Init

.travis.yml

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+env:
+  - ghc=7.6.1  cabal=1.18
+  - ghc=7.8.1  cabal=1.18
+  - ghc=7.10.1 cabal=1.22 benchmarks=1 tests=1
+
+install:
+  # Set up the Shell to treat the semicolon as &&
+  - set -o pipefail && set -e
+  # Install GHC and Cabal
+  -
+    ghc=${ghc=7.10.1}; 
+    cabal=${cabal=1.22};
+    travis_retry sudo add-apt-repository -y ppa:hvr/ghc;
+    travis_retry sudo apt-get update;
+    travis_retry sudo apt-get install cabal-install-$cabal ghc-$ghc;
+    export PATH=/opt/ghc/$ghc/bin:/opt/cabal/$cabal/bin:$PATH;
+  # Update the Cabal database
+  - cabal update
+  # Switch to the distro:
+  - 
+    export pkg_name=$(cabal info . | awk '{print $2;exit}');
+    cabal sdist;
+    cd dist;
+    tar xzvf $pkg_name.tar.gz;
+    cd $pkg_name;
+  # Install the lower bound dependencies
+  - 
+    if [ "$lower_bound_dependencies" = "1" ];
+    then
+      constraint_options=(
+      );
+    fi;
+  # Install the library dependencies
+  - cabal install --only-dependencies --reorder-goals --force-reinstalls
+      ${constraint_options[@]}
+      $([ "$tests" = "1" ] && echo "--enable-tests")
+      $([ "$benchmarks" = "1" ] && echo "--enable-benchmarks")
+  # Build the library
+  - cabal build
+  # Configure and build the remaining stuff
+  - cabal configure 
+      $([ "$tests" = "1" ] && echo "--enable-tests")
+      $([ "$benchmarks" = "1" ] && echo "--enable-benchmarks")
+      -f doctest
+  - cabal build
+
+script:
+  - |
+    if [ "$tests" = "1" ];
+    then
+      cabal clean;
+      cabal test --show-details=always;
+    fi;

LICENSE

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+Copyright (c) 2016, Nikita Volkov
+
+Permission is hereby granted, free of charge, to any person
+obtaining a copy of this software and associated documentation
+files (the "Software"), to deal in the Software without
+restriction, including without limitation the rights to use,
+copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the
+Software is furnished to do so, subject to the following
+conditions:
+
+The above copyright notice and this permission notice shall be
+included in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
+OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+OTHER DEALINGS IN THE SOFTWARE.

deque.cabal

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+name:
+  deque
+version:
+  0.1
+synopsis:
+
+homepage:
+  https://github.com/nikita-volkov/deque 
+bug-reports:
+  https://github.com/nikita-volkov/deque/issues 
+author:
+  Nikita Volkov <nikita.y.volkov@mail.ru>
+maintainer:
+  Nikita Volkov <nikita.y.volkov@mail.ru>
+copyright:
+  (c) 2016, Nikita Volkov
+license:
+  MIT
+license-file:
+  LICENSE
+build-type:
+  Simple
+cabal-version:
+  >=1.10
+
+
+source-repository head
+  type:
+    git
+  location:
+    git://github.com/nikita-volkov/deque.git
+
+
+library
+  hs-source-dirs:
+    library
+  default-extensions:
+    Arrows, BangPatterns, ConstraintKinds, DataKinds, DefaultSignatures, DeriveDataTypeable, DeriveFunctor, DeriveGeneric, EmptyDataDecls, FlexibleContexts, FlexibleInstances, FunctionalDependencies, GADTs, GeneralizedNewtypeDeriving, ImpredicativeTypes, LambdaCase, LiberalTypeSynonyms, MagicHash, MultiParamTypeClasses, MultiWayIf, NoImplicitPrelude, NoMonomorphismRestriction, OverloadedStrings, PatternGuards, ParallelListComp, PolyKinds, QuasiQuotes, RankNTypes, RecordWildCards, ScopedTypeVariables, StandaloneDeriving, TemplateHaskell, TupleSections, TypeFamilies, TypeOperators, UnboxedTuples
+  default-language:
+    Haskell2010
+  other-modules:
+
+  exposed-modules:
+    Deque
+  build-depends:
+    base-prelude < 2

library/Deque.hs

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+module Deque where
+
+import BasePrelude hiding (uncons, unsnoc, cons, snoc)
+
+
+data Deque a =
+  Deque ![a] ![a]
+
+shiftRight :: Deque a -> Deque a
+shiftRight (Deque snocList consList) =
+  case consList of
+    head : tail ->
+      Deque (head : snocList) tail
+    _ ->
+      case reverse snocList of
+        head : tail ->
+          Deque (head : []) tail
+        _ ->
+          Deque snocList consList
+
+shiftLeft :: Deque a -> Deque a
+shiftLeft deque =
+  fromMaybe deque $
+  fmap (\(a, b) -> cons a b) $
+  unsnoc deque
+
+cons :: a -> Deque a -> Deque a
+cons a (Deque snocList consList) =
+  Deque (snocList) (a : consList)
+
+snoc :: a -> Deque a -> Deque a
+snoc a (Deque snocList consList) =
+  Deque (a : snocList) (consList)
+
+uncons :: Deque a -> Maybe (a, Deque a)
+uncons deque =
+  unconsWithoutBalancing deque <|>
+  unconsWithoutBalancing (shiftRight deque)
+
+unconsWithoutBalancing :: Deque a -> Maybe (a, Deque a)
+unconsWithoutBalancing (Deque snocList consList) =
+  case consList of
+    head : tail ->
+      Just (head, Deque snocList tail)
+    _ ->
+      Nothing
+
+unsnoc :: Deque a -> Maybe (a, Deque a)
+unsnoc (Deque snocList consList) =
+  case snocList of
+    head : tail ->
+      Just (head, Deque tail consList)
+    _ ->
+      case reverse consList of
+        head : tail ->
+          Just (head, Deque tail [])
+        _ ->
+          Nothing