initial commit

LICENSE

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+Copyright (c) 2015, Andrzej Rybczak
+
+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 Andrzej Rybczak 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.

Setup.hs

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+import Distribution.Simple
+main = defaultMain

monad-time.cabal

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+name:                monad-time
+version:             0.1
+synopsis:            Type class for monads which carry the notion of the current time.
+homepage:            https://github.com/scrive/monad-time
+license:             BSD3
+license-file:        LICENSE
+author:              Andrzej Rybczak
+maintainer:          [email protected]
+category:            Control
+build-type:          Simple
+cabal-version:       >=1.10
+
+library
+  exposed-modules:   Control.Monad.Time,
+                     Control.Monad.Time.Instances
+
+  build-depends:     base < 5,
+                     mtl,
+                     time
+
+  hs-source-dirs:    src
+  default-language:  Haskell2010

src/Control/Monad/Time.hs

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+module Control.Monad.Time (MonadTime(..)) where
+
+import Control.Monad.Trans
+import Data.Time
+
+-- | Class of monads which carry the notion of the current time.
+class Monad m => MonadTime m where
+  currentTime :: m UTCTime
+
+instance MonadTime IO where
+  currentTime = getCurrentTime

src/Control/Monad/Time/Instances.hs

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+{-# LANGUAGE FlexibleContexts, FlexibleInstances, OverlappingInstances
+  , UndecidableInstances #-}
+module Control.Monad.Time.Instances () where
+
+import Control.Monad.Trans
+
+import Control.Monad.Time
+
+-- | Generic, overlapping instance.
+instance (
+    MonadTime m
+  , MonadTrans t
+  , Monad (t m)
+  ) => MonadTime (t m) where
+    currentTime = lift currentTime