More explicit error handling and messages when required environment variables are missing

src/Core/Configuration.hs

@@ -1,7 +1,11 @@
-module Core.Configuration where
+module Core.Configuration ( adminEmail, appPort 
+                          , confLinkLength, dataPath, dbPath
+                          , encKey, getRuntimeEnvironment 
+                          , keyFileInit, main) where 
 
 import qualified Data.ByteString               as B
 
+import           Control.Monad                 (mapM)
 import           Core.Types
 import           Crypto.Saltine.Core.SecretBox (newKey)
 import           Crypto.Saltine.Class          (encode)
@@ -9,13 +13,35 @@ import           Configuration.Dotenv
 import           System.Directory              (doesFileExist)
 import           System.Environment            (getEnv, lookupEnv)
 
--- Make the dotenv file configuration available if PURRNOFILE is not present
+-- Load environment variables from dotenv file if required
 main :: IO ()
 main = do
-  envFile <- lookupEnv "PURRNOFILE"
-  case envFile of
-    Nothing  -> loadFile defaultConfig
-    _        -> putStrLn "Not using dotenv file"
+  reqEnvLookup <- getRequiredEnv
+  if (Nothing `elem` reqEnvLookup)
+    then checkEnvFile requiredEnvVars
+    else pure ()
+  where
+    getRequiredEnv :: IO [Maybe String]
+    getRequiredEnv = mapM (\s -> lookupEnv s) requiredEnvVars
+
+    checkEnvFile :: [String] -> IO ()
+    checkEnvFile requiredEnv = do
+      dotEnvExists <- doesFileExist "./.env"
+      if dotEnvExists
+        then do
+          loadFile defaultConfig
+          fromEnvFile <- getRequiredEnv
+          if (Nothing `elem` fromEnvFile)
+             then error $ missingEnvMsg requiredEnv
+             else pure ()
+        else error $ "Cannot find .env file in application directory.\n"
+          ++ missingEnvMsg requiredEnv
+
+    missingEnvMsg :: [String] -> String
+    missingEnvMsg required = 
+      "Missing required environment variable(s).\n"
+      ++ "All required environment variables:\n" 
+      ++  unlines required
 
 -- Check if an encryption key exists on the filesystem and create one if not
 keyFileInit :: IO ()
@@ -53,3 +79,8 @@ dbPath = "data/Purr.sqlite"
 
 confLinkLength :: IO String
 confLinkLength = getEnv "LINKLENGTH"
+
+requiredEnvVars :: [String]
+requiredEnvVars = [ "ADMINEMAIL", "APPLICATIONHOST", "APPLICATIONPORT"
+                  , "DATADIR", "ENVIRONMENT", "LINKLENGTH"
+                  ]