February 2016 - Libraries

Proposal: merge either into transformers
by Roman Cheplyaka 03 Jan '17

03 Jan '17

I propose to add the sole module of the 'either' package[1], Control.Monad.Trans.Either, to the transformers package. It provides EitherT, a very basic and fundamental data type. The difference between EitherT and ErrorT is that the latter has an Error constraint, which is used to imlement 'fail'. Note that 'either' depends on the 'semigroupoids' and 'semigroup' packages to provide appropriate instances. The proposal is not to add those instances to 'transformers' to avoid additional dependencies. The instances can then be left in the 'either' package or moved to the 'semigroupoids' and 'semigroup' packages respectively. ('semigroupoids' already depends on 'transformers', while 'semigroups' does not). Compared to the 'either' package, Show, Read, Eq and Ord instances will be dropped to keep the code Haskell2010 (those instances require FlexibleInstances, FlexibleContexts, and UndecidableInstances). The patch is attached. [*] [*] against transformers-0.3.0.0, because the darcs version is not buildable (Control/Monad/Signatures.hs is not in the repository). Roman

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Proposal: add liftA4 and liftA5 to match liftM4 and liftM5
by David Feuer 14 Jul '16

14 Jul '16

Since Applicative is supposed to be important now, I figure we should get these in.

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Proposal: add chunksOf to Data.Sequence
by David Feuer 06 May '16

06 May '16

Data.List.Split.chunksOf n xs breaks up a list xs into chunks of n elements each. The internal "splitMap" function Data.Sequence uses to implement zipWith supports a simple and efficient implementation of chunksOf. All in favor, say "aye". Also, I'm still waiting for a response (any response) to my "cycleN" proposal. David

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Proposal: Export cycleN from Data.Sequence
by David Feuer 26 Apr '16

26 Apr '16

Yesterday I rewrote `*>` for Data.Sequence (again), using an internal function cycleN :: Int -> Seq a -> Seq a The name of the function is based on that of Data.Sequence.iterateN. cycleN takes a sequence and cycles it as many times as requested: cycleN 0 $ fromList [1,2] = [] cycleN 5 $ fromList [1,2] = [1,2,1,2,1,2,1,2,1,2] The function is written to maximize sharing in the result sequence and to minimize construction time. Specifically, cycleN n xs should take something like O(|xs| + log n) space (of which all but O(log |xs| + log n) is shared with the structure of xs) and O(log |xs| + log n) time. With current (on GitHub) Data.Sequence exports, the only way to get this functionality with these time and space bounds is to combine replicate with *> : cycleN n xs = replicate n () *> xs This strikes me as a bit unpleasant. David

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prelude-compat, prelude2010
by Henning Thielemann 16 Apr '16

16 Apr '16

Here is my attempt to make packages compile with GHC-7.10 and older compilers without warnings: https://hackage.haskell.org/package/prelude-compat-0.0/candidate https://hackage.haskell.org/package/prelude2010-0.0/candidate The idea is that prelude2010:Prelude maintains the export list of haskell2010:Prelude such that you are saved both from identifier conflicts and warnings about redundant imports of Data.Foldable etc. Unfortunately, when I try to use it on GHC-7.8.4 together with base-noprelude, I am told that base-noprelude cannot be installed with Cabal-1.22. :-( However, when thinking about adapting all my packages to this work-around I'd prefer to switch to the real solution, namely split base packages that can be upgraded independently from GHC.

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Proposal: Make Semigroup as a superclass of Monoid
by Jeremy 11 Apr '16

11 Apr '16

The proposal to make Semigroup a superclass of Monoid was discussed a while ago [1], and the conclusion was to "put this off until the dust has settled from the AMP and FT changes". Now that 7.10 is out, I would like to re-propose. The proposed plan is similar to AMP, but less invasive, as (in my subjective experience) user-defined Monoids are much less common than user-defined Monads. 1. GHC 7.12 will include Semigroup and NonEmpty in base. All Monoid instances, and anything else which forms a Semigroup, will have a Semigroup instance. GHC will issue a warning when it encounters an instance of Monoid which is not an instance of Semigroup. 2. GHC >7.12 will define Monoid as a subclass of Semigroup. Stage 2 could be delayed - or in the extreme case, cancelled - if the warnings following stage 1 indicate that the proposal would cause significant breakage of existing code, although this is not anticipated. The rationale for this change is: 1. Semigroup is a popular package (this is relevant in combination with the following point). 2. Using an existing Monoid as a Semigroup requires redefining it (or WrappedMonoid), leading to much boilerplate and duplication. 3. NonEmpty is often redefined by beginners (or those who don't think it's worth having an extra dependency for). [1] https://mail.haskell.org/pipermail/libraries/2013-June/020188.html -- View this message in context: http://haskell.1045720.n5.nabble.com/Proposal-Make-Semigroup-as-a-superclas… Sent from the Haskell - Libraries mailing list archive at Nabble.com.

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Re: Proposal: Add conspicuously missing Functor instances for tuples
by Ryan Scott 22 Mar '16

22 Mar '16

Strong +1 from me. We derive Eq/Ord/Read/Show/etc. for all tuple types of size up to 15 as it is, we might as well do the same for Functor. (Personally, I'd even argue for adding Foldable and Traversable instances as well, but the Foldable instance for pairs has proven to be pretty controversial...) Ryan S.

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Cabal on Mac OSX EL Capitan - forwarding DYLD_LIBRARY_PATH
by tamarind code 21 Mar '16

21 Mar '16

Hi, There was a discussion in github about stack (see link below) where the conclusion seems to be pointing towards a bug in Cabal on Mac OSX EL Capitan related to forwarding DYLD_LIBRARY_PATH https://github.com/commercialhaskell/stack/issues/1161#issuecomment-1584739… At the moment the suggested workaround is to disable System Integrity Protection which sounds a bit scary to me. So I thought this might be a good idea to reported to someone in a Cabal team. At the moment I am using an Ubuntu Virtual Machine to avoid having to mess with SIP on my mac. Hope some clever/kind person from the Haskell community would be able to fix this soon. Thanks Sam

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Re: Libraries Digest, Vol 150, Issue 32
by Michał J Gajda 27 Feb '16

27 Feb '16

-1(*inf) Downsides: 1. New way to make a Haskell expression that makes no semantic sense `length (1, 2)`, and it still evaluates. 2. New trick interview question that makes interviewers hated for being impractical. (Just like `join (*)`.) 3. Confuse hell out of beginners and practitioners alike. Upside: 1. One Canonical(tm) Functor instance for tuple, and pope's own claim that the other instance does not fit in our library. 2. More standard types that are Functor instances. 3. Easier to demonstrate that one can DO EVIL in Haskell too. -- Cheers Michal

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GHC documentation of modules exposed by the base package
by Andrés Sicard-Ramírez 26 Feb '16

26 Feb '16

Dear all, By running in GHC 7.10.3 $ ghc-pkg field base-4.8.2.0 exposed-modules I can see various modules exposed by base-4.8.2.0 which are not documented (e.g. GHC.NUm) in http://downloads.haskell.org/~ghc/7.10.3/docs/html/libraries/index.html Which is the criterion for adding GHC documentation to the modules exposed by base? All the best, -- Andrés

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