Moves minimum pg version to config, eliminates magic constant from main and improves docs.

src/PostgREST/Config.hs

@@ -8,15 +8,17 @@ It also can be used to define other middleware configuration that may be delegat
 external configuration.
 
 It currently includes a hardcoded CORS policy but this could easly be turned in configurable behaviour if needed.
+
+Other hardcoded options such as the minimum version number also belong here.
 -}
 module PostgREST.Config ( prettyVersion
                         , readOptions
                         , corsPolicy
+                        , minimumPgVersion
                         , AppConfig (..)
                         )
        where
 
-
 import           Control.Applicative
 import qualified Data.ByteString.Char8       as BS
 import qualified Data.CaseInsensitive        as CI
@@ -100,3 +102,7 @@ readOptions = customExecParser parserPrefs opts
                     <> " / create a REST API to an existing Postgres database"
                     )
     parserPrefs = prefs showHelpOnError
+
+-- | Tells the minimum PostgreSQL version required by this version of PostgREST
+minimumPgVersion :: Integer
+minimumPgVersion = 90200

src/PostgREST/Main.hs

@@ -26,12 +26,13 @@ import           System.IO                            (BufferMode (..),
 
 import           PostgREST.Config                     (AppConfig (..),
                                                        prettyVersion,
-                                                       readOptions)
+                                                       readOptions,
+                                                       minimumPgVersion)
 
 isServerVersionSupported :: H.Session P.Postgres IO Bool
 isServerVersionSupported = do
   Identity (row :: Text) <- H.tx Nothing $ H.singleEx $ [H.stmt|SHOW server_version_num|]
-  return $ read (cs row) >= (90200::Integer)
+  return $ read (cs row) >= minimumPgVersion
 
 main :: IO ()
 main = do