In Haskell: how to filter the result after the call to exec, so that you get a new environment that only contains output-variable-pairs

This is the BIPLE constructor: data BIPLE = BIPLE [String] [String] [Stmt] -- first tuple is input, second tuple is output

This is the function:

execBIPLE :: BIPLE -> Environment -> Environment execBIPLE (n1 n2 s) env = case checkInput i env True -> (exec n1 env) False -> error $ "unknown_variable_" -- this is the one I struggle with: -- execBIPLE (_ n2 _) env = filter (`elem` exec n1) env ???