Null Pointer Pattern

[David Feuer]

GHC.Ptr feels like too low-level a zone for it. If the objections I
predicted don't appear, Foreign.Ptr sure seems more logical.

On Mon, Aug 17, 2020, 11:07 AM Andrew Martin <andrew.thaddeus at gmail.com>
wrote:

> There is always the possibility of using GHC.Ptr as a home for it instead
> of Foreign.Ptr. The main reason I would want this in base rather than just
> some library is that, as a library author, I would never pick up an extra
> dependency for something that trivial.
>
> On Mon, Aug 17, 2020 at 10:52 AM David Feuer <david.feuer at gmail.com>
> wrote:
>
>> In the context of GHC Haskell, that's definitely the Right Thing. I think
>> what might concern some people is that pattern synonyms as we know them are
>> a very GHC thing, while the Ptr business is pretty much Report Haskell.
>>
>> On Mon, Aug 17, 2020, 10:44 AM Andrew Martin <andrew.thaddeus at gmail.com>
>> wrote:
>>
>>> Foreign.Ptr provides nullPtr. It would make some of my code more terse
>>> if this was additionally provided as a pattern synonym. The pattern synonym
>>> can be defined as:
>>>
>>>     {-# language ViewPatterns #-}
>>>     {-# language PatternSynonyms #-}
>>>     module NullPointerPattern
>>>       ( pattern Null
>>>       ) where
>>>     import Foreign.Ptr (Ptr,nullPtr)
>>>     pattern Null :: Ptr a
>>>     pattern Null <- ((\x -> x == nullPtr) -> True)
>>>       where Null = nullPtr
>>>
>>> Any here is example of code that becomes more terse once this is
>>> available:
>>>
>>>     foo :: IO (Either Error (Ptr Foo))
>>>     foo = do
>>>       p <- initialize mySettings
>>>       if p == nullPtr
>>>         then pure (Left InitializeFailure)
>>>         else pure (Right p)
>>>
>>> With the pattern synonym, we are able to take advantage of LambdaCase:
>>>
>>>     foo :: IO (Either Error (Ptr Foo))
>>>     foo = initialize mySettings >>= \case
>>>       Null -> pure (Left InitializeFailure)
>>>       p -> pure (Right p)
>>>
>>> I'm curious what others think.
>>>
>>> --
>>> -Andrew Thaddeus Martin
>>> _______________________________________________
>>> Libraries mailing list
>>> Libraries at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>>>
>>
>
> --
> -Andrew Thaddeus Martin
>
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