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@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@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@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

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--

-Andrew Thaddeus Martin

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