ptr>
>>>> XEvent
>>>> <http://hackage.haskell.org/package/X11-1.9/docs/Graphics-X11-Xlib-Event.html#t: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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>
>
>
> --
> brandon s allbery kf8nh
> allbery.b at gmail.com
>


-- 
-- Kim-Ee
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