Newtype wrappers
Edward Kmett
ekmett at gmail.com
Mon Jan 14 23:40:31 CET 2013
It sounds like the solution you are proposing then is to an issue largely
orthogonal to the one I'm talking about.
As far as I can tell, I derive no immediate benefit from this version.
-Edward
On Mon, Jan 14, 2013 at 4:09 PM, Simon Peyton-Jones
<simonpj at microsoft.com>wrote:
> If you are worrying about #1496, don’t worry; we must fix that, and the
> fix will apply to newtype wrappers.****
>
>
> If you are worrying about something else, can you articulate what the
> something else is?****
>
> ** **
>
> I don’t want to involve type classes, nor Functor. We don’t even have a
> good way to say “is a functor of its second type argument” for a type
> constructor of three arguments.****
>
> ** **
>
> Simon****
>
> ** **
>
> ** **
>
> ** **
>
> *From:* Edward Kmett [mailto:ekmett at gmail.com]
> *Sent:* 14 January 2013 18:39
> *To:* Simon Peyton-Jones
> *Cc:* GHC users
> *Subject:* Re: Newtype wrappers****
>
> ** **
>
> Many of us definitely care. =)****
>
> ** **
>
> The main concern that I would have is that the existing solutions to this
> problem could be implemented while retaining SafeHaskell, and I don't see
> how a library that uses this can ever recover its SafeHaskell guarantee.**
> **
>
> ** **
>
> Here is a straw man example of a solution that permits SafeHaskell in the
> resulting code that may be useful in addition to or in lieu of your
> proposed approach:****
>
> ** **
>
> We could extend Data.Functor with an fmap# operation that was only, say,
> exposed via Data.Functor.Unsafe:****
>
> ** **
>
> {-# LANGUAGE Unsafe, MagicHash #-}****
>
> module Data.Functor.Unsafe where****
>
> class Functor f where****
>
> fmap# :: (a -> b) -> f a -> f b****
>
> fmap :: (a -> b) -> f a -> f b****
>
> (<$) :: b -> f a -> f b****
>
> fmap# = \f -> \fa -> fa `seq` fmap f p****
>
> ** **
>
> Then we flag Data.Functor as Trustworthy and export just the safe subset:*
> ***
>
> ** **
>
> {-# LANGUAGE Trustworthy #-}****
>
> module Data.Functor (Functor(fmap,(<$))) where****
>
> import Data.Functor.Unsafe****
>
> ** **
>
> then fmap# from Data.Functor.Unsafe is allowed to be fmap# _ =
> unsafeCoerce for any Functor that doesn't perform GADT-like interrogation
> of its argument (this could be assumed automatically in DeriveFunctor,
> which can't handle those cases anyways!)****
>
> ** **
>
> Then any user who wants to enable a more efficient fmap for fmapping over
> his data type with a newtype instantiates fmap# for his Functor. They'd
> have to claim Trustworthy (or use the enhanced DeriveFunctor), to discharge
> the obligation that they aren't introducing an unsafeCoerce that is visible
> to the user. (After all the user has to import another Unsafe module to get
> access to fmap# to invoke it.)****
>
> ** **
>
> Finally then code that is willing to trust other trustworthy code can
> claim to be Trustworthy in turn, import Data.Functor.Unsafe and use fmap#
> for newtypes and impossible arguments:****
>
> ** **
>
> {-# LANGUAGE Trustworthy #-}****
>
> module Data.Void where****
>
> ** **
>
> import Data.Functor.Unsafe****
>
> ** **
>
> newtype Void = Void Void deriving Functor****
>
> ** **
>
> absurd :: Void -> a****
>
> absurd (Void a) = absurd a****
>
> ** **
>
> vacuous :: Functor f => f Void -> f a****
>
> vacuous = fmap# absurd****
>
> ** **
>
> This becomes valuable when data types like Void are used to mark the
> absence of variables in a syntax tree, which could be quite large.****
>
> ** **
>
> Currently we have to fmap absurd over the tree, paying an asymptotic cost
> for not using (forall a. Expr a) or some newtype wrapped equivalent as our
> empty-expression type.****
>
> ** **
>
> This would dramatically improve the performance of libraries like bound
> which commonly use constructions like Expr Void.****
>
> ** **
>
> Its safety could be built upon by making another class for tracking
> newtypes etc so we can know whats safe to pass to fmap#, and you might be
> able to spot opportunities to rewrite an explicit fmap of something that is
> a `cast` in the core to a call to fmap#.****
>
> ** **
>
> -Edward****
>
> ** **
>
> On Mon, Jan 14, 2013 at 1:09 PM, Simon Peyton-Jones <simonpj at microsoft.com>
> wrote:****
>
> Friends****
>
> ****
>
> I’d like to propose a way to “promote” newtypes over their enclosing
> type. Here’s the writeup****
>
> http://hackage.haskell.org/trac/ghc/wiki/NewtypeWrappers****
>
> ****
>
> Any comments? Below is the problem statement, taken from the above page.*
> ***
>
> ****
>
> I’d appreciate****
>
> · A sense of whether you care. Does this matter?****
>
> · Improvements to the design I propose****
>
> ****
>
> Simon****
>
> ****
>
> ****
>
> ****
>
> *The problem*****
>
> Suppose we have ****
>
> newtype Age = MkAge Int****
>
> Then if n :: Int, we can convert n to an Age thus: MkAge n :: Age.
> Moreover, this conversion is a type conversion only, and involves no
> runtime instructions whatsoever. This cost model -- that newtypes are free
> -- is important to Haskell programmers, and encourages them to use newtypes
> freely to express type distinctions without introducing runtime overhead.
> ****
>
> Alas, the newtype cost model breaks down when we involve other data
> structures. Suppose we have these declarations ****
>
> data T a = TLeaf a | TNode (Tree a) (Tree a)****
>
> data S m a = SLeaf (m a) | SNode (S m a) (S m a)****
>
> and we have these variables in scope ****
>
> x1 :: [Int]****
>
> x2 :: Char -> Int****
>
> x3 :: T Int****
>
> x4 :: S IO Int****
>
> Can we convert these into the corresponding forms where the Int is
> replaced by Age? Alas, not easily, and certainly not without overhead. ***
> *
>
> - For x1 we can write map MkAge x1 :: [Age]. But this does not follow
> the newtype cost model: there will be runtime overhead from executing the
> map at runtime, and sharing will be lost too. Could GHC optimise the
> map somehow? This is hard; apart from anything else, how would GHC
> know that map was special? And it it gets worse. ****
>
>
> - For x2 we'd have to eta-expand: (\y -> MkAge (x2 y)) :: Char -> Age.
> But this isn't good either, because eta exapansion isn't semantically valid
> (if x2 was bottom, seq could distinguish the two). See #7542<http://hackage.haskell.org/trac/ghc/ticket/7542>for a real life example.
> ****
>
>
> - For x3, we'd have to map over T, thus mapT MkAge x3. But what if mapTdidn't exist? We'd have to make it. And not all data types have maps.
> S is a harder one: you could only map over S-values if m was a
> functor. There's a lot of discussion abou this on #2110<http://hackage.haskell.org/trac/ghc/ticket/2110>.
> ****
>
> ****
>
>
> _______________________________________________
> Glasgow-haskell-users mailing list
> Glasgow-haskell-users at haskell.org
> http://www.haskell.org/mailman/listinfo/glasgow-haskell-users****
>
> ** **
>
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