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November 2010

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Haskell Platform Proposal: add transformers and revise the mtl package to depend on it
by Ross Paterson 03 Nov '10

03 Nov '10

The proposal is that version 2 of the mtl package be the current contents of the monads-fd package, which depends on the transformers package: http://hackage.haskell.org/package/monads-fd http://hackage.haskell.org/package/transformers An up to date copy of this text is kept at: http://trac.haskell.org/haskell-platform/wiki/Proposals/transformers Everyone is invited to review this proposal, following the standard procedure for proposing and reviewing packages. http://trac.haskell.org/haskell-platform/wiki/AddingPackages Review comments should be sent to the libraries mailing list by October 15 so that we have time to discuss and resolve issues before the final deadline on November 1. http://trac.haskell.org/haskell-platform/wiki/ReleaseTimetable == Proposer == * Ross Paterson (maintainer of transformers and monads-fd) If this proposal is accepted, monads-fd would be obsoleted and transformers would be turned over to community control (like the current mtl package). == Rationale == Monad transformers are widely used, but the MTL interface is tied to functional dependencies, whose future is in doubt. Many people want to try out interfaces based on type functions, or without using experimental type class features, but their interfaces will then be incompatible with libraries using mtl. The idea is to factor out the monad transformers as the Haskell 98 package. The objectives for this package are: 1. that it be as useful as possible by itself, so that it can replace many uses of mtl. 2. that it serve as a base for other packages adding type classes based on either functional dependencies (e.g. mtl) or type functions, with interfaces referring to the monad transformers being compatible across the different libraries. The transformers and monads-fd packages have been in use for over 18 months, and are used by 112 and 23 packages respectively. The split structure has been shown to be viable, but its existence in parallel with mtl has meant extra effort for authors of client packages. It is time to complete the transition. == Structure == The current mtl is to be split in two: * transformers is a Haskell 98 package containing * base functors (Data.Functor.Constant and Data.Functor.Identity), * operations on functors (Data.Functor.Compose and Data.Functor.Product), * transformer classes (Control.Monad.Trans.Class and Control.Monad.IO.Class) and * concrete monad transformers with code to lift operators (Control.Monad.Trans.*). The package can be used on its own (see the Control.Monad.Trans.Class documentation for examples), or with packages adding type classes. * mtl-2 (the current monads-fd) depends on transformers and adds type classes using functional dependencies. It has the same modules as mtl-1 and usage is very close, except for the differences listed below. == Incompatibilities == The proposed interface of mtl-2 is close to that of mtl-1, but with the following differences (illustrated with Reader): * instances of Applicative and Alternative have been added as appropriate, e.g. instance (Applicative m) => Applicative (ReaderT r m) where ... instance (Alternative m) => Alternative (ReaderT r m) where ... Rationale: These classes postdate the MTL, and such instances have been repeatedly defined in various packages. They belong together with the type constructors. * Functor instances for monad transformers no longer require Monad where Functor is sufficient. Unfortunately this is incompatible because Functor is not a superclass of Monad, e.g. instance (Monad m) => Functor (ReaderT r m) where ... is replaced by instance (Functor m) => Functor (ReaderT r m) where ... Rationale: These instances are more general, and are consistent with the instances of other classes. * simple monads are now aliases for monad trasformers applied to Identity, e.g. newtype Reader r a = Reader { runReader :: r -> a } is replaced by type Reader r = ReaderT r Identity reader :: (r -> a) -> Reader r a reader f = ReaderT (Identity . f) runReader :: Reader r a -> r -> a runReader m = runIdentity . runReaderT m Rationale: This avoids repetition in the interfaces of both transformers and the proposed mtl-2. It makes transformers more useful on its own, and also saves clients of mtl from defining instances for both Reader r and ReaderT r and ensuring that they are consistent. * The instance Error String is restructured to avoid a type synonym instance: instance Error String where noMsg = "" strMsg = id is replaced by instance ErrorList a => Error [a] where strMsg = listMsg class ErrorList a where listMsg :: String -> [a] instance ErrorList Char where listMsg = id Rationale: This makes the instance Haskell 98, so it can be included in the transformers package. == Transition issues == In early September 2010, of the 510 buildable packages in hackage that directly depended on mtl, * 312 built unchanged with the proposed mtl-2 * 102 had a bounded mtl dependency (or a dependent package had) that excluded mtl-2 * 2 failed because of the recent move of the Monad (Either e) instance from mtl to base * 41 failed with the new mtl: * 11 because they defined their own Applicative instances (which can now be deleted) * 11 because of the changed constraint on Functor instances * 15 that used the constructors of base monads (which can be trivially replaced) * 3 that defined instances for base monads * 1 that defined an overlapping Error instance * 53 failed because they depended on one of the other failures The attached test report lists the packages involved. == Other issues == Some other issues have been raised with mtl over the years, but they are orthogonal to this proposal: * The MonadCont instance for StateT is not compatible with the monad transformer. The transformers package provides the correct lifting (in which callcc causes the state to rollback on entering the saved continuation), but also provides the MTL lifting for compatibility, and this is used by monads-fd. It could be switched to the correct lifting later. * The ErrorT monad transformer has an Error constraint, so that errors can be passed to the fail method of the Monad class. * ListT only works on commutative monads. Attachments * mtl-2.0.0.0.tar.gz (13.4 kB) -"proposed mtl-2.0.0.0.tar.gz", added by ross on 09/14/10 19:01:21. * test-Sep-2010 (21.3 kB) -"test of mtl-using packages in Sep 2010", added by ross on 09/15/10 20:05:56.

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Merging network and network-bytestring: license question
by Johan Tibell 01 Nov '10

01 Nov '10

Hi, While looking into merging the network and network-bytestring packages, I noticed that their licenses differ slightly. I've attached both licenses in the end of this email for reference. How do I merge the two licenses? I own the copyright on network-bytestring, so I could change that license to be the same as the one used for network. Is this the right way to go? Is it possible to move the whole resulting library to the two-clause BSD license, which is more common on Hackage? Would that mean getting approval from everyone who has ever contributed to the code base? Johan ------------------------------ network license ============== The Glasgow Haskell Compiler License Copyright 2002, The University Court of the University of Glasgow. All rights reserved. Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met: - Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer. - Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution. - Neither name of the University nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission. THIS SOFTWARE IS PROVIDED BY THE UNIVERSITY COURT OF THE UNIVERSITY OF GLASGOW AND THE CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE UNIVERSITY COURT OF THE UNIVERSITY OF GLASGOW OR THE CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. network-bytestring license ========================= Copyright Johan Tibell 2007 All rights reserved. Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met: * Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer. * Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution. THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

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