dhall-to-nix: Only double-quote symbols if they have symbols

A small improvement in the generation logic.
The hnix should really just do this for us.

dhall-nix/src/Dhall/Nix.hs

@@ -604,7 +604,7 @@ dhallToNix e =
         -- see https://github.com/dhall-lang/dhall-haskell/issues/2414
         nixAttrs pairs =
           Fix $ NSet NonRecursive $
-          (\(key, val) -> NamedVar ((mkDoubleQuoted key) :| []) val Nix.nullPos)
+          (\(key, val) -> NamedVar ((mkDoubleQuotedIfNecessary key) :| []) val Nix.nullPos)
           <$> pairs
     loop (Union _) = return untranslatable
     loop (Combine _ _ a b) = do
@@ -697,7 +697,7 @@ dhallToNix e =
             _ -> return (unionChoice k Nothing)
     loop (Field a (Dhall.Core.fieldSelectionLabel -> b)) = do
         a' <- loop a
-        return (Fix (Nix.NSelect Nothing a' (mkDoubleQuoted b :| [])))
+        return (Fix (Nix.NSelect Nothing a' (mkDoubleQuotedIfNecessary b :| [])))
     loop (Project a (Left b)) = do
         a' <- loop a
         return (Nix.mkNonRecSet [ Nix.inheritFrom a' (fmap VarName b) ])
@@ -736,7 +736,7 @@ dhallToNix e =
 -- If passArgument is @Just@, pass the argument to the union selector.
 unionChoice :: Text -> Maybe NExpr -> NExpr
 unionChoice chosenKey passArgument =
-  let selector = Fix (Nix.NSelect Nothing (Nix.mkSym "u") (mkDoubleQuoted chosenKey :| []))
+  let selector = Fix (Nix.NSelect Nothing (Nix.mkSym "u") (mkDoubleQuotedIfNecessary chosenKey :| []))
   in Nix.Param "u" ==>
      case passArgument of
        Nothing -> selector
@@ -750,8 +750,15 @@ unionChoice chosenKey passArgument =
 -- where
 --
 -- @{ foo/bar = 42; }.foo/bar@ is not syntactically valid nix.
-mkDoubleQuoted :: Text -> NKeyName r
-mkDoubleQuoted key = DynamicKey (Plain (DoubleQuoted [Plain key]))
+--
+-- This is only done if necessary (where “necessary” is not super defined right now).
+mkDoubleQuotedIfNecessary :: Text -> NKeyName r
+mkDoubleQuotedIfNecessary key =
+    if Text.all simpleChar key
+    then StaticKey (VarName key)
+    else DynamicKey (Plain (DoubleQuoted [Plain key]))
+    where
+        simpleChar c = isAsciiLower c || isAsciiUpper c
 
 
 -- | Nix does not support symbols like @foo/bar@, but they are allowed in dhall.