Re: Null Pointer Pattern

There’s an interesting idea or two in here. Like, should we support having non-nullable pointers? On Mon, Aug 17, 2020 at 11:13 AM David Feuer wrote:

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 wrote:

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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 wrote:

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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 wrote:

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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@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

-- -Andrew Thaddeus Martin

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