diff --git a/ChangeLog.md b/ChangeLog.md
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+# Changelog for polysemy-methodology
+
+## v0.1.0.0
+
+* Imported `KVStore` effect from polysemy-zoo.
diff --git a/LICENSE b/LICENSE
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--- /dev/null
+++ b/LICENSE
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+Copyright Sandy Maguire (c) 2019
+Copyright Daniel Firth (c) 2021
+
+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 Sandy Maguire 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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+++ b/README.md
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+# polysemy-kvstore
+
+KVStore effect for polysemy, originally taken from
+[polysemy-zoo](https://hackage.haskell.org/package/polysemy-zoo).
diff --git a/polysemy-kvstore.cabal b/polysemy-kvstore.cabal
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+++ b/polysemy-kvstore.cabal
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+cabal-version: 1.12
+
+-- This file has been generated from package.yaml by hpack version 0.34.4.
+--
+-- see: https://github.com/sol/hpack
+
+name:           polysemy-kvstore
+version:        0.1.0.0
+synopsis:       KVStore effect for 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
+
+library
+  exposed-modules:
+      Polysemy.KVStore
+  other-modules:
+      Paths_polysemy_kvstore
+  hs-source-dirs:
+      src
+  ghc-options: -Wall -Wcompat -Wincomplete-record-updates -Wincomplete-uni-patterns -Wredundant-constraints
+  build-depends:
+      base >=4.7 && <5
+    , containers
+    , polysemy >=1.3.0.0 && <1.7
+  default-language: Haskell2010
diff --git a/src/Polysemy/KVStore.hs b/src/Polysemy/KVStore.hs
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--- /dev/null
+++ b/src/Polysemy/KVStore.hs
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+{-# LANGUAGE AllowAmbiguousTypes #-}
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE ExplicitForAll #-}
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE LambdaCase #-}
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TemplateHaskell #-}
+{-# LANGUAGE TypeApplications #-}
+{-# LANGUAGE TypeOperators #-}
+
+module Polysemy.KVStore
+  ( -- * Effect
+    KVStore (..),
+
+    -- * Actions
+    lookupKV,
+    lookupOrThrowKV,
+    existsKV,
+    writeKV,
+    deleteKV,
+    updateKV,
+    modifyKV,
+
+    -- * Interpretations
+    runKVStoreAsState,
+    runKVStorePure,
+  )
+where
+
+import qualified Data.Map as M
+import Data.Maybe (isJust)
+import Polysemy
+import Polysemy.Error
+import Polysemy.State
+
+-- | Models things like Redis, HTTP GET/POST, etc. Things that are keyed, have
+-- a value, and may or may not be there.
+data KVStore k v m a where
+  LookupKV :: k -> KVStore k v m (Maybe v)
+  UpdateKV :: k -> Maybe v -> KVStore k v m ()
+
+makeSem ''KVStore
+
+-- |
+--
+-- @since 0.1.0.0
+writeKV :: Member (KVStore k v) r => k -> v -> Sem r ()
+writeKV k = updateKV k . Just
+{-# INLINE writeKV #-}
+
+-- |
+--
+-- @since 0.1.0.0
+deleteKV :: forall k v r. Member (KVStore k v) r => k -> Sem r ()
+deleteKV k = updateKV k (Nothing @v)
+{-# INLINE deleteKV #-}
+
+-- |
+--
+-- @since 0.1.0.0
+lookupOrThrowKV ::
+  Members
+    '[ KVStore k v,
+       Error e
+     ]
+    r =>
+  (k -> e) ->
+  k ->
+  Sem r v
+lookupOrThrowKV f k =
+  fromEither . maybe (Left $ f k) Right =<< lookupKV k
+
+-- |
+--
+-- @since 0.1.0.0
+existsKV :: forall k v r. Member (KVStore k v) r => k -> Sem r Bool
+existsKV = fmap isJust . lookupKV @k @v
+
+-- |
+--
+-- @since 0.1.0.0
+modifyKV ::
+  Member (KVStore k v) r =>
+  -- | Default value if the key isn't present
+  v ->
+  (v -> v) ->
+  k ->
+  Sem r ()
+modifyKV d f k =
+  lookupKV k >>= \case
+    Just v -> writeKV k $ f v
+    Nothing -> writeKV k $ f d
+
+-- | Run a `KVStore` as a `State` effect containing a `Map`.
+--
+-- @since 0.1.0.0
+runKVStoreAsState :: Ord k => Sem (KVStore k v ': r) a -> Sem (State (M.Map k v) ': r) a
+runKVStoreAsState = reinterpret $ \case
+  LookupKV k -> gets $ M.lookup k
+  UpdateKV k v -> modify $ M.alter (const v) k
+{-# INLINE runKVStoreAsState #-}
+
+-- | Run a `KVStore` purely as a `Map`.
+--
+-- @since 0.1.0.0
+runKVStorePure ::
+  Ord k =>
+  M.Map k v ->
+  Sem (KVStore k v ': r) a ->
+  Sem r (M.Map k v, a)
+runKVStorePure m = runState m . runKVStoreAsState
+{-# INLINE runKVStorePure #-}