f# - How to find index of last element in list matching predicate? -

i last index of element in list matching predicate. know can use following code array, wondering if there way list (without converting array , using below function):

let tryfindlastindex f (arr: 'a []) =     let rec loop n (arr: 'a []) =         match n         | -1 -> none         | n ->             if f arr.[n] n             else loop (n-1) arr     loop (arr.length - 1) arr 

list in f# linked list can't iterate in last->first direction. because of have iterate beginning , keep track of both current index , index of last element matches predicate.

you still use local recursive function that.

let tryfindlastindex f source =     let rec loop index lastfoundindex source =         let newindex = index+1         match source         | [] -> lastfoundindex         | head :: tail ->             if f head loop newindex (some index) tail             else loop newindex lastfoundindex tail     loop 0 none source