Pull port and host from environment

semantic.cabal

@@ -153,6 +153,7 @@ library
                      , Semantic.CLI
                      , Semantic.Diff
                      , Semantic.Distribute
+                     , Semantic.Env
                      , Semantic.Graph
                      , Semantic.IO
                      , Semantic.Log

src/Semantic/Env.hs

@@ -0,0 +1,15 @@
+module Semantic.Env where
+
+import Control.Monad.IO.Class
+import Prologue
+import System.Environment
+import Text.Read (readMaybe)
+
+envLookupHost :: MonadIO io => String -> String -> io String
+envLookupHost defaultHost k = liftIO $ fromMaybe defaultHost <$> lookupEnv k
+
+envLookupPort :: MonadIO io => Int -> String -> io Int
+envLookupPort defaultPort k = liftIO $ parsePort <$> lookupEnv k
+  where parsePort x | Just s <- x
+                    , Just p <- readMaybe s = p
+                    | otherwise = defaultPort