SML tail-recursive length-of-list function

Question

I am trying to write a tail-recursive function in Standard ML to find the length of a list; so, for example, len([1,2,3,4,5]) should be 5.

I was able to do it in Scheme no problem:

(define (len1 lis sofar)
 (if
  (null? lis) sofar
  (len1 (cdr lis) (+ sofar 1))))

(define (len lis)
 (len1 lis 0))

(len (list 2 3 4 5 6 7)) = 6

But I can't seem to get it in SML. Here is what I have so far:

fun len [] = raise Fail "len: empty list"
 | len [x] = (x,x)
 | len (x::xs) =

or

len1([]:list, curLen:int) =  []null? then curLen | len1(tl[], curLen+1);

len([]) =  len1([], 0);

correction: ignore the first 3 lines listed under Sceme. They were an attempt on SML. — G F
– G F, Commented Mar 16, 2015 at 7:39
If you were able to do it in Scheme, then I can only presume you don't know the syntax of SML, in which case all you have to do it read a tutorial, like this one: homepages.inf.ed.ac.uk/stg/NOTES/node2.html. — Ionuț G. Stan
– Ionuț G. Stan, Commented Mar 16, 2015 at 10:17

Inaimathi · Accepted Answer · 2015-03-16 14:44:09Z

1

In Standard ML, you can use pattern-matching instead of an explicit null? check.

fun length lst = let
    fun recur [] acc = acc
           (* we don't care about the value of the element *)
      | recur (_::rest) acc = recur rest (1 + acc)
  in recur lst 0
end

Trying it out in the REPL gives you the expected result.

Standard ML of New Jersey v110.76 [built: Thu Feb 19 00:37:13 2015]
- fun length lst = let
    fun recur [] acc = acc
           (* we don't care about the value of the element *)
      | recur (_::rest) acc = recur rest (1 + acc)
  in recur lst 0
end ;;
- length [] ;;
= val it = 0 : int
- length [1, 2, 3] ;;
= val it = 3 : int
-

Agreed with the commenter to the OP, by the way. You managed fine in Scheme, which means you're probably having a bit of trouble with SML syntax. I suggest the appropriate Learn X in Y page.

edited Mar 16, 2015 at 14:44

answered Mar 16, 2015 at 14:32

Inaimathi

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