Data.List haddocks mention $dmnull, $dmlength, etc.
Something weird just happened to haddocks for Data.List
: https://ghc.gitlab.haskell.org/ghc/doc/libraries/base-4.19.0.0-inplace/Data-List.html#v:null lists
null :: Foldable t => t a -> Bool
$dmnull :: Foldable t => forall a. t a -> Bool
length :: Foldable t => t a -> Int
$dmlength :: Foldable t => forall a. t a -> Int