Add a comment to Pretty.hs

src/Nix/Pretty.hs

@@ -158,7 +158,12 @@ prettyNix = withoutParens . cata phi where
   phi (NAbs args body) = leastPrecedence $
    nest 2 ((prettyParams args <> colon) <$> withoutParens body)
   phi (NBinary NApp fun arg)
-    = NixDoc (wrapParens appOp fun <+> wrapParens appOpNonAssoc arg) appOp
+    -- jww (2018-04-19): If 'arg' was a binary operator, it will not have
+    -- parens itself, and yet its precedence is very likely to be lower than
+    -- function application. However, we've lost the knowledge of whether it
+    -- should or shouldn't be surrounded by parens, so we do it always for
+    -- now.
+    = NixDoc (wrapParens appOp fun <+> parens (withoutParens arg)) appOp
   phi (NBinary op r1 r2) = flip NixDoc opInfo $ hsep
     [ wrapParens (f NAssocLeft) r1
     , text $ unpack $ operatorName opInfo