diff --git a/Changelog.md b/Changelog.md
--- a/Changelog.md
+++ b/Changelog.md
@@ -1,3 +1,7 @@
+### 0.5.8.0
+
+- Using *Data.HashSet* for *pcGauges* and *pcScale1000* in Prometheus metrics.
+
 ### 0.5.7.0
 
 - Show histogram *error* counters in parameterized Prometheus metrics.
diff --git a/NgxExport/Tools/Prometheus.hs b/NgxExport/Tools/Prometheus.hs
--- a/NgxExport/Tools/Prometheus.hs
+++ b/NgxExport/Tools/Prometheus.hs
@@ -37,6 +37,8 @@
 
 import           Data.Map.Strict (Map)
 import qualified Data.Map.Strict as M
+import           Data.HashSet (HashSet)
+import qualified Data.HashSet as HS
 import           Data.ByteString (ByteString)
 import qualified Data.ByteString.Char8 as C8
 import qualified Data.ByteString.Lazy as L
@@ -72,8 +74,8 @@
 
 data PrometheusConf =
     PrometheusConf { pcMetrics :: Map MetricsName MetricsHelp
-                   , pcGauges :: [MetricsName]
-                   , pcScale1000 :: [MetricsName]
+                   , pcGauges :: HashSet MetricsName
+                   , pcScale1000 :: HashSet MetricsName
                    } deriving Read
 
 data HistogramLayout =
@@ -187,8 +189,10 @@
 --                     ,(\"cnt_uptime_reload\", \"Nginx master uptime after reload\")
 --                     ,(\"hst_request_time\", \"Request duration\")
 --                     ]
---                 , __/pcGauges/__ = [\"cnt_stub_status_active\"]
---                 , __/pcScale1000/__ = [\"hst_request_time_sum\"]
+--                 , __/pcGauges/__ = fromList
+--                     [\"cnt_stub_status_active\"]
+--                 , __/pcScale1000/__ = fromList
+--                     [\"hst_request_time_sum\"]
 --                 }';
 --
 --     haskell_var_empty_on_error $hs_prom_metrics;
@@ -440,7 +444,7 @@
 toPrometheusMetrics' :: PrometheusConf -> AllMetrtics -> PrometheusMetrics
 toPrometheusMetrics' PrometheusConf {..} (srv, cnts, hs, ocnts) =
     let toValues = M.mapWithKey
-            (\k v -> (if k `elem` pcScale1000
+            (\k v -> (if k `HS.member` pcScale1000
                           then (/ 1000)
                           else id
                      ) $ fromIntegral v
@@ -476,7 +480,7 @@
                                      in ((k == v1 || k == v2) :)
                                 ) [] ks
                       )
-          gCounter = const . flip elem pcGauges
+          gCounter = const . flip HS.member pcGauges
           toHistogram cs hk rs =
               let ranges = M.mapWithKey
                       (\k ->
@@ -518,10 +522,8 @@
           renameErrCounter k =
               let s = "_err"
                   (b, (a, e)) =
-                      second (\v -> if s `T.isSuffixOf` v
-                                        then (fromJust $ T.stripSuffix s v, s)
-                                        else (v, "")
-                             ) $ T.breakOn "@" k
+                      second (\v -> maybe (v, "") (, s) $ T.stripSuffix s v) $
+                          T.breakOn "@" k
               in T.concat [b, e, a]
 
 showPrometheusMetrics :: PrometheusMetrics -> L.ByteString
@@ -745,10 +747,12 @@
 --                     ,(\"__/hst_u_response_time/__\"
 --                      ,\"Response time from all servers in a single upstream\")
 --                     ]
---                 , __/pcGauges/__ = [\"cnt_stub_status_active\"]
---                 , __/pcScale1000/__ = [\"hst_request_time_sum\"
---                                 ,\"__/hst_u_response_time_sum/__\"
---                                 ]
+--                 , __/pcGauges/__ = fromList
+--                     [\"cnt_stub_status_active\"]
+--                 , __/pcScale1000/__ = fromList
+--                     [\"hst_request_time_sum\"
+--                     ,\"__/hst_u_response_time_sum/__\"
+--                     ]
 --                 }\';
 -- @
 -- @
@@ -921,10 +925,12 @@
 --                     ,(\"cnt_uptime_reload\", \"Nginx master uptime after reload\")
 --                     ,(\"__/hst_request_time/__\", \"Request duration\")
 --                     ]
---                 , pcGauges = [\"cnt_stub_status_active\"]
---                 , pcScale1000 = [\"__/hst_request_time\@scope=(total)_sum/__\"
---                                 ,\"__/hst_request_time\@scope=(in_upstreams)_sum/__\"
---                                 ]
+--                 , pcGauges = fromList
+--                     [\"cnt_stub_status_active\"]
+--                 , pcScale1000 = fromList
+--                     [\"__/hst_request_time@\scope=(total)_sum/__\"
+--                     ,\"__/hst_request_time\@scope=(in_upstreams)_sum/__\"
+--                     ]
 --                 }\';
 -- 
 -- @
diff --git a/ngx-export-tools-extra.cabal b/ngx-export-tools-extra.cabal
--- a/ngx-export-tools-extra.cabal
+++ b/ngx-export-tools-extra.cabal
@@ -1,5 +1,5 @@
 name:                       ngx-export-tools-extra
-version:                    0.5.7.0
+version:                    0.5.8.0
 synopsis:                   More extra tools for Nginx haskell module
 description:                More extra tools for
         <https://github.com/lyokha/nginx-haskell-module Nginx haskell module>.