Returning a recursive function in ML

Question

How can I return a recursive function in ML?

As far as I know recursive anonymous functions cannot be returned and only anonymous functions can be used as returning value(if the returning value is function...).

Can you rephrase the question or give an example? Why do you think that you cannot return a recursive function? — Andreas Rossberg
– Andreas Rossberg, Commented Nov 19, 2012 at 22:34

Andreas Rossberg · Accepted Answer · 2012-11-21 13:54:53Z

3

Would this be the kind of example you are wondering about?

fun f n =
   let
      fun g k =
         if k = n then [] else k :: g (k-1)
   in
      g
   end

You can only make a recursive definition by naming it, but that's not a problem, because you can write a let expression anywhere.

Update to answer comment more specifically:

fun f g =
   let
      fun h 0 = g 0
        | h i = h (i-1) + g i
   in
      h
   end

(A more efficient implementation would make h tail-recursive.)

edited Nov 21, 2012 at 13:54

answered Nov 19, 2012 at 22:42

Andreas Rossberg

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1 Comment

poli Over a year ago

I am tying to implement a function f: (int->int) -> (int->int) that receives a function g as parameter and return h, where: h= $sum_{i=0}^{i}g(i)$ I tried to use "let" and after a lot of trying still it doesn't work well.

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