Possibly the world’s first functional pearl (by David Barron and Christopher Strachey) - Cartesian product

cp.ml

open ListLabels 

let product xss = 
    let f xs yss = 
        let g x xs = 
            let h a b = (x :: a) :: b in 
            List.fold_right ~f:h ~init:xs yss
        in 
        List.fold_right ~f:g ~init:[] xs
    in  
    List.fold_right ~f:f ~init:[[]] xss
    
(* PS: it looks way nicer in Haskell because of where clauses *)