-- β-reduction
--------------------------------
beta, betatop :: Transform
--- Substitute arg for x in expr
-beta (App (Lam x expr) arg) = setChanged >> substitute x arg expr
+-- Substitute arg for x in expr. For value lambda's, also clone before
+-- substitution.
+beta (App (Lam x expr) arg) | CoreSyn.isTyVar x = setChanged >> substitute x arg expr
+ | otherwise = setChanged >> substitute_clone x arg expr
-- Propagate the application into the let
beta (App (Let binds expr) arg) = change $ Let binds (App expr arg)
-- Propagate the application into each of the alternatives