Add some type annotations

yesod-form/Yesod/Form/Fields.hs

@@ -3,6 +3,7 @@
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE CPP #-}
 -- | Field functions allow you to easily create and validate forms, cleanly handling the uncertainty of parsing user input.
@@ -681,7 +682,7 @@ optionsPairs opts = do
 --
 -- @since 1.7.0
 optionsPairsGrouped
-  :: (MonadHandler m, RenderMessage (HandlerSite m) msg)
+  :: forall m msg a. (MonadHandler m, RenderMessage (HandlerSite m) msg)
   => [(msg, [(msg, a)])] -> m (OptionList a)
 optionsPairsGrouped opts = do
   mr <- getMessageRender
@@ -690,14 +691,14 @@ optionsPairsGrouped opts = do
                  , optionInternalValue = internal
                  , optionExternalValue = pack $ show external
                  }
-      opts' = enumerateSublists opts -- :: [(msg, [(Int, (msg, a))])]
+      opts' = enumerateSublists opts :: [(msg, [(Int, (msg, a))])]
       opts'' = map (\(x, ys) -> (mr x, map mkOption ys)) opts'
   return $ mkOptionListGrouped opts''
 
 -- | Helper to enumerate sublists with one consecutive index.
-enumerateSublists :: [(a, [b])] -> [(a, [(Int, b)])]
+enumerateSublists :: forall a b. [(a, [b])] -> [(a, [(Int, b)])]
 enumerateSublists xss =
-  let -- yss :: [(Int, (a, [b]))]
+  let yss :: [(Int, (a, [b]))]
       yss = snd $ foldl (\(i, res) xs -> (i + (length.snd) xs, res ++ [(i, xs)])) (1, []) xss
    in map (\(i, (x, ys)) -> (x, zip [i :: Int ..] ys)) yss