diff --git a/Example.hs b/Example.hs
--- a/Example.hs
+++ b/Example.hs
@@ -2,19 +2,20 @@
 
 module Example where
 
-import           Control.Monad.IO.Class                         (liftIO)
-import           System.Metrics.Prometheus.Http.Scrape          (serveMetricsT)
-import           System.Metrics.Prometheus.Concurrent.RegistryT
-import           System.Metrics.Prometheus.Metric.Counter       (inc)
-import           System.Metrics.Prometheus.MetricId
+import Control.Monad.IO.Class (liftIO)
+import System.Metrics.Prometheus.Concurrent.RegistryT
+import System.Metrics.Prometheus.Http.Scrape (serveMetricsT)
+import System.Metrics.Prometheus.Metric.Counter (inc)
+import System.Metrics.Prometheus.MetricId
 
+
 main :: IO ()
 main = runRegistryT $ do
     -- Labels can be defined as lists or added to an empty label set
     connectSuccessGauge <- registerGauge "example_connections" (fromList [("login", "success")])
     connectFailureGauge <- registerGauge "example_connections" (addLabel "login" "failure" mempty)
     connectCounter <- registerCounter "example_connection_total" mempty
-    latencyHistogram <- registerHistogram "example_round_trip_latency_ms" mempty [10, 20..100]
+    latencyHistogram <- registerHistogram "example_round_trip_latency_ms" mempty [10, 20 .. 100]
 
     liftIO $ inc connectCounter -- increment a counter
 
diff --git a/README.md b/README.md
--- a/README.md
+++ b/README.md
@@ -42,7 +42,7 @@
 
     -- [...] pass metric handles to the rest of the app
 
-    serveMetricsT 8080 ["metrics"] -- http://localhost:8080/metric server
+    serveMetricsT 8080 ["metrics"] -- http://localhost:8080/metrics server
 ```
 
 ## Advanced Usage
diff --git a/prometheus.cabal b/prometheus.cabal
--- a/prometheus.cabal
+++ b/prometheus.cabal
@@ -1,5 +1,5 @@
 name:                prometheus
-version:             2.2.3
+version:             2.2.4
 synopsis:            Prometheus Haskell Client
 homepage:            http://github.com/bitnomial/prometheus
 bug-reports:         http://github.com/bitnomial/prometheus/issues
@@ -90,8 +90,8 @@
                , http-client-tls >= 0.3  && < 0.4
                , http-types      >= 0.8  && < 0.13
                , network-uri     >= 2.5  && < 2.7
-               , text            >= 1.2  && < 1.3
-               , transformers    >= 0.4  && < 0.6
+               , text            >= 1.2  && < 2.1
+               , transformers    >= 0.4  && < 0.7
                , wai             >= 3.2  && < 3.3
                , warp            >= 3.2  && < 3.5
 
diff --git a/src/System/Metrics/Prometheus/Concurrent/Registry.hs b/src/System/Metrics/Prometheus/Concurrent/Registry.hs
--- a/src/System/Metrics/Prometheus/Concurrent/Registry.hs
+++ b/src/System/Metrics/Prometheus/Concurrent/Registry.hs
@@ -1,28 +1,39 @@
-module System.Metrics.Prometheus.Concurrent.Registry
-       ( Registry
-       , new
-       , registerCounter
-       , registerGauge
-       , registerHistogram
-       , sample
-       ) where
-
+module System.Metrics.Prometheus.Concurrent.Registry (
+    Registry,
+    new,
+    registerCounter,
+    registerGauge,
+    registerHistogram,
+    listMetricIds,
+    removeMetric,
+    sample,
+) where
 
-import           Control.Applicative                        ((<$>))
-import           Control.Concurrent.MVar                    (MVar,
-                                                             modifyMVarMasked,
-                                                             newMVar, withMVar)
-import           Data.Tuple                                 (swap)
+import Control.Applicative ((<$>))
+import Control.Concurrent.MVar (
+    MVar,
+    modifyMVarMasked,
+    newMVar,
+    readMVar,
+    withMVar,
+ )
+import Data.Tuple (swap)
 
-import           System.Metrics.Prometheus.Metric.Counter   (Counter)
-import           System.Metrics.Prometheus.Metric.Gauge     (Gauge)
-import           System.Metrics.Prometheus.Metric.Histogram (Histogram,
-                                                             UpperBound)
-import           System.Metrics.Prometheus.MetricId         (Labels, Name)
-import qualified System.Metrics.Prometheus.Registry         as R
+import System.Metrics.Prometheus.Metric.Counter (Counter)
+import System.Metrics.Prometheus.Metric.Gauge (Gauge)
+import System.Metrics.Prometheus.Metric.Histogram (
+    Histogram,
+    UpperBound,
+ )
+import System.Metrics.Prometheus.MetricId (
+    Labels,
+    MetricId,
+    Name,
+ )
+import qualified System.Metrics.Prometheus.Registry as R
 
 
-newtype Registry = Registry { unRegistry :: MVar R.Registry }
+newtype Registry = Registry {unRegistry :: MVar R.Registry}
 
 
 new :: IO Registry
@@ -45,6 +56,16 @@
 registerHistogram name labels buckets = flip modifyMVarMasked register . unRegistry
   where
     register = fmap swap . R.registerHistogram name labels buckets
+
+
+removeMetric :: MetricId -> Registry -> IO ()
+removeMetric i = flip modifyMVarMasked remove . unRegistry
+  where
+    remove reg = pure (R.removeMetric i reg, ())
+
+
+listMetricIds :: Registry -> IO [MetricId]
+listMetricIds = fmap R.listMetricIds . readMVar . unRegistry
 
 
 sample :: Registry -> IO R.RegistrySample
diff --git a/src/System/Metrics/Prometheus/Concurrent/RegistryT.hs b/src/System/Metrics/Prometheus/Concurrent/RegistryT.hs
--- a/src/System/Metrics/Prometheus/Concurrent/RegistryT.hs
+++ b/src/System/Metrics/Prometheus/Concurrent/RegistryT.hs
@@ -2,24 +2,32 @@
 
 module System.Metrics.Prometheus.Concurrent.RegistryT where
 
-import           Control.Applicative                           ((<$>))
-import           Control.Monad.IO.Class                        (MonadIO, liftIO)
-import           Control.Monad.Trans.Class                     (MonadTrans)
-import           Control.Monad.Trans.Reader                    (ReaderT (..),
-                                                                ask)
+import Control.Applicative ((<$>))
+import Control.Monad.IO.Class (MonadIO, liftIO)
+import Control.Monad.Trans.Class (MonadTrans)
+import Control.Monad.Trans.Reader (
+    ReaderT (..),
+    ask,
+ )
 
-import           System.Metrics.Prometheus.Concurrent.Registry (Registry, new)
+import System.Metrics.Prometheus.Concurrent.Registry (Registry, new)
 import qualified System.Metrics.Prometheus.Concurrent.Registry as R
-import           System.Metrics.Prometheus.Metric.Counter      (Counter)
-import           System.Metrics.Prometheus.Metric.Gauge        (Gauge)
-import           System.Metrics.Prometheus.Metric.Histogram    (Histogram)
-import qualified System.Metrics.Prometheus.Metric.Histogram    as Histogram
-import           System.Metrics.Prometheus.MetricId            (Labels, Name)
-import           System.Metrics.Prometheus.Registry            (RegistrySample)
+import System.Metrics.Prometheus.Metric.Counter (Counter)
+import System.Metrics.Prometheus.Metric.Gauge (Gauge)
+import System.Metrics.Prometheus.Metric.Histogram (Histogram)
+import qualified System.Metrics.Prometheus.Metric.Histogram as Histogram
+import System.Metrics.Prometheus.MetricId (
+    Labels,
+    MetricId,
+    Name,
+ )
+import System.Metrics.Prometheus.Registry (
+    RegistrySample,
+    listMetricIds,
+ )
 
 
-newtype RegistryT m a =
-    RegistryT { unRegistryT :: ReaderT Registry m a }
+newtype RegistryT m a = RegistryT {unRegistryT :: ReaderT Registry m a}
     deriving (Monad, MonadTrans, Applicative, Functor, MonadIO)
 
 
@@ -37,6 +45,14 @@
 
 registerHistogram :: MonadIO m => Name -> Labels -> [Histogram.UpperBound] -> RegistryT m Histogram
 registerHistogram n l b = RegistryT ask >>= liftIO . R.registerHistogram n l b
+
+
+removeMetric :: MonadIO m => MetricId -> RegistryT m ()
+removeMetric i = RegistryT ask >>= liftIO . R.removeMetric i
+
+
+listMetricIds :: MonadIO m => RegistryT m [MetricId]
+listMetricIds = RegistryT ask >>= liftIO . R.listMetricIds
 
 
 sample :: Monad m => RegistryT m (IO RegistrySample)
diff --git a/src/System/Metrics/Prometheus/Encode/Text.hs b/src/System/Metrics/Prometheus/Encode/Text.hs
--- a/src/System/Metrics/Prometheus/Encode/Text.hs
+++ b/src/System/Metrics/Prometheus/Encode/Text.hs
@@ -1,45 +1,61 @@
-module System.Metrics.Prometheus.Encode.Text
-       ( encodeMetrics
-       ) where
+module System.Metrics.Prometheus.Encode.Text (
+    encodeMetrics,
+) where
 
-import           Data.ByteString.Builder                         (Builder)
-import           Data.Function                                   (on)
-import           Data.List                                       (groupBy,
-                                                                  intersperse)
-import qualified Data.Map                                        as Map
-import           Data.Monoid                                     ((<>))
+import Data.ByteString.Builder (Builder)
+import Data.Function (on)
+import Data.List (
+    groupBy,
+    intersperse,
+ )
+import qualified Data.Map as Map
+import Data.Monoid ((<>))
 
-import           System.Metrics.Prometheus.Encode.Text.Histogram (encodeHistogram)
-import           System.Metrics.Prometheus.Encode.Text.MetricId  (encodeDouble,
-                                                                  encodeHeader,
-                                                                  encodeInt,
-                                                                  encodeMetricId,
-                                                                  newline,
-                                                                  space)
-import           System.Metrics.Prometheus.Metric                (MetricSample (..),
-                                                                  metricSample)
-import           System.Metrics.Prometheus.Metric.Counter        (CounterSample (..))
-import           System.Metrics.Prometheus.Metric.Gauge          (GaugeSample (..))
-import           System.Metrics.Prometheus.MetricId              (MetricId (..))
-import           System.Metrics.Prometheus.Registry              (RegistrySample (..))
+import System.Metrics.Prometheus.Encode.Text.Histogram (encodeHistogram)
+import System.Metrics.Prometheus.Encode.Text.MetricId (
+    encodeDouble,
+    encodeHeader,
+    encodeInt,
+    encodeMetricId,
+    newline,
+    space,
+ )
+import System.Metrics.Prometheus.Metric (
+    MetricSample (..),
+    metricSample,
+ )
+import System.Metrics.Prometheus.Metric.Counter (CounterSample (..))
+import System.Metrics.Prometheus.Metric.Gauge (GaugeSample (..))
+import System.Metrics.Prometheus.MetricId (MetricId (..))
+import System.Metrics.Prometheus.Registry (RegistrySample (..))
 
 
 encodeMetrics :: RegistrySample -> Builder
-encodeMetrics = (<> newline) . mconcat . intersperse newline . map encodeMetricGroup
-    . groupByName . Map.toList . unRegistrySample
-  where groupByName = groupBy ((==) `on` (name . fst))
+encodeMetrics =
+    (<> newline) . mconcat . intersperse newline . map encodeMetricGroup
+        . groupByName
+        . Map.toList
+        . unRegistrySample
+  where
+    groupByName = groupBy ((==) `on` (name . fst))
 
 
 encodeMetricGroup :: [(MetricId, MetricSample)] -> Builder
-encodeMetricGroup group = encodeHeader mid sample <> newline
-    <> mconcat (intersperse newline $ map encodeMetric group)
+encodeMetricGroup group =
+    encodeHeader mid sample <> newline
+        <> mconcat (intersperse newline $ map encodeMetric group)
   where
     (mid, sample) = head group
 
 
 encodeMetric :: (MetricId, MetricSample) -> Builder
-encodeMetric (mid, sample) = metricSample (encodeCounter mid) (encodeGauge mid)
-    (encodeHistogram mid) (encodeSummary mid) sample
+encodeMetric (mid, sample) =
+    metricSample
+        (encodeCounter mid)
+        (encodeGauge mid)
+        (encodeHistogram mid)
+        (encodeSummary mid)
+        sample
   where
     encodeSummary = undefined
 
diff --git a/src/System/Metrics/Prometheus/Encode/Text/Histogram.hs b/src/System/Metrics/Prometheus/Encode/Text/Histogram.hs
--- a/src/System/Metrics/Prometheus/Encode/Text/Histogram.hs
+++ b/src/System/Metrics/Prometheus/Encode/Text/Histogram.hs
@@ -1,44 +1,61 @@
 {-# LANGUAGE OverloadedStrings #-}
 
-module System.Metrics.Prometheus.Encode.Text.Histogram
-       ( encodeHistogram
-       ) where
-
+module System.Metrics.Prometheus.Encode.Text.Histogram (
+    encodeHistogram,
+) where
 
-import           Data.ByteString.Builder                    (Builder)
-import           Data.List                                  (intersperse)
-import qualified Data.Map                                   as Map
-import           Data.Monoid                                ((<>))
+import Data.ByteString.Builder (Builder)
+import Data.List (intersperse)
+import qualified Data.Map as Map
+import Data.Monoid ((<>))
 
-import           System.Metrics.Prometheus.Encode.Text.MetricId  (encodeDouble,
-                                                             encodeInt,
-                                                             encodeLabels,
-                                                             encodeName,
-                                                             newline, space,
-                                                             textValue)
-import           System.Metrics.Prometheus.Metric.Histogram (HistogramSample (..),
-                                                             UpperBound)
-import           System.Metrics.Prometheus.MetricId         (MetricId (..),
-                                                             addLabel)
+import System.Metrics.Prometheus.Encode.Text.MetricId (
+    encodeDouble,
+    encodeInt,
+    encodeLabels,
+    encodeName,
+    newline,
+    space,
+    textValue,
+ )
+import System.Metrics.Prometheus.Metric.Histogram (
+    HistogramSample (..),
+    UpperBound,
+ )
+import System.Metrics.Prometheus.MetricId (
+    MetricId (..),
+    addLabel,
+ )
 
 
 encodeHistogram :: MetricId -> HistogramSample -> Builder
-encodeHistogram mid histogram
-    =  encodeHistogramBuckets mid histogram <> newline
-    <> n <> "_sum"   <> ls <> space <> encodeDouble (histSum histogram) <> newline
-    <> n <> "_count" <> ls <> space <> encodeInt (histCount histogram)
+encodeHistogram mid histogram =
+    encodeHistogramBuckets mid histogram <> newline
+        <> n
+        <> "_sum"
+        <> ls
+        <> space
+        <> encodeDouble (histSum histogram)
+        <> newline
+        <> n
+        <> "_count"
+        <> ls
+        <> space
+        <> encodeInt (histCount histogram)
   where
     n = encodeName $ name mid
     ls = encodeLabels $ labels mid
 
 
 encodeHistogramBuckets :: MetricId -> HistogramSample -> Builder
-encodeHistogramBuckets mid = mconcat . intersperse newline . map snd . Map.toList
-    . Map.mapWithKey (encodeHistogramBucket mid) . histBuckets
+encodeHistogramBuckets mid =
+    mconcat . intersperse newline . map snd . Map.toList
+        . Map.mapWithKey (encodeHistogramBucket mid)
+        . histBuckets
 
 
 encodeHistogramBucket :: MetricId -> UpperBound -> Double -> Builder
-encodeHistogramBucket mid upperBound count
-    = encodeName (name mid) <> "_bucket" <> encodeLabels labels' <> space <> encodeDouble count
+encodeHistogramBucket mid upperBound count =
+    encodeName (name mid) <> "_bucket" <> encodeLabels labels' <> space <> encodeDouble count
   where
     labels' = addLabel "le" (textValue upperBound) (labels mid)
diff --git a/src/System/Metrics/Prometheus/Encode/Text/MetricId.hs b/src/System/Metrics/Prometheus/Encode/Text/MetricId.hs
--- a/src/System/Metrics/Prometheus/Encode/Text/MetricId.hs
+++ b/src/System/Metrics/Prometheus/Encode/Text/MetricId.hs
@@ -1,46 +1,65 @@
 {-# LANGUAGE OverloadedStrings #-}
 
-module System.Metrics.Prometheus.Encode.Text.MetricId
-       ( encodeHeader
-       , encodeMetricId
-       , encodeLabels
-       , encodeName
-       , textValue
-       , encodeDouble
-       , encodeInt
-       , escape
-       , newline
-       , space
-       ) where
+module System.Metrics.Prometheus.Encode.Text.MetricId (
+    encodeHeader,
+    encodeMetricId,
+    encodeLabels,
+    encodeName,
+    textValue,
+    encodeDouble,
+    encodeInt,
+    escape,
+    newline,
+    space,
+) where
 
-import           Data.ByteString.Builder            (Builder, byteString, char8,
-                                                     intDec)
-import           Data.List                          (intersperse)
-import           Data.Monoid                        ((<>))
-import           Data.Text                          (Text, replace)
-import           Data.Text.Encoding                 (encodeUtf8)
-import           Data.Text.Lazy                     (toStrict)
-import           Data.Text.Lazy.Builder             (toLazyText)
-import           Data.Text.Lazy.Builder.RealFloat   (FPFormat (Generic),
-                                                     formatRealFloat)
-import           Prelude                            hiding (null)
+import Data.ByteString.Builder (
+    Builder,
+    byteString,
+    char8,
+    intDec,
+ )
+import Data.List (intersperse)
+import Data.Monoid ((<>))
+import Data.Text (Text, replace)
+import Data.Text.Encoding (encodeUtf8)
+import Data.Text.Lazy (toStrict)
+import Data.Text.Lazy.Builder (toLazyText)
+import Data.Text.Lazy.Builder.RealFloat (
+    FPFormat (Generic),
+    formatRealFloat,
+ )
+import Prelude hiding (null)
 
-import           System.Metrics.Prometheus.Metric   (MetricSample (..),
-                                                     metricSample)
-import           System.Metrics.Prometheus.MetricId (Labels (..), MetricId (..),
-                                                     Name (..), null, toList)
+import System.Metrics.Prometheus.Metric (
+    MetricSample (..),
+    metricSample,
+ )
+import System.Metrics.Prometheus.MetricId (
+    Labels (..),
+    MetricId (..),
+    Name (..),
+    null,
+    toList,
+ )
 
 
 encodeHeader :: MetricId -> MetricSample -> Builder
-encodeHeader mid sample
-    = "# TYPE " <> nm <> space <> encodeSampleType sample
+encodeHeader mid sample =
+    "# TYPE " <> nm <> space <> encodeSampleType sample
+  where
     -- <> "# HELP " <> nm <> space <> escape "help" <> newline <>
-  where nm = encodeName (name mid)
+    nm = encodeName (name mid)
 
 
 encodeSampleType :: MetricSample -> Builder
-encodeSampleType = byteString . metricSample (const "counter")
-    (const "gauge") (const "histogram") (const "summary")
+encodeSampleType =
+    byteString
+        . metricSample
+            (const "counter")
+            (const "gauge")
+            (const "histogram")
+            (const "summary")
 
 
 encodeMetricId :: MetricId -> Builder
@@ -55,9 +74,9 @@
 encodeLabels ls
     | null ls = space
     | otherwise =
-             openBracket
-          <> (mconcat . intersperse comma . map encodeLabel $ toList ls)
-          <> closeBracket
+        openBracket
+            <> (mconcat . intersperse comma . map encodeLabel $ toList ls)
+            <> closeBracket
 
 
 encodeLabel :: (Text, Text) -> Builder
@@ -65,10 +84,11 @@
 
 
 textValue :: RealFloat f => f -> Text
-textValue x | isInfinite x && x > 0 = "+Inf"
-            | isInfinite x && x < 0 = "-Inf"
-            | isNaN x = "NaN"
-            | otherwise = toStrict . toLazyText $ formatRealFloat Generic Nothing x
+textValue x
+    | isInfinite x && x > 0 = "+Inf"
+    | isInfinite x && x < 0 = "-Inf"
+    | isNaN x = "NaN"
+    | otherwise = toStrict . toLazyText $ formatRealFloat Generic Nothing x
 
 
 encodeDouble :: RealFloat f => f -> Builder
diff --git a/src/System/Metrics/Prometheus/Http/Push.hs b/src/System/Metrics/Prometheus/Http/Push.hs
--- a/src/System/Metrics/Prometheus/Http/Push.hs
+++ b/src/System/Metrics/Prometheus/Http/Push.hs
@@ -1,33 +1,38 @@
 {-# LANGUAGE OverloadedStrings #-}
 
-module System.Metrics.Prometheus.Http.Push
-       ( pushMetrics
-       , parseURI
-       )
-       where
+module System.Metrics.Prometheus.Http.Push (
+    pushMetrics,
+    parseURI,
+) where
 
-import           Control.Concurrent                    (threadDelay)
-import           Control.Monad                         (forever)
-import           Data.ByteString.Builder               (toLazyByteString)
-import           Data.Map                              (foldMapWithKey)
-import           Data.Text                             (Text, unpack)
-import           Network.HTTP.Client                   (Request (..),
-                                                        RequestBody (..),
-                                                        getUri,
-                                                        httpNoBody,
-                                                        parseRequest,
-                                                        requestBody,
-                                                        requestFromURI,
-                                                        requestHeaders)
-import           Network.HTTP.Types                    (hContentType, methodPut)
-import           Network.HTTP.Client.TLS               (newTlsManager)
-import           Network.URI                           (URI (..), URIAuth,
-                                                        nullURI)
+import Control.Concurrent (threadDelay)
+import Control.Monad (forever)
+import Data.ByteString.Builder (toLazyByteString)
+import Data.Map (foldMapWithKey)
+import Data.Text (Text, unpack)
+import Network.HTTP.Client (
+    Request (..),
+    RequestBody (..),
+    getUri,
+    httpNoBody,
+    parseRequest,
+    requestBody,
+    requestFromURI,
+    requestHeaders,
+ )
+import Network.HTTP.Client.TLS (newTlsManager)
+import Network.HTTP.Types (hContentType, methodPut)
+import Network.URI (
+    URI (..),
+    URIAuth,
+    nullURI,
+ )
 
-import           System.Metrics.Prometheus.Encode.Text (encodeMetrics)
-import           System.Metrics.Prometheus.MetricId    (Labels (..))
-import           System.Metrics.Prometheus.Registry    (RegistrySample)
+import System.Metrics.Prometheus.Encode.Text (encodeMetrics)
+import System.Metrics.Prometheus.MetricId (Labels (..))
+import System.Metrics.Prometheus.Registry (RegistrySample)
 
+
 -- | Parses a uri such that
 -- @
 --   parseURI "https://example.com"
@@ -37,29 +42,40 @@
 parseURI :: String -> Maybe URI
 parseURI = fmap getUri . parseRequest
 
-pushMetrics :: URI               -- ^ PushGateway URI name, including port number (ex: @parseUri https://myGateway.com:8080@)
-            -> Text              -- ^ Job name
-            -> Labels            -- ^ Label set to use as a grouping key for metrics
-            -> Int               -- ^ Microsecond push frequency
-            -> IO RegistrySample -- ^ Action to get latest metrics
-            -> IO ()
+
+pushMetrics ::
+    -- | PushGateway URI name, including port number (ex: @parseUri https://myGateway.com:8080@)
+    URI ->
+    -- | Job name
+    Text ->
+    -- | Label set to use as a grouping key for metrics
+    Labels ->
+    -- | Microsecond push frequency
+    Int ->
+    -- | Action to get latest metrics
+    IO RegistrySample ->
+    IO ()
 pushMetrics gatewayURI jobName labels frequencyMicros getSample = do
-    manager    <- newTlsManager
-    gn         <- maybe (error "Invalid URI Authority") pure gatewayName
+    manager <- newTlsManager
+    gn <- maybe (error "Invalid URI Authority") pure gatewayName
     requestUri <- requestFromURI $ buildUri scheme gn jobName labels
     forever $ getSample >>= flip httpNoBody manager . request requestUri >> threadDelay frequencyMicros
   where
     URI scheme gatewayName _ _ _ = gatewayURI
-    request req sample = req
-        { method         = methodPut
-        , requestBody    = RequestBodyLBS . toLazyByteString $ encodeMetrics sample
-        , requestHeaders = [(hContentType, "text/plain; version=0.0.4")]
-        }
+    request req sample =
+        req
+            { method = methodPut
+            , requestBody = RequestBodyLBS . toLazyByteString $ encodeMetrics sample
+            , requestHeaders = [(hContentType, "text/plain; version=0.0.4")]
+            }
 
+
 buildUri :: String -> URIAuth -> Text -> Labels -> URI
-buildUri scheme gatewayName jobName (Labels ls) = nullURI
-    { uriScheme    = scheme
-    , uriAuthority = Just gatewayName
-    , uriPath      = "/metrics/job/" ++ unpack jobName ++ foldMapWithKey labelPath ls
-    }
-  where labelPath k v = "/" ++ unpack k ++ "/" ++ unpack v
+buildUri scheme gatewayName jobName (Labels ls) =
+    nullURI
+        { uriScheme = scheme
+        , uriAuthority = Just gatewayName
+        , uriPath = "/metrics/job/" ++ unpack jobName ++ foldMapWithKey labelPath ls
+        }
+  where
+    labelPath k v = "/" ++ unpack k ++ "/" ++ unpack v
diff --git a/src/System/Metrics/Prometheus/Http/Scrape.hs b/src/System/Metrics/Prometheus/Http/Scrape.hs
--- a/src/System/Metrics/Prometheus/Http/Scrape.hs
+++ b/src/System/Metrics/Prometheus/Http/Scrape.hs
@@ -1,34 +1,41 @@
 {-# LANGUAGE OverloadedStrings #-}
 
-module System.Metrics.Prometheus.Http.Scrape
-       ( Path
-       , serveMetrics
-       , serveMetricsT
-       , prometheusApp
-       )
-       where
+module System.Metrics.Prometheus.Http.Scrape (
+    Path,
+    serveMetrics,
+    serveMetricsT,
+    prometheusApp,
+) where
 
-import           Control.Applicative                            ((<$>))
-import           Control.Monad.IO.Class                         (MonadIO,
-                                                                 liftIO)
-import           Data.Text                                      (Text)
-import           Network.HTTP.Types                             (hContentType,
-                                                                 methodGet,
-                                                                 status200,
-                                                                 status404)
-import           Network.Wai                                    (Application,
-                                                                 Request,
-                                                                 Response,
-                                                                 pathInfo,
-                                                                 requestMethod,
-                                                                 responseBuilder,
-                                                                 responseLBS)
-import           Network.Wai.Handler.Warp                       (Port, run)
+import Control.Applicative ((<$>))
+import Control.Monad.IO.Class (
+    MonadIO,
+    liftIO,
+ )
+import Data.Text (Text)
+import Network.HTTP.Types (
+    hContentType,
+    methodGet,
+    status200,
+    status404,
+ )
+import Network.Wai (
+    Application,
+    Request,
+    Response,
+    pathInfo,
+    requestMethod,
+    responseBuilder,
+    responseLBS,
+ )
+import Network.Wai.Handler.Warp (Port, run)
 
-import           System.Metrics.Prometheus.Concurrent.RegistryT (RegistryT,
-                                                                 sample)
-import           System.Metrics.Prometheus.Encode.Text          (encodeMetrics)
-import           System.Metrics.Prometheus.Registry             (RegistrySample)
+import System.Metrics.Prometheus.Concurrent.RegistryT (
+    RegistryT,
+    sample,
+ )
+import System.Metrics.Prometheus.Encode.Text (encodeMetrics)
+import System.Metrics.Prometheus.Registry (RegistrySample)
 
 
 -- | The HTTP web route on which to serve data
diff --git a/src/System/Metrics/Prometheus/Metric.hs b/src/System/Metrics/Prometheus/Metric.hs
--- a/src/System/Metrics/Prometheus/Metric.hs
+++ b/src/System/Metrics/Prometheus/Metric.hs
@@ -1,18 +1,22 @@
 module System.Metrics.Prometheus.Metric where
 
-import           System.Metrics.Prometheus.Metric.Counter   (Counter,
-                                                             CounterSample)
-import           System.Metrics.Prometheus.Metric.Gauge     (Gauge, GaugeSample)
-import           System.Metrics.Prometheus.Metric.Histogram (Histogram,
-                                                             HistogramSample)
-import           System.Metrics.Prometheus.Metric.Summary   (SummarySample)
+import System.Metrics.Prometheus.Metric.Counter (
+    Counter,
+    CounterSample,
+ )
+import System.Metrics.Prometheus.Metric.Gauge (Gauge, GaugeSample)
+import System.Metrics.Prometheus.Metric.Histogram (
+    Histogram,
+    HistogramSample,
+ )
+import System.Metrics.Prometheus.Metric.Summary (SummarySample)
 
 
 data Metric
     = CounterMetric Counter
     | GaugeMetric Gauge
-    -- | Summary S.Summary
-    | HistogramMetric Histogram
+    | -- | Summary S.Summary
+      HistogramMetric Histogram
 
 
 data MetricSample
@@ -22,10 +26,14 @@
     | SummaryMetricSample SummarySample
 
 
-metricSample :: (CounterSample -> a) -> (GaugeSample -> a)
-             -> (HistogramSample -> a) -> (SummarySample -> a)
-             -> MetricSample -> a
-metricSample f _ _ _ (CounterMetricSample s)   = f s
-metricSample _ f _ _ (GaugeMetricSample s)     = f s
+metricSample ::
+    (CounterSample -> a) ->
+    (GaugeSample -> a) ->
+    (HistogramSample -> a) ->
+    (SummarySample -> a) ->
+    MetricSample ->
+    a
+metricSample f _ _ _ (CounterMetricSample s) = f s
+metricSample _ f _ _ (GaugeMetricSample s) = f s
 metricSample _ _ f _ (HistogramMetricSample s) = f s
-metricSample _ _ _ f (SummaryMetricSample s)   = f s
+metricSample _ _ _ f (SummaryMetricSample s) = f s
diff --git a/src/System/Metrics/Prometheus/Metric/Counter.hs b/src/System/Metrics/Prometheus/Metric/Counter.hs
--- a/src/System/Metrics/Prometheus/Metric/Counter.hs
+++ b/src/System/Metrics/Prometheus/Metric/Counter.hs
@@ -1,20 +1,21 @@
-module System.Metrics.Prometheus.Metric.Counter
-       ( Counter
-       , CounterSample (..)
-       , new
-       , add
-       , inc
-       , sample
-       , addAndSample
-       ) where
-
+module System.Metrics.Prometheus.Metric.Counter (
+    Counter,
+    CounterSample (..),
+    new,
+    add,
+    inc,
+    sample,
+    addAndSample,
+    set,
+) where
 
-import           Control.Applicative  ((<$>))
-import           Data.Atomics.Counter (AtomicCounter, incrCounter, newCounter)
+import Control.Applicative ((<$>))
+import Control.Monad (when)
+import Data.Atomics.Counter (AtomicCounter, incrCounter, newCounter, writeCounter)
 
 
-newtype Counter = Counter { unCounter :: AtomicCounter }
-newtype CounterSample = CounterSample { unCounterSample :: Int }
+newtype Counter = Counter {unCounter :: AtomicCounter}
+newtype CounterSample = CounterSample {unCounterSample :: Int}
 
 
 new :: IO Counter
@@ -22,8 +23,9 @@
 
 
 addAndSample :: Int -> Counter -> IO CounterSample
-addAndSample by | by >= 0   = fmap CounterSample . incrCounter by . unCounter
-                | otherwise = error "must be >= 0"
+addAndSample by
+    | by >= 0 = fmap CounterSample . incrCounter by . unCounter
+    | otherwise = error "must be >= 0"
 
 
 add :: Int -> Counter -> IO ()
@@ -36,3 +38,15 @@
 
 sample :: Counter -> IO CounterSample
 sample = addAndSample 0
+
+
+-- | Write @i@ to the counter, if @i@ is more than the current value. This is
+-- useful for when the count is maintained by a separate system (e.g. GHC's GC
+-- counter).
+--
+-- WARNING: For multiple writers, the most recent one wins, which may not
+-- preserve the increasing property. If you have stronger requirements than this,
+-- please check with the maintainers. 
+-- See <https://github.com/bitnomial/prometheus/pull/44> for discussion.
+set :: Int -> Counter -> IO ()
+set i (Counter c) = writeCounter c i
diff --git a/src/System/Metrics/Prometheus/Metric/Gauge.hs b/src/System/Metrics/Prometheus/Metric/Gauge.hs
--- a/src/System/Metrics/Prometheus/Metric/Gauge.hs
+++ b/src/System/Metrics/Prometheus/Metric/Gauge.hs
@@ -1,22 +1,22 @@
-module System.Metrics.Prometheus.Metric.Gauge
-       ( Gauge
-       , GaugeSample (..)
-       , new
-       , add
-       , sub
-       , inc
-       , dec
-       , set
-       , sample
-       , modifyAndSample
-       ) where
+module System.Metrics.Prometheus.Metric.Gauge (
+    Gauge,
+    GaugeSample (..),
+    new,
+    add,
+    sub,
+    inc,
+    dec,
+    set,
+    sample,
+    modifyAndSample,
+) where
 
-import           Control.Applicative ((<$>))
-import           Data.IORef          (IORef, atomicModifyIORef', newIORef)
+import Control.Applicative ((<$>))
+import Data.IORef (IORef, atomicModifyIORef', newIORef)
 
 
-newtype Gauge = Gauge { unGauge :: IORef Double }
-newtype GaugeSample = GaugeSample { unGaugeSample :: Double }
+newtype Gauge = Gauge {unGauge :: IORef Double}
+newtype GaugeSample = GaugeSample {unGaugeSample :: Double}
 
 
 new :: IO Gauge
@@ -25,7 +25,8 @@
 
 modifyAndSample :: (Double -> Double) -> Gauge -> IO GaugeSample
 modifyAndSample f = flip atomicModifyIORef' g . unGauge
-  where g v = (f v, GaugeSample $ f v)
+  where
+    g v = (f v, GaugeSample $ f v)
 
 
 add :: Double -> Gauge -> IO ()
diff --git a/src/System/Metrics/Prometheus/Metric/Histogram.hs b/src/System/Metrics/Prometheus/Metric/Histogram.hs
--- a/src/System/Metrics/Prometheus/Metric/Histogram.hs
+++ b/src/System/Metrics/Prometheus/Metric/Histogram.hs
@@ -1,45 +1,47 @@
 {-# LANGUAGE TupleSections #-}
 
-module System.Metrics.Prometheus.Metric.Histogram
-       ( Histogram
-       , HistogramSample (..)
-       , Buckets
-       , UpperBound
-       , new
-       , observe
-       , sample
-       , observeAndSample
-       ) where
-
+module System.Metrics.Prometheus.Metric.Histogram (
+    Histogram,
+    HistogramSample (..),
+    Buckets,
+    UpperBound,
+    new,
+    observe,
+    sample,
+    observeAndSample,
+) where
 
-import           Control.Applicative ((<$>))
-import           Control.Monad       (void)
-import           Data.Bool           (bool)
-import           Data.IORef          (IORef, atomicModifyIORef', newIORef,
-                                      readIORef)
-import           Data.Map.Strict     (Map)
-import qualified Data.Map.Strict     as Map
+import Control.Applicative ((<$>))
+import Control.Monad (void)
+import Data.Bool (bool)
+import Data.IORef (
+    IORef,
+    atomicModifyIORef',
+    newIORef,
+    readIORef,
+ )
+import Data.Map.Strict (Map)
+import qualified Data.Map.Strict as Map
 
 
-newtype Histogram = Histogram { unHistogram :: IORef HistogramSample }
+newtype Histogram = Histogram {unHistogram :: IORef HistogramSample}
 
 
 type UpperBound = Double -- Inclusive upper bounds
 type Buckets = Map UpperBound Double
 
 
-data HistogramSample =
-    HistogramSample
+data HistogramSample = HistogramSample
     { histBuckets :: !Buckets
-    , histSum     :: !Double
-    , histCount   :: !Int
+    , histSum :: !Double
+    , histCount :: !Int
     }
 
 
 new :: [UpperBound] -> IO Histogram
 new buckets = Histogram <$> newIORef empty
   where
-    empty = HistogramSample (Map.fromList $ map (, 0) (read "Infinity" : buckets)) zeroSum zeroCount
+    empty = HistogramSample (Map.fromList $ map (,0) (read "Infinity" : buckets)) zeroSum zeroCount
     zeroSum = 0.0
     zeroCount = 0
 
@@ -49,10 +51,11 @@
   where
     update histData = (hist' histData, histData)
     hist' histData =
-        histData { histBuckets = updateBuckets x $ histBuckets histData
-                 , histSum = histSum histData + x
-                 , histCount = histCount histData + 1
-                 }
+        histData
+            { histBuckets = updateBuckets x $ histBuckets histData
+            , histSum = histSum histData + x
+            , histCount = histCount histData + 1
+            }
 
 
 observe :: Double -> Histogram -> IO ()
@@ -61,7 +64,8 @@
 
 updateBuckets :: Double -> Buckets -> Buckets
 updateBuckets x = Map.mapWithKey updateBucket
-  where updateBucket key val = bool val (val + 1) (x <= key)
+  where
+    updateBucket key val = bool val (val + 1) (x <= key)
 
 
 sample :: Histogram -> IO HistogramSample
diff --git a/src/System/Metrics/Prometheus/Metric/Summary.hs b/src/System/Metrics/Prometheus/Metric/Summary.hs
--- a/src/System/Metrics/Prometheus/Metric/Summary.hs
+++ b/src/System/Metrics/Prometheus/Metric/Summary.hs
@@ -1,10 +1,10 @@
 module System.Metrics.Prometheus.Metric.Summary where
 
-import           Data.Map.Strict (Map)
+import Data.Map.Strict (Map)
 
-data SummarySample =
-    SummarySample
+
+data SummarySample = SummarySample
     { sumQuantiles :: !(Map Double Int)
-    , sumSum       :: !Int
-    , sumCount     :: !Int
+    , sumSum :: !Int
+    , sumCount :: !Int
     }
diff --git a/src/System/Metrics/Prometheus/MetricId.hs b/src/System/Metrics/Prometheus/MetricId.hs
--- a/src/System/Metrics/Prometheus/MetricId.hs
+++ b/src/System/Metrics/Prometheus/MetricId.hs
@@ -1,33 +1,36 @@
-{-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE OverloadedStrings #-}
 
 module System.Metrics.Prometheus.MetricId where
 
-import           Data.Char (isDigit)
-import           Data.Bifunctor (first)
-import           Data.Map       (Map)
-import qualified Data.Map       as Map
-import           Data.Monoid    (Monoid)
-import           Data.Semigroup (Semigroup)
-import           Data.String    (IsString(..))
-import           Data.Text      (Text)
+import Data.Bifunctor (first)
+import Data.Char (isDigit)
+import Data.Map (Map)
+import qualified Data.Map as Map
+import Data.Monoid (Monoid)
+import Data.Semigroup (Semigroup)
+import Data.String (IsString (..))
+import Data.Text (Text)
 import qualified Data.Text as Text
-import           Prelude        hiding (null)
+import Prelude hiding (null)
 
+
 -- | Construct with 'makeName' to ensure that names use only valid characters
-newtype Name = Name { unName :: Text } deriving (Show, Eq, Ord, Monoid, Semigroup)
+newtype Name = Name {unName :: Text} deriving (Show, Eq, Ord, Monoid, Semigroup)
 
+
 instance IsString Name where
-  fromString = makeName . Text.pack
+    fromString = makeName . Text.pack
 
-newtype Labels = Labels { unLabels :: Map Text Text } deriving (Show, Eq, Ord, Monoid, Semigroup)
 
+newtype Labels = Labels {unLabels :: Map Text Text} deriving (Show, Eq, Ord, Monoid, Semigroup)
 
-data MetricId =
-    MetricId
-    { name   :: Name
+
+data MetricId = MetricId
+    { name :: Name
     , labels :: Labels
-    } deriving (Eq, Ord, Show)
+    }
+    deriving (Eq, Ord, Show)
 
 
 addLabel :: Text -> Text -> Labels -> Labels
@@ -53,7 +56,7 @@
 -- first character is only valid as a later character.
 makeValid :: Text -> Text
 makeValid "" = "_"
-makeValid txt = prefix_ <> Text.map (\c -> if allowedChar c then c else '_' ) txt
+makeValid txt = prefix_ <> Text.map (\c -> if allowedChar c then c else '_') txt
   where
     prefix_ = if isDigit (Text.head txt) then "_" else ""
     allowedChar :: Char -> Bool
diff --git a/src/System/Metrics/Prometheus/Registry.hs b/src/System/Metrics/Prometheus/Registry.hs
--- a/src/System/Metrics/Prometheus/Registry.hs
+++ b/src/System/Metrics/Prometheus/Registry.hs
@@ -1,37 +1,47 @@
 {-# LANGUAGE DeriveDataTypeable #-}
 
-module System.Metrics.Prometheus.Registry
-       ( Registry
-       , RegistrySample (..)
-       , new
-       , registerCounter
-       , registerGauge
-       , registerHistogram
-       , sample
-       ) where
+module System.Metrics.Prometheus.Registry (
+    Registry,
+    RegistrySample (..),
+    new,
+    registerCounter,
+    registerGauge,
+    registerHistogram,
+    listMetricIds,
+    removeMetric,
+    sample,
+) where
 
-import           Control.Applicative                        ((<$>))
-import           Control.Exception                          (Exception, throw)
-import           Data.Map                                   (Map)
-import qualified Data.Map                                   as Map
-import           Data.Typeable                              (Typeable)
+import Control.Applicative ((<$>))
+import Control.Exception (Exception, throw)
+import Data.Map (Map)
+import qualified Data.Map as Map
+import Data.Typeable (Typeable)
 
-import           System.Metrics.Prometheus.Metric           (Metric (..),
-                                                             MetricSample (..))
-import           System.Metrics.Prometheus.Metric.Counter   (Counter)
-import qualified System.Metrics.Prometheus.Metric.Counter   as Counter
-import           System.Metrics.Prometheus.Metric.Gauge     (Gauge)
-import qualified System.Metrics.Prometheus.Metric.Gauge     as Gauge
-import           System.Metrics.Prometheus.Metric.Histogram (Histogram,
-                                                             UpperBound)
+import System.Metrics.Prometheus.Metric (
+    Metric (..),
+    MetricSample (..),
+ )
+import System.Metrics.Prometheus.Metric.Counter (Counter)
+import qualified System.Metrics.Prometheus.Metric.Counter as Counter
+import System.Metrics.Prometheus.Metric.Gauge (Gauge)
+import qualified System.Metrics.Prometheus.Metric.Gauge as Gauge
+import System.Metrics.Prometheus.Metric.Histogram (
+    Histogram,
+    UpperBound,
+ )
 import qualified System.Metrics.Prometheus.Metric.Histogram as Histogram
-import           System.Metrics.Prometheus.MetricId         (Labels (..),
-                                                             MetricId (MetricId),
-                                                             Name (..))
+import System.Metrics.Prometheus.MetricId (
+    Labels (..),
+    MetricId (MetricId),
+    Name (..),
+ )
 
-newtype Registry = Registry { unRegistry :: Map MetricId Metric }
-newtype RegistrySample = RegistrySample { unRegistrySample :: Map MetricId MetricSample }
 
+newtype Registry = Registry {unRegistry :: Map MetricId Metric}
+newtype RegistrySample = RegistrySample {unRegistrySample :: Map MetricId MetricSample}
+
+
 newtype KeyError = KeyError MetricId deriving (Show, Typeable)
 instance Exception KeyError
 
@@ -45,8 +55,8 @@
     counter <- Counter.new
     return (counter, Registry $ Map.insertWithKey collision mid (CounterMetric counter) (unRegistry registry))
   where
-      mid = MetricId name labels
-      collision k _ _ = throw (KeyError k)
+    mid = MetricId name labels
+    collision k _ _ = throw (KeyError k)
 
 
 registerGauge :: Name -> Labels -> Registry -> IO (Gauge, Registry)
@@ -54,8 +64,8 @@
     gauge <- Gauge.new
     return (gauge, Registry $ Map.insertWithKey collision mid (GaugeMetric gauge) (unRegistry registry))
   where
-      mid = MetricId name labels
-      collision k _ _ = throw (KeyError k)
+    mid = MetricId name labels
+    collision k _ _ = throw (KeyError k)
 
 
 registerHistogram :: Name -> Labels -> [UpperBound] -> Registry -> IO (Histogram, Registry)
@@ -63,8 +73,16 @@
     histogram <- Histogram.new buckets
     return (histogram, Registry $ Map.insertWithKey collision mid (HistogramMetric histogram) (unRegistry registry))
   where
-      mid = MetricId name labels
-      collision k _ _ = throw (KeyError k)
+    mid = MetricId name labels
+    collision k _ _ = throw (KeyError k)
+
+
+removeMetric :: MetricId -> Registry -> Registry
+removeMetric i (Registry m) = Registry . Map.delete i $ m
+
+
+listMetricIds :: Registry -> [MetricId]
+listMetricIds = Map.keys . unRegistry
 
 
 sample :: Registry -> IO RegistrySample
diff --git a/src/System/Metrics/Prometheus/RegistryT.hs b/src/System/Metrics/Prometheus/RegistryT.hs
--- a/src/System/Metrics/Prometheus/RegistryT.hs
+++ b/src/System/Metrics/Prometheus/RegistryT.hs
@@ -2,26 +2,34 @@
 
 module System.Metrics.Prometheus.RegistryT where
 
-import           Control.Applicative                        ((<$>))
-import           Control.Monad.IO.Class                     (MonadIO, liftIO)
-import           Control.Monad.Trans.Class                  (MonadTrans)
-import           Control.Monad.Trans.State.Strict           (StateT (..),
-                                                             evalStateT,
-                                                             execStateT, get)
+import Control.Applicative ((<$>))
+import Control.Monad.IO.Class (MonadIO, liftIO)
+import Control.Monad.Trans.Class (MonadTrans)
+import Control.Monad.Trans.State.Strict (
+    StateT (..),
+    evalStateT,
+    execStateT,
+    get,
+ )
 
-import           System.Metrics.Prometheus.Metric.Counter   (Counter)
-import           System.Metrics.Prometheus.Metric.Gauge     (Gauge)
-import           System.Metrics.Prometheus.Metric.Histogram (Histogram)
+import System.Metrics.Prometheus.Metric.Counter (Counter)
+import System.Metrics.Prometheus.Metric.Gauge (Gauge)
+import System.Metrics.Prometheus.Metric.Histogram (Histogram)
 import qualified System.Metrics.Prometheus.Metric.Histogram as Histogram
-import           System.Metrics.Prometheus.MetricId         (Labels, Name)
-import           System.Metrics.Prometheus.Registry         (Registry,
-                                                             RegistrySample,
-                                                             new)
-import qualified System.Metrics.Prometheus.Registry         as R
+import System.Metrics.Prometheus.MetricId (
+    Labels,
+    MetricId,
+    Name,
+ )
+import System.Metrics.Prometheus.Registry (
+    Registry,
+    RegistrySample,
+    new,
+ )
+import qualified System.Metrics.Prometheus.Registry as R
 
 
-newtype RegistryT m a =
-    RegistryT { unRegistryT :: StateT Registry m a }
+newtype RegistryT m a = RegistryT {unRegistryT :: StateT Registry m a}
     deriving (Monad, MonadTrans, Applicative, Functor, MonadIO)
 
 
@@ -51,6 +59,14 @@
 
 registerHistogram :: MonadIO m => Name -> Labels -> [Histogram.UpperBound] -> RegistryT m Histogram
 registerHistogram n l u = withRegistry (liftIO . R.registerHistogram n l u)
+
+
+removeMetric :: MonadIO m => MetricId -> RegistryT m ()
+removeMetric i = withRegistry (pure . (,) () . R.removeMetric i)
+
+
+listMetricIds :: MonadIO m => RegistryT m [MetricId]
+listMetricIds = R.listMetricIds <$> RegistryT get
 
 
 sample :: Monad m => RegistryT m (IO RegistrySample)