January 2017 - Libraries

MonadZip for Data.Tree
by David Feuer 08 Jan '17

08 Jan '17

MonadZip doesn't really explain how strict mzipWith and (especially) munzip should be. For example, we could have munzip (Node (a, b) ts) = (Node a as, Node b bs) where (as, bs) = Data.List.unzip (map munzip ts) or we could make some or all of the pattern matches lazy, or we could use something lazier than Data.List.unzip, or we could make everything completely spine-strict (surely unwise). Does anyone have a particular preference, or a particular reason to prefer one choice over others? If not, I think we should go with the simple version above. David Feuer

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Proposal: Add NonEmpty constructors to the Prelude
by Yitzchak Gale 05 Jan '17

05 Jan '17

In continuation of Simon Jakobi's proposal for a small incremental change to the Prelude to make it more conducive to partial-function-free programming, https://mail.haskell.org/pipermail/libraries/2016-December/027496.html and after some encouragement from the community on reddit, I propose adding the basic constructors of NonEmpty to the Prelude: nonEmpty, (:|), and the type constructor NonEmpty. In practice, users of NonEmpty will often still need a qualified import of Data.List.NonEmpty. But having the constructors available in the Prelude will make the use of NonEmpty more common, and will make programming without partial functions feel more natural and standard. Note: This proposal is separate from Simon's and should not interfere with the acceptance of Simon's. But it is related in that it attempts to achieve the same goals. Discussion period: Until the end of the discussion period of Simon's proposal. Thanks, Yitz

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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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