cached-traversable (empty) → 0.1.0.0

raw patch · 4 files changed

+120/−0 lines, 4 filesdep +basedep +binarydep +bytestringsetup-changed

Dependencies added: base, binary, bytestring, containers, directory, filepath, mtl

Files

• LICENSE +30/−0
• Setup.hs +2/−0
• cached-traversable.cabal +32/−0
• src/Data/Traversable/Cached.hs +56/−0

@@ -0,0 +1,30 @@+Copyright (c) 2014, Thomas Tuegel++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 Thomas Tuegel 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 +1,32 @@+name:                cached-traversable+version:             0.1.0.0+synopsis:            Transparent, persistent caching of lazy, traversable structures+description:+  cached-traversable provides persistent (on-disk) caching of lazy, traversable+  structures for any element type which has a Binary instance. Think of it as+  a poor man's acid-state for deterministic programs.+license:             BSD3+license-file:        LICENSE+author:              Thomas Tuegel+maintainer:          ttuegel@gmail.com+copyright:           2014 Thomas Tuegel+category:            Data+build-type:          Simple+cabal-version:       >=1.8++source-repository head+  type: git+  location: https://github.com/ttuegel/cached-traversable.git++library+  exposed-modules:+    Data.Traversable.Cached+  build-depends:+    base >=4.6 && <5,+    mtl ==2.1.*,+    binary ==0.5.*,+    bytestring ==0.10.*,+    containers ==0.5.*,+    directory ==1.2.*,+    filepath ==1.3.*+  hs-source-dirs: src

@@ -0,0 +1,56 @@+module Data.Traversable.Cached where++import Control.Applicative+import Control.Arrow+import Control.Monad (when)+import qualified Control.Monad.State.Strict as State+import Control.Monad.Trans (liftIO)+import Data.Binary+import qualified Data.ByteString as BS+import qualified Data.ByteString.Lazy as LBS+import Data.Foldable+import Data.Sequence ((|>))+import qualified Data.Sequence as Seq+import Data.Traversable+import System.Directory (doesFileExist, removeFile, renameFile)+import System.FilePath ((<.>))++-- | Traverse the given structure, retrieving values from and storing values in+-- the cache at the given path. Only the element type need be serializable. The+-- struture must be lazy in the values for this to be effective. This does not+-- work well for large structures because the entire cache is rewritten every+-- time a new value is computed. Works best when values are small, but expensive+-- to compute. Atomicity and consistency are weakly guaranteed by writing the+-- cache to a new file every time, and renaming the file into place.+cached :: (Binary a, Traversable t) => FilePath -> t a -> IO (t a)+cached path dat = do+    cacheExists <- doesFileExist path+    cache <- if cacheExists then LBS.readFile path else return LBS.empty+    newExists <- doesFileExist newPath+    when newExists $ removeFile newPath+    as <- State.evalStateT (traverse cached_go dat) (cache, Seq.empty)+    return as+  where+    newPath = path <.> "new"++    appendEncoded = flip (|>) . LBS.toStrict . encode++    flushCache = do+        (oldCache, newCache) <- State.get+        -- When there is data yet to be read in oldCache, the cache is not stale+        -- so there is no reason to write a new one.+        when (LBS.null oldCache) $ liftIO $ do+            forM_ newCache (BS.appendFile newPath)+            renameFile newPath path++    cached_go computed = do+        decoded <- decodeOrFail . fst <$> State.get+        case decoded of+            Left _ -> do+                State.modify $ const LBS.empty *** appendEncoded computed+                seq computed flushCache+                return computed+            Right (nextCache, _, retrieved) -> do+                State.modify $ const nextCache *** appendEncoded retrieved+                seq retrieved flushCache+                return retrieved