diff --git a/CHANGELOG.md b/CHANGELOG.md new file mode 100644 --- /dev/null +++ b/CHANGELOG.md @@ -0,0 +1,3 @@ +## 0.1 + +- Initial revision. diff --git a/LICENSE b/LICENSE new file mode 100644 --- /dev/null +++ b/LICENSE @@ -0,0 +1,30 @@ +Copyright (c) 2015, Dimitri Sabadie + +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 the name of Dimitri Sabadie nor the names of other + contributors may be used to endorse or promote products derived + from this software without specific prior written permission. + +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. diff --git a/Setup.hs b/Setup.hs new file mode 100644 --- /dev/null +++ b/Setup.hs @@ -0,0 +1,2 @@ +import Distribution.Simple +main = defaultMain diff --git a/src/Data/Zero.hs b/src/Data/Zero.hs new file mode 100644 --- /dev/null +++ b/src/Data/Zero.hs @@ -0,0 +1,72 @@ +----------------------------------------------------------------------------- +-- | +-- Copyright : (C) 2015 Dimitri Sabadie +-- License : BSD3 +-- +-- Maintainer : Dimitri Sabadie +-- Stability : experimental +-- Portability : portable +----------------------------------------------------------------------------- + +module Data.Zero where + +import Control.Monad.Fix ( MonadFix ) +import Data.Semigroup ( Semigroup(..) ) +import GHC.Generics ( Generic ) + +-- |'Semigroup' with a 'zero' element. It’s important to understand that the +-- standard 'Semigroup' types – i.e. 'Maybe' and so on – are already biased, +-- because they’re 'Monoid's. That’s why you’ll find a few 'Zero' instances. +-- +-- Should satisfies the following laws: +-- +-- ==== Annhilation +-- +-- @ a '<>' 'zero' = 'zero' '<>' a = 'zero' @ +-- +-- ==== Associativity +-- +-- @ a '<>' b '<>' c = (a '<>' b) '<>' c = a '<>' (b '<>' c) @ +class (Semigroup a) => Zero a where + -- |The zero element. + zero :: a + -- |Concat all the elements according to ('<>') and 'zero'. + zconcat :: [a] -> a + + default zconcat :: (Semigroup a) => [a] -> a + zconcat [] = zero + zconcat (x:xs) = foldr (<>) x xs + {-# MINIMAL zero #-} + +instance Zero () where + zero = () + +-- |'Zero' under multiplication. +newtype Product a = Product { getProduct :: a } + deriving (Bounded,Eq,Generic,Num,Ord,Read,Show) + +instance (Num a) => Semigroup (Product a) where + Product a <> Product b = Product $ a * b + +instance (Num a) => Zero (Product a) where + zero = Product 0 + +-- |'Zero' under boolean logical and. +newtype All = All { getAll :: Bool} deriving (Bounded,Eq,Generic,Ord,Read,Show) + +instance Semigroup All where + All a <> All b = All $ a && b + +instance Zero All where + zero = All False + +-- |'Zero' for 'Maybe'. +newtype Success a = Success { getSuccess :: Maybe a } + deriving (Applicative,Eq,Foldable,Functor,Monad,MonadFix,Ord,Traversable,Read,Show) + +instance (Semigroup a) => Semigroup (Success a) where + Success (Just a) <> Success (Just b) = Success . Just $ a <> b + _ <> _ = zero + +instance (Semigroup a) => Zero (Success a) where + zero = Success Nothing diff --git a/zero.cabal b/zero.cabal new file mode 100644 --- /dev/null +++ b/zero.cabal @@ -0,0 +1,46 @@ +name: zero +version: 0.1 +synopsis: Semigroups with absorption +description: 'Monoid' is a 'Semigroup' glued with a neutral element + called 'mempty'. In the same idea, 'Zero' is a 'Semigroup' + glued with an absorbing element called 'zero'. + . + Keep in mind that 'Zero' requires 'Semigroup'. If you have + 'Semigroup' defined to work with 'Monoid', you might end up + with no way to implement 'Zero'. That’s why the 'Semigroup' + instance for 'Maybe' is confusing, because it relies on + 'Monoid', and cannot be used with 'Zero'. 'Success' is the + 'Zero' equivalent of 'Maybe' + 'Monoid'. +license: BSD3 +license-file: LICENSE +author: Dimitri Sabadie +maintainer: Dimitri Sabadie +copyright: Dimitri Sabadie +category: Data +extra-source-files: CHANGELOG.md +homepage: https://github.com/phaazon/zero +bug-reports: https://github.com/phaazon/zero/issues + +build-type: Simple +cabal-version: >= 1.10 + +source-repository head + type: git + location: https://github.com/phaazon/zero.git + +library + ghc-options: -W -Wall + + exposed-modules: Data.Zero + + default-extensions: DefaultSignatures + , DeriveGeneric + , DeriveTraversable + , GeneralizedNewtypeDeriving + + build-depends: base >= 4.8 && < 4.9 + , semigroups >= 0.16 && < 0.17 + + hs-source-dirs: src + + default-language: Haskell2010