ocaml - List.iter & List.fold_right used together -

i trying use fold_right , list.iter functions in list module. there anyway use them in conjunction 1 another?

let step nfa start transition =    let transition_list = get_transition nfa in   list.iter ( fun state ->     list.fold_right (fun ct nl ->        if ((get_pre_trans transition)= state && (get_trans ct) = transition)          (get_post_transition transition)::nl        else          nl     ) transition_list []   ) start ;; 

** get_xxx functions values tuple there pre-transition, transition value, , post-transition.

return error: error: expression has type 'a list expression expected of type unit.

not sure do.

the body of function pass iter contains 1 expression, call fold_right, evaluates value of type list, iter signature requires pass function, returns value of type unit. compiler tries you. if you're not interested in value fold_right evaluated, can ignore using ignore function, takes value of type , returns value of type unit. on other hand, if don't want discard it, shouldn't use iter, , use fold_right or, better, fold_left.

and, finally, answering question, yes, there're ways combine them together, usually, if you're applying fold inside iter, you're doing wrong.