diff --git a/.gitignore b/.gitignore
index 080b6e1..4e9bcbb 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,7 +3,6 @@
 *.swp
 examples/gmotest
 examples/gmodump
-*.mo
 log
 hgettext
 messages.po
diff --git a/Text/Localize/Locale.hs b/Text/Localize/Locale.hs
deleted file mode 100644
index 0546129..0000000
--- a/Text/Localize/Locale.hs
+++ /dev/null
@@ -1,17 +0,0 @@
--- | This module contains definitions related to
--- obtaining current localization settings from
--- process's locale.
-module Text.Localize.Locale where
-
-import Data.Maybe
-import System.Locale.SetLocale
-
-import Text.Localize.Types
-
--- | Obtain language to be used from process's locale.
-languageFromLocale :: IO LanguageId
-languageFromLocale = do
-  mbLocale <- setLocale LC_MESSAGES (Just "")
-  let locale = fromMaybe "C" mbLocale
-  return $ takeWhile (/= '_') locale
-
diff --git a/examples/Makefile b/examples/Makefile
index 0cd9462..e5a7504 100644
--- a/examples/Makefile
+++ b/examples/Makefile
@@ -21,4 +21,3 @@ clean:
 	find . -name \*.hi -delete
 	find . -name \*.mo -delete
 	rm -f gmotest gmodump
-
diff --git a/examples/gmotest.hs b/examples/gmotest.hs
index 74704ae..0697142 100644
--- a/examples/gmotest.hs
+++ b/examples/gmotest.hs
@@ -14,7 +14,7 @@ import Data.Text.Format.Heavy (Single (..))
 import Text.Localize
 
 data LocState = LocState {
-  lsTranslations :: Translations, 
+  lsTranslations :: Translations,
   lsLanguage :: LanguageId }
   deriving (Show)
 
@@ -49,4 +49,3 @@ main = runLoc $ do
   hello name
   setLang "fr"
   hello name
-  
diff --git a/examples/iotest.hs b/examples/iotest.hs
index 00f0824..bbd5534 100644
--- a/examples/iotest.hs
+++ b/examples/iotest.hs
@@ -26,8 +26,7 @@ main = do
   TLIO.putStr =<< __ "Your name: "
   hFlush stdout
   name <- liftIO $ TLIO.getLine
-  
+
   withLanguage "en" $ hello name
   withLanguage "ru" $ hello name
   withLanguage "fr" $ hello name
-
diff --git a/localize.cabal b/localize.cabal
index 766a9ec..a1d3f20 100644
--- a/localize.cabal
+++ b/localize.cabal
@@ -1,44 +1,58 @@
+cabal-version:       2.2
 name:                localize
 version:             0.2.0.1
 synopsis:            GNU Gettext-based messages localization library
-description:         More or less fully functional translation framework,
-                     based on @haskell-gettext@ and @text-format-heavy@
-                     packages.
-license:             BSD3
+description:         Translation framework based on @haskell-gettext@ and
+                     @text-format-heavy@ packages.
+license:             BSD-3-Clause
 license-file:        LICENSE
 author:              IlyaPortnov
 maintainer:          portnov84@rambler.ru
--- copyright:           
+copyright:           © 2013–2022
 category:            Text
 build-type:          Simple
 extra-doc-files:     README.md
-cabal-version:       1.18
+extra-source-files:  test/mock-program/test.hs,
+                     test/mock-program/en_GB.mo
+
+common shared
+  build-depends:    base == 4.*,
+  ghc-options:      -Wall
+  default-language: Haskell2010
 
 library
+  import:              shared
   exposed-modules:     Text.Localize,
-                       Text.Localize.Types,
+                       Text.Localize.IO
+  other-modules:       Text.Localize.Types,
                        Text.Localize.Load,
-                       Text.Localize.State,
-                       Text.Localize.IO,
-                       Text.Localize.Locale
-  build-depends:       base > 4 && < 5,
-                       mtl >= 2.2.1,
-                       binary >=0.7,
+                       Text.Localize.State
+  build-depends:       binary >=0.7 && < 0.9,
+                       containers >=0.5 && < 0.9,
                        bytestring >= 0.10 && < 0.13,
-                       text >= 1.2,
-                       data-default >= 0.7,
-                       containers >=0.5,
+                       data-default >= 0.7 && < 0.9,
+                       directory >= 1.3 && < 1.4,
+                       filepath >= 1.4 && < 1.6,
+                       Glob >= 0.7.14 && < 0.11,
+                       haskell-gettext >= 0.1.1.0 && < 0.2,
+                       mtl >= 2.2.1 && < 2.4,
+                       text >= 1.2 && < 2.2,
                        text-format-heavy >= 0.1.4.0 && < 1.6,
-                       haskell-gettext >= 0.1.1.0,
-                       time >=1.4,
-                       transformers >=0.3,
-                       filepath >= 1.4,
-                       directory >= 1.3,
-                       Glob >= 0.7.14,
-                       setlocale >= 1.0
-  default-language:    Haskell2010
+                       time >=1.4 && < 1.15,
+                       transformers >=0.3 && < 0.7,
+                       setlocale >= 1.0 && < 1.1
+  hs-source-dirs:      src
+
+test-suite test
+  import:             shared
+  main-is:            Test.hs
+  other-modules:      Unit.LocalizeSpec
+  build-depends:      localize,
+                      hspec >= 2.11 && < 2.12
+  build-tool-depends: hspec-discover:hspec-discover
+  hs-source-dirs:     test
+  type:               exitcode-stdio-1.0
 
-Source-repository head
+source-repository head
   type: git
   location: https://github.com/portnov/localize.git
-
diff --git a/Text/Localize.hs b/src/Text/Localize.hs
similarity index 87%
rename from Text/Localize.hs
rename to src/Text/Localize.hs
index 6d70cd3..5e19262 100644
--- a/Text/Localize.hs
+++ b/src/Text/Localize.hs
@@ -8,34 +8,54 @@ module Text.Localize
    -- * Most used functions
    __, __n, __f,
    -- * Basic functions
-   translate, translateN, translateNFormat, 
+   translate, translateN, translateNFormat,
    lookup, withTranslation,
+
    -- * Reexports
-   module Text.Localize.Types,
-   module Text.Localize.Load,
-   module Text.Localize.Locale
+
+   -- ** Locale
+   languageFromLocale,
+
+   -- ** Load
+   Facet,
+   LocatePolicy(..),
+   loadTranslations,
+   locateTranslations,
+   linuxLocation,
+   localLocation,
+
+   -- ** Types
+   LanguageId,
+   Context,
+   TranslationSource,
+   Translations(..),
+   availableLanguages,
+   Localized(..),
+   toText,
+
+   -- ** Monad
+   LocState(..),
+   Localize,
+   runLocalize,
+   LocalizeT,
+   runLocalizeT,      -- TODO add execLocalizeT and execLocalize
+   setLanguage,
+   withLanguage,
+   setContext,
+   withContext
+
   ) where
 
 import Prelude hiding (lookup)
-import Control.Applicative
-import Control.Monad
 import qualified Data.Map as M
-import qualified Data.ByteString as B
-import qualified Data.ByteString.Lazy as L
 import qualified Data.Text.Lazy as T
-import qualified Data.Text.Lazy.Encoding as TLE
-import qualified Data.Text.Encoding as TE
-import Data.String
-import Data.List hiding (lookup)
-import Data.Monoid
-import Data.Typeable
 import qualified Data.Gettext as Gettext
 import qualified Data.Text.Format.Heavy as F
 import Data.Text.Format.Heavy.Parse (parseFormat)
 
 import Text.Localize.Types
 import Text.Localize.Load
-import Text.Localize.Locale
+import Text.Localize.State
 
 -- $description
 --
@@ -54,16 +74,16 @@ import Text.Localize.Locale
 -- import qualified Data.Text as T
 -- import qualified Data.Text.Lazy.IO as TLIO
 -- import Text.Localize
--- 
+--
 -- newtype MyMonad a = MyMonad {unMyMonad :: ... }
 --   deriving (Monad)
--- 
+--
 -- instance Localized MyMonad where
 --   ...
 --
 -- runMyMonad :: Translations -> MyMonad a -> IO a
 -- runMyMonad = ...
--- 
+--
 -- hello :: T.Text -> MyMonad ()
 -- hello name = do
 --   liftIO $ TLIO.putStrLn =<< __ "Your name: "
@@ -116,15 +136,15 @@ translate orig = do
         Nothing -> return $ Gettext.gettext gmo orig
         Just ctxt -> return $ Gettext.cgettext gmo ctxt orig
 
--- | Short alias for @translate@.
+-- | Short alias for 'translate'.
 __ ::  (Localized m) => TranslationSource -> m T.Text
 __ = translate
 
 -- | Translate a string, taking plural forms into account.
 translateN :: (Localized m)
-           => TranslationSource -- ^ Single form in original language
-           -> TranslationSource -- ^ Plural form in original language
-           -> Int               -- ^ Number
+           => TranslationSource -- ^ Single form in original language.
+           -> TranslationSource -- ^ Plural form in original language.
+           -> Int               -- ^ Number.
            -> m T.Text
 translateN orig plural n = do
   t <- getTranslations
@@ -140,8 +160,8 @@ translateN orig plural n = do
 -- | Translate a string and substitute variables into it.
 -- Data.Text.Format.Heavy.format syntax is used.
 translateFormat :: (Localized m, MonadFail m, F.VarContainer vars)
-                => TranslationSource -- ^ Original formatting string
-                -> vars              -- ^ Substitution variables
+                => TranslationSource -- ^ Original formatting string.
+                -> vars              -- ^ Substitution variables.
                 -> m T.Text
 translateFormat orig vars = do
   fmtStr <- translate orig
@@ -157,10 +177,10 @@ __f = translateFormat
 -- and substitute variables into it.
 -- Data.Text.Format.Heavy.format syntax is used.
 translateNFormat :: (Localized m, MonadFail m, F.VarContainer vars)
-                 => TranslationSource -- ^ Single form of formatting string in original language
-                 -> TranslationSource -- ^ Plural form of formatting string in original language
-                 -> Int               -- ^ Number
-                 -> vars              -- ^ Substitution variables
+                 => TranslationSource -- ^ Single form of formatting string in original language.
+                 -> TranslationSource -- ^ Plural form of formatting string in original language.
+                 -> Int               -- ^ Number.
+                 -> vars              -- ^ Substitution variables.
                  -> m T.Text
 translateNFormat orig plural n vars = do
   fmtStr <- translateN orig plural n
@@ -171,4 +191,3 @@ translateNFormat orig plural n vars = do
 -- | Short alias for @translateNFormat@.
 __n :: (Localized m, MonadFail m, F.VarContainer c) => TranslationSource -> TranslationSource -> Int -> c -> m T.Text
 __n = translateNFormat
-
diff --git a/Text/Localize/IO.hs b/src/Text/Localize/IO.hs
similarity index 72%
rename from Text/Localize/IO.hs
rename to src/Text/Localize/IO.hs
index 8bff6a1..2e85317 100644
--- a/Text/Localize/IO.hs
+++ b/src/Text/Localize/IO.hs
@@ -11,30 +11,8 @@ module Text.Localize.IO
   ) where
 
 import Data.IORef
-import System.IO.Unsafe (unsafePerformIO)
-
 import Text.Localize.Types
 import Text.Localize.Load
-import Text.Localize.Locale
-
-currentContext :: IORef (Maybe Context)
-currentContext = unsafePerformIO $ newIORef Nothing
-{-# NOINLINE currentContext #-}
-
-currentLanguage :: IORef LanguageId
-currentLanguage = unsafePerformIO $ do
-    language <- languageFromLocale
-    newIORef language
-{-# NOINLINE currentLanguage #-}
-
-currentTranslations :: IORef Translations
-currentTranslations = unsafePerformIO $ newIORef undefined
-{-# NOINLINE currentTranslations #-}
-
-instance Localized IO where
-  getLanguage = readIORef currentLanguage
-  getTranslations = readIORef currentTranslations
-  getContext = readIORef currentContext
 
 -- | This function must be called before any translation function call,
 -- otherwise you will get runtime error.
@@ -73,4 +51,3 @@ withContext ctxt actions = do
   result <- actions
   setContext oldContext
   return result
-
diff --git a/Text/Localize/Load.hs b/src/Text/Localize/Load.hs
similarity index 95%
rename from Text/Localize/Load.hs
rename to src/Text/Localize/Load.hs
index 33195ac..e5cd52a 100644
--- a/Text/Localize/Load.hs
+++ b/src/Text/Localize/Load.hs
@@ -1,11 +1,11 @@
 {-# LANGUAGE TypeSynonymInstances, FlexibleInstances, ExistentialQuantification, DeriveDataTypeable, OverloadedStrings, RecordWildCards #-}
 -- | This module contains definitions for loading translation catalogs.
 module Text.Localize.Load
-  ( -- * Data types
+  ( -- * Data types.
     LocatePolicy (..), Facet,
-    -- * Main functions
+    -- * Main functions.
     loadTranslations, locateTranslations,
-    -- * Commonly used location policies
+    -- * Commonly used location policies.
     linuxLocation, localLocation
   ) where
 
@@ -17,12 +17,12 @@ import qualified Data.Text.Lazy as T
 import qualified Data.Gettext as Gettext
 import Data.Text.Format.Heavy
 import System.Directory
-import System.FilePath
 import System.FilePath.Glob
 
 import Text.Localize.Types
 
--- | Load translations when path to each translation file is known.
+-- | Load compiled translations (@mo/@) when path to each translation file
+-- is known.
 loadTranslations :: [(LanguageId, FilePath)] -> IO Translations
 loadTranslations pairs = do
   res <- forM pairs $ \(lang, path) -> do
@@ -61,13 +61,13 @@ instance Default LocatePolicy where
 
 -- | Usual Linux translations location policy.
 -- Catalog files are found under @\/usr\/[local\/]share\/locale\/{language}\/LC_MESSAGES\/{name}.mo@.
-linuxLocation :: String        -- ^ Catalog file name (text domain)
+linuxLocation :: String        -- ^ Catalog file name (text domain).
               -> LocatePolicy
 linuxLocation name = def {lcBasePaths = ["/usr/share/locale", "/usr/local/share/locale"], lcName = name}
 
 -- | Simple translations location polciy, assuming all catalog files located at
 -- @{base}\/{language}.mo@.
-localLocation :: FilePath      -- ^ Path to directory with translations
+localLocation :: FilePath      -- ^ Path to directory with translations.
               -> LocatePolicy
 localLocation base = def {lcBasePaths = [base], lcFormat = "{base}/{language}.mo"}
 
@@ -98,8 +98,7 @@ locateTranslations (LocatePolicy {..}) = liftIO $ do
       let (hd, tl) = break isLanguage items
       in  case tl of
             [] -> (hd, [])
-            _  -> (hd, tail tl)
+            (_:tl') -> (hd, tl')
 
     isLanguage (FVariable name _) = name == "language"
     isLanguage _ = False
-
diff --git a/Text/Localize/State.hs b/src/Text/Localize/State.hs
similarity index 70%
rename from Text/Localize/State.hs
rename to src/Text/Localize/State.hs
index 307590c..01fba94 100644
--- a/Text/Localize/State.hs
+++ b/src/Text/Localize/State.hs
@@ -6,28 +6,33 @@
 module Text.Localize.State
   (-- * Data types
    LocState (..),
+   Localize (..),
    LocalizeT (..),
    -- * Functions
+   runLocalize,
    runLocalizeT,
    setLanguage, withLanguage,
    setContext, withContext
   ) where
 
-import Control.Applicative
 import Control.Monad.State
-import Control.Monad.Trans
 
 import Text.Localize.Types
 
--- | Localization state
+import qualified Control.Monad.Identity as I
+
+-------------------------------------------------------------------------------
+-- Types
+
+-- | Localization state.
 data LocState = LocState {
-    lsTranslations :: Translations, 
+    lsTranslations :: Translations,
     lsLanguage :: LanguageId,
     lsContext :: Maybe Context
   }
   deriving (Show)
 
--- | Localization monad transformer
+-- | Localization monad transformer.
 newtype LocalizeT m a = LocalizeT {
     unLocalizeT :: StateT LocState m a
   }
@@ -42,7 +47,25 @@ instance Monad m => Localized (LocalizeT m) where
 runLocalizeT :: Monad m => LocalizeT m a -> LocState -> m a
 runLocalizeT actions st = evalStateT (unLocalizeT actions) st
 
--- | Set current language
+-- | Localization monad.
+newtype Localize a = Localize {
+    unLocalize :: StateT LocState I.Identity a
+  }
+  deriving (Functor, Applicative, Monad, MonadState LocState)
+
+instance Localized Localize where
+  getTranslations = gets lsTranslations
+  getLanguage = gets lsLanguage
+  getContext = gets lsContext
+
+-- | Run a computation inside 'LocalizeT'.
+runLocalize :: Localize a -> LocState -> a
+runLocalize actions st = evalState (unLocalize actions) st
+
+-------------------------------------------------------------------------------
+-- Operate
+
+-- | Set current language.
 setLanguage :: Monad m => LanguageId -> LocalizeT m ()
 setLanguage lang = modify $ \st -> st {lsLanguage = lang}
 
@@ -67,4 +90,3 @@ withContext ctxt actions = do
   result <- actions
   setContext oldContext
   return result
-
diff --git a/Text/Localize/Types.hs b/src/Text/Localize/Types.hs
similarity index 52%
rename from Text/Localize/Types.hs
rename to src/Text/Localize/Types.hs
index e4441f9..f67ee4b 100644
--- a/Text/Localize/Types.hs
+++ b/src/Text/Localize/Types.hs
@@ -2,32 +2,46 @@
 -- | This module contains data type definitions for the @localize@ package.
 module Text.Localize.Types where
 
-import Control.Applicative
+import qualified Data.IORef as IR
+import qualified Data.Maybe as MB
 import qualified Data.Map as M
 import qualified Data.ByteString as B
 import qualified Data.Text.Lazy as T
 import qualified Data.Text.Encoding as TE
 import qualified Data.Gettext as Gettext
+import qualified System.IO.Unsafe as UI
+import qualified System.Locale.SetLocale as SL
 
--- | Language identifier
+
+-- | Language identifier.
 type LanguageId = String
 
--- | Context name
+-- | Context name.
 type Context = B.ByteString
 
--- | String to be translated
+-- | String to be translated.
 type TranslationSource = B.ByteString
 
--- | Stores translation catalogs for all supported languages
+-- | Stores translation catalogs for all supported languages.
 data Translations = Translations {
   tMap :: M.Map LanguageId Gettext.Catalog }
 
+instance Semigroup Translations where
+  (<>) (Translations a) (Translations b) = Translations (a <> b)
+
+instance Monoid Translations where
+  mempty = Translations M.empty
+
+-- | List available languages.
+availableLanguages :: Translations -> [LanguageId]
+availableLanguages ts = M.keys (tMap ts)
+
 instance Show Translations where
   show t = "<Translations to languages: " ++ (unwords $ M.keys $ tMap t) ++ ">"
 
 -- | This is the main type class of the package.
 -- All functions work with any instance of this type class.
--- 
+--
 -- Note that this class only supports **getting** current language
 -- and translation, not setting them. Though concrete implementation
 -- can have its own ways to change language or context, these ways are
@@ -36,7 +50,7 @@ instance Show Translations where
 class (Monad m, Applicative m) => Localized m where
   -- | Obtain currently selected language ID.
   getLanguage :: m LanguageId
-  
+
   -- | Obtain currently loaded translations.
   getTranslations :: m Translations
 
@@ -49,3 +63,28 @@ class (Monad m, Applicative m) => Localized m where
 toText :: TranslationSource -> T.Text
 toText bstr = T.fromStrict $ TE.decodeUtf8 bstr
 
+instance Localized IO where
+  getLanguage = IR.readIORef currentLanguage
+  getTranslations = IR.readIORef currentTranslations
+  getContext = IR.readIORef currentContext
+
+currentContext :: IR.IORef (Maybe Context)
+currentContext = UI.unsafePerformIO $ IR.newIORef Nothing
+{-# NOINLINE currentContext #-}
+
+currentLanguage :: IR.IORef LanguageId
+currentLanguage = UI.unsafePerformIO $ do
+    language <- languageFromLocale
+    IR.newIORef language
+{-# NOINLINE currentLanguage #-}
+
+currentTranslations :: IR.IORef Translations
+currentTranslations = UI.unsafePerformIO $ IR.newIORef undefined
+{-# NOINLINE currentTranslations #-}
+
+-- | Obtain language to be used from process's locale.
+languageFromLocale :: IO LanguageId
+languageFromLocale = do
+  mbLocale <- SL.setLocale SL.LC_MESSAGES (Just "")
+  let locale = MB.fromMaybe "C" mbLocale
+  return $ takeWhile (/= '_') locale
diff --git a/test/Test.hs b/test/Test.hs
new file mode 100644
index 0000000..a824f8c
--- /dev/null
+++ b/test/Test.hs
@@ -0,0 +1 @@
+{-# OPTIONS_GHC -F -pgmF hspec-discover #-}
diff --git a/test/Unit/LocalizeSpec.hs b/test/Unit/LocalizeSpec.hs
new file mode 100644
index 0000000..20e56a5
--- /dev/null
+++ b/test/Unit/LocalizeSpec.hs
@@ -0,0 +1,20 @@
+{-# Language OverloadedStrings #-}
+
+module Unit.LocalizeSpec where
+
+import Text.Localize as L
+
+import Test.Hspec
+
+main :: IO ()
+main = hspec spec
+
+spec :: Spec
+spec = do
+
+  describe "lookup" $ do
+    it "returns source string if the translation is not found" $
+      L.lookup mempty "en" "prova" `shouldBe` "prova"
+    it "returns a translated string if the translation is present" $ do
+      ts <- loadTranslations [("en_GB", "test/mock-program/en_GB.mo")]
+      L.lookup ts "en_GB" "prova" `shouldBe` "test"
diff --git a/test/mock-program/en_GB.mo b/test/mock-program/en_GB.mo
new file mode 100644
index 0000000..3145d6a
Binary files /dev/null and b/test/mock-program/en_GB.mo differ
diff --git a/test/mock-program/test.hs b/test/mock-program/test.hs
new file mode 100644
index 0000000..a7e2e36
--- /dev/null
+++ b/test/mock-program/test.hs
@@ -0,0 +1,4 @@
+-- Run `hgettext -o messages.pot test.hs` to obtain `messages.pot`.
+-- Then `msginit --locale=en_GB` to get `en_GB.po.
+-- Finally `msgfmt en_GB.po -o en_GB.mo`.
+string = __ "prova"