October 2015 - Libraries

Re: Monad of no `return` Proposal (MRP): Moving `return` out of `Monad`
by Thomas Miedema 04 Oct '15

04 Oct '15

> > > So there's not that much immediate breakage due to MRP specifically after > > all. > > Then count me again as +1, thanks for the clarification, Herbert. > +1 as well - The books are already outdated. Libraries can be updated. - This proposal simplifies the language and the next Haskell report.

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Re: Monad of no `return` Proposal (MRP): Moving `return` out of `Monad`
by Andrés Sicard-Ramírez 04 Oct '15

04 Oct '15

Hi, In Agda, we added support for GHC 7.10.* following the migration guide available at https://ghc.haskell.org/trac/ghc/wiki/Migration/7.10 In particular, we added instances of Applicative using instance Applicative Foo where pure = return (<*>) = ap -- defined in Control.Monad After reading the "Monad of no `return` proposal" below, I have two questions: 1. Shouldn't the above migration guide to be changed to something like instance Applicative Foo where pure = <copy-the-definition-of-return> ... instance Monad Foo where return = pure ? 2. Since in Agda code `return` is used everywhere, does follow (1) has some performance cost? All the best, On 24 September 2015 at 16:43, Herbert Valerio Riedel <hvr(a)gnu.org> wrote: > Hello *, > > Concluding AMP and MFP, We (David and I) proudly present you the final > installment of the Monad trilogy: > > > Monad of no `return` Proposal > ============================= > > TLDR: To complete the AMP, turn `Monad(return)` method into a > top-level binding aliasing `Applicative(pure)`. > > > Current Situation > ----------------- > > With the implementation of Functor-Applicative-Monad Proposal (AMP)[1] and > (at some point) the MonadFail proposal (MFP)[2] the AMP class hierarchy > becomes > > > class Functor f where > fmap :: (a -> b) -> f a -> f b > > > class Functor f => Applicative f where > pure :: a -> f a > (<*>) :: f (a -> b) -> f a -> f b > > (*>) :: f a -> f b -> f b > u *> v = … > > (<*) :: f a -> f b -> f a > u <* v = … > > > class Applicative m => Monad m where > (>>=) :: m a -> (a -> m b) -> m b > > return :: a -> m a > return = pure > > (>>) :: m a -> m b -> m b > m >> k = … > > > class Monad m => MonadFail m where > fail :: String -> m a > > > Consequently, the `Monad` class is left with a now redundant `return` > method as a historic artifact, as there's no compelling reason to > have `pure` and `return` implemented differently. > > Traditionally, `return` is often used where `pure` would suffice > today, forcing a `Monad` constraint even if a weaker `Applicative` > would have sufficed. > > As a result, language extensions like `ApplicativeDo`[3] have to > rewrite `return` to weaken its `Monad m =>` constraint to > `Applicative m =>` in order to benefit existing code at the cost > of introducing magic behavior at the type level. > > Finally, this redundancy becomes even more significant when viewed in > light of the renewed Haskell standardisation process[7]: The next > Haskell Report will almost certainly incorporate the AMP (and MFP) > changes, and there's no justification for the Report to retain > `return` as a method of `Monad`. A good reason would have been to > retain backward compatibility with Haskell 2010. However, as the AMP > superclass hierarchy requires `Monad` instances to be accompanied by > `Applicative` instances (which aren't part of Haskell 2010, c.f. [6]), > backward compatibility with Haskell 2010 goes out the window when it > comes to defining `Monad` instances (unless via use of `-XCPP` or > similar). Consequently, meeting the high bar for a formal document > such as the Haskell Report demands that `Monad` shall not carry a > redundant `return` method that serves no purpose anymore. Moreover, > getting `return` out of the way is desirable to facilitate > standardising potential candidates such as the earlier mentioned > `ApplicativeDo` in the future and avoids the technical debt incurred > by keeping around this language wart. > > > Proposed Change > --------------- > > Remove `return` as a method from the `Monad` class and in its place > define a top-level binding with the weaker `Applicative` typeclass > constraint: > > > -- | Legacy alias for 'pure' > return :: Applicative f => a -> f a > return = pure > > > This allows existing code using `return` to benefit from a weaker > typeclass constraint as well as cleaning the `Monad` class from a > redundant method in the post-AMP world. > > A possible migration strategy is described further below. > > > Compatibility Considerations > ---------------------------- > > Generalizing the type signature of a function from a `Monad` > constraint to its superclass `Applicative` doesn't cause new > type-errors in existing code. > > However, moving a method to a top-level binding obviously breaks code > that assumes `return` to be a class method. Foremost, code that > defines `Monad` instances it at risk: > > ### Instance Definitions > > Code defining `return` as part of an instance definition > breaks. However, we had the foresight to provide a default > implementation in `base-4.8` for `return` so that the following > represents a proper minimal instance definition post-AMP: > > > instance Functor Foo where > fmap g foo = … > > instance Applicative Foo where > pure x = … > a1 <*> a2 = … > > instance Monad Foo where > m >>= f = … > > -- NB: No mention of `return` > > > Consequently, it is possible to write forward-compatible instances > that are valid under this proposal starting with GHC 7.10/`base-4.8`. > > Heuristically `grep`ing through Hackage source-code reveals a > non-negligible number of packages defining `Monad` instances with > explicit `return` definitions[4]. This has a comparable impact to the > AMP, and similarly will require a transition scheme aided by compiler > warnings. > > ### Module Import/Export Specifications > > A second source of incompatibility may be due to > `import`s. Specifically module import that assert `return` to be a > method of `Monad`, e.g.: > > import Control.Monad (Monad ((>>=), return)) > > or > > import Prelude hiding (Monad(..)) > import Control.Monad (Monad(..)) as Monad > > f = Monad.return () > > The dual situation can occur when re-exporting `return` via module > export specifications. > > However, given that `return` is exported by `Prelude` and the examples > above are rather artificial, we don't expect this to be a major source > of breakage in the case of `return`. In fact, a heuristic grep[5] over > Hackage source-code revealed only 21 packages affected. > > ### Example for writing compatible code > > > instance Functor Foo where > fmap g foo = … > > instance Applicative Foo where > pure x = … > a1 <*> a2 = … > > instance Monad Foo where > m >>= f = … > > #if !(MIN_VERSION_base(4,8,0)) > return = pure > #endif > > > Migration Strategy > ------------------ > > The migration strategy is straightforward: > > **Phase 1** *(GHC 8.0)*: Implement new warning in GHC which gets > triggered when `Monad` instances explicitly override the > default `return` method implementation. > > **Phase 2** *(GHC 8.2 or later)*: When we're confident that the > majority of Hackage has reacted to the warning (with the help of > Stackage actively pursuing maintainers to update their packages) we > turn the `return` method into a top-level binding and remove the > warning implemented in Phase 1 from GHC again. > > > Discussion period > ----------------- > > A discussion period of three weeks (until 2015-10-15) should be enough > to allow everyone to chime in as well as leave enough time to make the > required preparations for GHC 8.0 should this proposal pass as we hope. > > ---- > > [1]: https://wiki.haskell.org/Functor-Applicative-Monad_Proposal > [2]: https://wiki.haskell.org/MonadFail_Proposal > [3]: https://ghc.haskell.org/trac/ghc/wiki/ApplicativeDo > [4]: https://gist.github.com/hvr/b0e34463d85b58f169d9 > [5]: https://gist.github.com/hvr/afcd040783d980594883 > [6]: https://ghc.haskell.org/trac/ghc/ticket/9590 > [7]: https://mail.haskell.org/pipermail/haskell-prime/2015-September/003936.html > > -- > > _______________________________________________ > Libraries mailing list > Libraries(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries > -- Andrés

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Can we simplify Dynamic?
by David Feuer 02 Oct '15

02 Oct '15

Currently, data Dynamic = Dynamic TypeRep Obj deriving Typeable where type Obj = Any As a result, all of the operations must be implemented "by hand" using unsafeCoerce. The more obvious representation these days would seem to be data Dynamic where Dynamic :: Typeable a => a -> Dynamic Most of the operations then become trivial applications of Typeable functions. The only exceptions seem to be dynApply and dynApp. That there are exceptions strikes me as quite unfortunate. The easiest fix is inspired by the fact that Data.Dynamic uses funResultTy :: TypeRep -> TypeRep -> Maybe TypeRep from Data.Typeable to decide whether to coerce. It seems reasonable to add a more informative version, something like applyTypeable :: (Typeable f, Typeable a) => proxy f -> proxy a -> (forall b . (Typeable b, f ~ (a -> b)) => r) -> Maybe r On the other hand, it would be really cool if there were some more general way to get type-level information out of Typeable instances, pattern matching on the type constructors.

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[unix] POSIX Message Queue support
by Elliot Robinson 01 Oct '15

01 Oct '15

We've had a request for info regarding message queue support in `unix` [0]. The related headers are currently listed in `System/Posix.hs` under the "Unsupported interfaces" category. Is the community aware of a reason that we have chosen not to support this interface going forward, or is this an artifact from when the library was originally ported (late 2002 by Simon Marlow) which we can now ignore? -- Elliot Robinson GPG Key: 9FEDE59A [0] https://github.com/haskell/unix/issues/47

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