diff --git a/ChangeLog.md b/ChangeLog.md
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+++ b/ChangeLog.md
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+# Changelog for polysemy-kvstore-jsonfile
+
+## v0.1.0.0
+
+* Add interpreters for running a kvstore as a JSON file.
diff --git a/LICENSE b/LICENSE
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--- /dev/null
+++ b/LICENSE
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+Copyright Daniel Firth (c) 2020
+
+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 Author name here 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/README.md b/README.md
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--- /dev/null
+++ b/README.md
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+# polysemy-kvstore-jsonfile
+
+Polysemy.JSONFileKVStore is an insane way to interpret a KVStore in polysemy,
+by running it as a single JSON file serialized as a Map.
diff --git a/polysemy-kvstore-jsonfile.cabal b/polysemy-kvstore-jsonfile.cabal
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+++ b/polysemy-kvstore-jsonfile.cabal
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+cabal-version: 1.12
+
+-- This file has been generated from package.yaml by hpack version 0.34.2.
+--
+-- see: https://github.com/sol/hpack
+
+name:           polysemy-kvstore-jsonfile
+version:        0.1.0.0
+synopsis:       Run a KVStore as a single json file in polysemy.
+category:       Polysemy
+author:         Daniel Firth
+maintainer:     dan.firth@homotopic.tech
+copyright:      2020 Daniel Firth
+license:        MIT
+license-file:   LICENSE
+build-type:     Simple
+extra-source-files:
+    README.md
+    ChangeLog.md
+
+source-repository head
+  type: git
+  location: https://gitlab.com/homotopic-tech/polysemy-kvstore-jsonfile
+
+library
+  exposed-modules:
+      Polysemy.JSONFileKVStore
+  other-modules:
+      Paths_polysemy_kvstore_jsonfile
+  hs-source-dirs:
+      src
+  build-depends:
+      aeson
+    , base >=4.7 && <5
+    , containers
+    , exceptions
+    , extra
+    , path
+    , polysemy
+    , polysemy-zoo
+    , unliftio-path
+  default-language: Haskell2010
diff --git a/src/Polysemy/JSONFileKVStore.hs b/src/Polysemy/JSONFileKVStore.hs
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--- /dev/null
+++ b/src/Polysemy/JSONFileKVStore.hs
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+{-# LANGUAGE BlockArguments      #-}
+{-# LANGUAGE DataKinds           #-}
+{-# LANGUAGE DeriveGeneric       #-}
+{-# LANGUAGE GADTs               #-}
+{-# LANGUAGE LambdaCase          #-}
+{-# LANGUAGE PolyKinds           #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TypeApplications    #-}
+{-# LANGUAGE TypeOperators       #-}
+module Polysemy.JSONFileKVStore where
+
+import Control.Monad.Catch
+import Control.Monad.Extra
+import Control.Monad.IO.Class
+import Data.Aeson
+import qualified Data.Map as Map
+import Data.Map (Map)
+import GHC.Generics
+import Polysemy
+import Polysemy.Error
+import Polysemy.Input
+import Polysemy.KVStore
+import Path
+import qualified UnliftIO.Path.Directory as U
+
+newtype JSONParseException = JSONParseException String
+  deriving (Show, Eq, Generic)
+
+instance Exception JSONParseException where
+  displayException (JSONParseException x) = x
+
+eitherDecodeOrCreate :: (ToJSON a, FromJSON a, MonadIO m) => Path b File -> a -> m (Either String a)
+eitherDecodeOrCreate f x = do
+  whenM (fmap not . U.doesFileExist $ f) $ liftIO $ encodeFile (toFilePath f) x
+  liftIO $ eitherDecodeFileStrict' (toFilePath f)
+
+runKVStoreAsJSONFileStore :: (Members '[Embed IO, Error JSONParseException] r,
+                              FromJSONKey k, ToJSONKey k, FromJSON v, ToJSON v, Ord k)
+                          => Path b File
+                          -> Sem (KVStore k v ': r) a -> Sem r a
+runKVStoreAsJSONFileStore d = interpret \case
+  LookupKV k -> do
+    z <- eitherDecodeOrCreate d mempty
+    case z of
+      Left x  -> throw @JSONParseException $ JSONParseException x
+      Right x -> return $ Map.lookup k x
+  UpdateKV k v -> do
+    z <- eitherDecodeOrCreate d mempty
+    case z of
+      Left x -> throw $ JSONParseException x
+      Right (x :: Map k v) -> embed $ encodeFile (toFilePath d) (Map.alter (const v) k x)