diff --git a/Data/Text/Buildable.hs b/Data/Text/Buildable.hs
index 4888eb4..549a118 100644
--- a/Data/Text/Buildable.hs
+++ b/Data/Text/Buildable.hs
@@ -1,4 +1,6 @@
-{-# LANGUAGE CPP, FlexibleInstances, OverloadedStrings #-}
+{-# LANGUAGE CPP               #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE OverloadedStrings #-}
 
 -- |
 -- Module      : Data.Text.Buildable
@@ -17,26 +19,31 @@ module Data.Text.Buildable
     ) where
 
 #if MIN_VERSION_base(4,8,0)
-import Data.Void (Void, absurd)
+import           Data.Void                   (Void, absurd)
 #endif
 
-import Data.Monoid (mempty)
-import Data.Int (Int8, Int16, Int32, Int64)
-import Data.Fixed (Fixed, HasResolution, showFixed)
-import Data.Ratio (Ratio, denominator, numerator)
-import Data.Text.Format.Functions ((<>))
-import Data.Text.Format.Int (decimal, hexadecimal)
-import Data.Text.Format.Types (Hex(..), Shown(..))
-import Data.Text.Lazy.Builder
-import Data.Time.Calendar (Day, showGregorian)
-import Data.Time.Clock (DiffTime, NominalDiffTime, UTCTime, UniversalTime)
-import Data.Time.Clock (getModJulianDate)
-import Data.Time.LocalTime (LocalTime, TimeOfDay, TimeZone, ZonedTime)
-import Data.Word (Word, Word8, Word16, Word32, Word64)
-import Foreign.Ptr (IntPtr, WordPtr, Ptr, ptrToWordPtr)
 import qualified Data.Double.Conversion.Text as C
-import qualified Data.Text as ST
-import qualified Data.Text.Lazy as LT
+import           Data.Fixed                  (Fixed, HasResolution, showFixed)
+import           Data.Int                    (Int16, Int32, Int64, Int8)
+import           Data.Monoid                 (mempty)
+import           Data.Ratio                  (Ratio, denominator, numerator)
+import qualified Data.Text                   as ST
+import           Data.Text.Format.Functions  ((<>))
+import           Data.Text.Format.Int        (decimal, hexadecimal)
+import           Data.Text.Format.Types      (Hex (..), Shown (..))
+import qualified Data.Text.Lazy              as LT
+import           Data.Text.Lazy.Builder
+import           Data.Time.Calendar          (Day, showGregorian)
+import           Data.Time.Clock             (DiffTime, NominalDiffTime,
+                                              UTCTime, UniversalTime)
+import           Data.Time.Clock             (getModJulianDate)
+import           Data.Time.LocalTime         (LocalTime, TimeOfDay, TimeZone,
+                                              ZonedTime)
+import           Data.Word                   (Word, Word16, Word32, Word64,
+                                              Word8)
+import           Foreign.Ptr                 (IntPtr, Ptr, WordPtr,
+                                              ptrToWordPtr)
+import           Prelude                     hiding ((<>))
 
 -- | The class of types that can be rendered to a 'Builder'.
 class Buildable p where
@@ -155,7 +162,7 @@ instance (Show a) => Buildable (Shown a) where
     {-# INLINE build #-}
 
 instance (Buildable a) => Buildable (Maybe a) where
-    build Nothing = mempty
+    build Nothing  = mempty
     build (Just v) = build v
     {-# INLINE build #-}
 
@@ -185,5 +192,5 @@ instance Buildable (Ptr a) where
     build = build . ptrToWordPtr
 
 instance Buildable Bool where
-    build True = fromText "True"
+    build True  = fromText "True"
     build False = fromText "False"
diff --git a/Data/Text/Format.hs b/Data/Text/Format.hs
index 7a7f725..07a4b36 100644
--- a/Data/Text/Format.hs
+++ b/Data/Text/Format.hs
@@ -1,4 +1,5 @@
-{-# LANGUAGE OverloadedStrings, RelaxedPolyRec #-}
+{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE RelaxedPolyRec    #-}
 
 -- |
 -- Module      : Data.Text.Format
@@ -35,19 +36,20 @@ module Data.Text.Format
     , shortest
     ) where
 
-import Control.Monad.IO.Class (MonadIO(liftIO))
-import Data.Text.Format.Functions ((<>))
-import Data.Text.Format.Params (Params(..))
-import Data.Text.Format.Types.Internal (Format(..), Only(..), Shown(..))
-import Data.Text.Format.Types.Internal (Hex(..))
-import Data.Text.Lazy.Builder
-import Prelude hiding (exp, print)
-import System.IO (Handle)
-import qualified Data.Double.Conversion.Text as C
-import qualified Data.Text as ST
-import qualified Data.Text.Buildable as B
-import qualified Data.Text.Lazy as LT
-import qualified Data.Text.Lazy.IO as LT
+import           Control.Monad.IO.Class          (MonadIO (liftIO))
+import qualified Data.Double.Conversion.Text     as C
+import qualified Data.Text                       as ST
+import qualified Data.Text.Buildable             as B
+import           Data.Text.Format.Functions      ((<>))
+import           Data.Text.Format.Params         (Params (..))
+import           Data.Text.Format.Types.Internal (Format (..), Only (..),
+                                                  Shown (..))
+import           Data.Text.Format.Types.Internal (Hex (..))
+import qualified Data.Text.Lazy                  as LT
+import           Data.Text.Lazy.Builder
+import qualified Data.Text.Lazy.IO               as LT
+import           Prelude                         hiding (exp, print, (<>))
+import           System.IO                       (Handle)
 
 -- Format strings are almost always constants, and they're expensive
 -- to interpret (which we refer to as "cracking" here).  We'd really
diff --git a/Data/Text/Format/Functions.hs b/Data/Text/Format/Functions.hs
index f10dd77..3068e34 100644
--- a/Data/Text/Format/Functions.hs
+++ b/Data/Text/Format/Functions.hs
@@ -17,9 +17,10 @@ module Data.Text.Format.Functions
     , i2d
     ) where
 
-import Data.Monoid (mappend)
-import Data.Text.Lazy.Builder (Builder)
-import GHC.Base
+import           Data.Monoid            (mappend)
+import           Data.Text.Lazy.Builder (Builder)
+import           GHC.Base               hiding ((<>))
+import           Prelude                hiding ((<>))
 
 -- | Unsafe conversion for decimal digits.
 {-# INLINE i2d #-}
diff --git a/Data/Text/Format/Int.hs b/Data/Text/Format/Int.hs
index 37858c9..80d8a29 100644
--- a/Data/Text/Format/Int.hs
+++ b/Data/Text/Format/Int.hs
@@ -1,4 +1,7 @@
-{-# LANGUAGE BangPatterns, CPP, MagicHash, UnboxedTuples #-}
+{-# LANGUAGE BangPatterns  #-}
+{-# LANGUAGE CPP           #-}
+{-# LANGUAGE MagicHash     #-}
+{-# LANGUAGE UnboxedTuples #-}
 
 -- Module:      Data.Text.Format.Int
 -- Copyright:   (c) 2011 MailRank, Inc.
@@ -16,20 +19,22 @@ module Data.Text.Format.Int
     , minus
     ) where
 
-import Data.Int (Int8, Int16, Int32, Int64)
-import Data.Monoid (mempty)
-import Data.Text.Format.Functions ((<>), i2d)
-import Data.Text.Lazy.Builder
-import Data.Word (Word, Word8, Word16, Word32, Word64)
-import GHC.Base (quotInt, remInt)
-import GHC.Num (quotRemInteger)
-import GHC.Types (Int(..))
+import           Data.Int                   (Int16, Int32, Int64, Int8)
+import           Data.Monoid                (mempty)
+import           Data.Text.Format.Functions (i2d, (<>))
+import           Data.Text.Lazy.Builder
+import           Data.Word                  (Word, Word16, Word32, Word64,
+                                             Word8)
+import           GHC.Base                   (quotInt, remInt)
+import           GHC.Num                    (quotRemInteger)
+import           GHC.Types                  (Int (..))
+import           Prelude                    hiding ((<>))
 
 #ifdef  __GLASGOW_HASKELL__
 # if __GLASGOW_HASKELL__ < 611
-import GHC.Integer.Internals
+import           GHC.Integer.Internals
 # else
-import GHC.Integer.GMP.Internals
+import           GHC.Integer.GMP.Internals
 # endif
 #endif
 
diff --git a/Data/Text/Format/Params.hs b/Data/Text/Format/Params.hs
index 1e5756d..85c45dd 100644
--- a/Data/Text/Format/Params.hs
+++ b/Data/Text/Format/Params.hs
@@ -14,9 +14,9 @@ module Data.Text.Format.Params
       Params(..)
     ) where
 
-import Data.Text.Buildable
-import Data.Text.Format.Types
-import Data.Text.Lazy.Builder
+import           Data.Text.Buildable
+import           Data.Text.Format.Types
+import           Data.Text.Lazy.Builder
 
 -- | The class of types that can be used as a collection of arguments
 -- for formatting.
diff --git a/Data/Text/Format/Types.hs b/Data/Text/Format/Types.hs
index 8972606..b4a9446 100644
--- a/Data/Text/Format/Types.hs
+++ b/Data/Text/Format/Types.hs
@@ -1,4 +1,5 @@
-{-# LANGUAGE DeriveDataTypeable, GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE DeriveDataTypeable         #-}
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
 
 -- |
 -- Module      : Data.Text.Format.Types
@@ -20,4 +21,4 @@ module Data.Text.Format.Types
     , Hex(..)
     ) where
 
-import Data.Text.Format.Types.Internal
+import           Data.Text.Format.Types.Internal
diff --git a/Data/Text/Format/Types/Internal.hs b/Data/Text/Format/Types/Internal.hs
index 2eaccb2..58e6fc5 100644
--- a/Data/Text/Format/Types/Internal.hs
+++ b/Data/Text/Format/Types/Internal.hs
@@ -1,4 +1,5 @@
-{-# LANGUAGE DeriveDataTypeable, GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE DeriveDataTypeable         #-}
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
 
 -- |
 -- Module      : Data.Text.Format.Types.Internal
@@ -20,10 +21,11 @@ module Data.Text.Format.Types.Internal
     , Hex(..)
     ) where
 
-import Data.Monoid (Monoid(..))
-import Data.String (IsString(..))
-import Data.Text (Text)
-import Data.Typeable (Typeable)
+import           Data.Monoid    (Monoid (..))
+import           Data.Semigroup (Semigroup (..))
+import           Data.String    (IsString (..))
+import           Data.Text      (Text)
+import           Data.Typeable  (Typeable)
 
 -- | A format string. This is intentionally incompatible with other
 -- string types, to make it difficult to construct a format string by
@@ -46,8 +48,10 @@ import Data.Typeable (Typeable)
 newtype Format = Format { fromFormat :: Text }
     deriving (Eq, Ord, Typeable, Show)
 
+instance Semigroup Format where
+    Format a <> Format b = Format (a <> b)
+
 instance Monoid Format where
-    Format a `mappend` Format b = Format (a `mappend` b)
     mempty = Format mempty
 
 instance IsString Format where
diff --git a/stack.yaml b/stack.yaml
index 3a4ea0b..5eefadc 100644
--- a/stack.yaml
+++ b/stack.yaml
@@ -1,7 +1,9 @@
+resolver: lts-12.0
+
 flags:
   text-format:
     developer: false
 packages:
 - '.'
 extra-deps: []
-resolver: lts-3.0
+