F# sum of a list of lists -

i created own datatype , i'm trying create sum of numbers in datatype list of lists. don't want use f# libraries

my datatype

type elist = | l of int * elist 

i'm beginner in f# , trying grasp head on it. want recursively. thinking going traverse end of list , start sum , go front , add each one.

example:

let l = l(4, l(3, l(6, l(3, a)))) 

that should return

val : int 16 

here's code , know wrong:

let rec sum l =     let = 0    match l    | -> 0    | l(head,a) -> head     | l(head,tail) -> sum tail + 

you're there. need lose a:

let rec sum l =     match l    | -> 0    | l(head,a) -> head     | l(head,tail) -> head + sum tail 

then evaluating sum l when l = l(4, l(3, l(6, l(3, a)))) gives

val : int = 16 

as required.