Empty (empty) → 0.1.0.0

raw patch · 5 files changed

+150/−0 lines, 5 filesdep +basedep +containerssetup-changed

Dependencies added: base, containers

Files

• ChangeLog.md +5/−0
• Empty.cabal +25/−0
• LICENSE +30/−0
• Setup.hs +2/−0
• src/Control/Empty.hs +88/−0

@@ -0,0 +1,5 @@+# Revision history for Empty++## 0.1.0.0  -- YYYY-mm-dd++* First version. Released on an unsuspecting world.

@@ -0,0 +1,25 @@+-- Initial Empty.cabal generated by cabal init.  For further documentation,+--  see http://haskell.org/cabal/users-guide/++name:                Empty+version:             0.1.0.0+synopsis:            A type class for empty containers+description:         HasEmpty f has a member empty of type forall a. f a+license:             BSD3+license-file:        LICENSE+author:              Isaac Shapira+maintainer:          fresheyeball@gmail.com+-- copyright:+category:            Control+build-type:          Simple+extra-source-files:  ChangeLog.md+cabal-version:       >=1.10++library+  exposed-modules:     Control.Empty+  -- other-modules:+  -- other-extensions:+  build-depends:       base >=4.9 && <4.10+                     , containers+  hs-source-dirs:      src+  default-language:    Haskell2010

@@ -0,0 +1,30 @@+Copyright (c) 2017, Isaac Shapira++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 Isaac Shapira 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.

@@ -0,0 +1,2 @@+import Distribution.Simple+main = defaultMain

@@ -0,0 +1,88 @@+{-# LANGUAGE DefaultSignatures #-}+{-# LANGUAGE FlexibleInstances #-}+{-# LANGUAGE RankNTypes        #-}+{-# LANGUAGE TypeOperators     #-}++module Control.Empty where++import qualified Control.Applicative             as AP+import           Data.Functor.Compose+import qualified Data.Functor.Product            as FP+import qualified Data.IntMap                     as IntMap+import qualified Data.Map                        as Map+import           Data.Proxy+import           Data.Semigroup+import qualified Data.Sequence                   as Seq+import qualified Data.Set                        as Set+import           GHC.Conc+import           Text.ParserCombinators.ReadP+import           Text.ParserCombinators.ReadPrec++{-| The class of type of @* -> *@ which may be empty.+    There is only one law for HasEmpty and its enforced by the type.+    The law is that there exists a value in type f for which a is fully generic.+    This alone is often enough to uniquely determine the value for a given,+    data type.++    Additional laws for HasEmpty work backwards, @empty@ has a relationships+    with other classes. So if @f@ is a member of one of these classes,+    the following should hold.++    [/Functor identity/]++        @g \<$\> empty = empty@++    [/Applicative annililation left/]++        @x \<*\> empty = empty@++    [/Applicative annililation right/]++        @empty \<*\> x = empty@++    [/Monad identity/]++        @empty >>= f = empty@++    [/Alternative empty/]++        @empty = Alternative.empty@++    [/MonadPlus mzero/]++        @empty = mzero@++    [/Foldable identity/]++        @foldr f x empty = empty@++-}+class HasEmpty f where+    empty :: forall a. f a+    default empty :: AP.Alternative f => f a+    empty = AP.empty++instance HasEmpty Maybe+instance HasEmpty []+instance HasEmpty IO+instance HasEmpty STM+instance HasEmpty Proxy+instance HasEmpty Option+instance HasEmpty ReadP+instance HasEmpty ReadPrec++instance (AP.Alternative f, AP.Alternative g) => HasEmpty (FP.Product f g)+instance (AP.Alternative f, AP.Applicative g) => HasEmpty (Compose f g)++instance HasEmpty IntMap.IntMap where empty = mempty+instance HasEmpty (Map.Map k)   where empty = Map.empty+instance HasEmpty Seq.Seq       where empty = Seq.empty+instance HasEmpty Set.Set       where empty = Set.empty+++coerce :: HasEmpty f => a -> f b+coerce _ = empty+++isEmpty :: (HasEmpty f, Eq (f a)) => f a -> Bool+isEmpty = (== empty)