Ocaml: calling recursive function again

Question

So I have a recursive function that takes in 2 ints, and a out_channel and basically prints line(a,a+1). It should do this until value of a is equal to b. I.e if a = 1, b = 5

line(1,2) line(2,3) ...line(4,5)

> let rec print_line (out:out_channel)(a:int)(b:int) : unit =
  if (a < b) then output_string out ("line("^string_of_int(a)^","^string_of_int(a+1)^")\n")
>    ;;

I want to make it recursive where it keeps printing the line(a,a+1) until a is no longer less than b. How exactly do I call it again?

Any help would be appreciated.

akoprowski · Accepted Answer · 2011-04-04 22:47:10Z

5

So: first check whether a >= b in which case you are done and can return (). Otherwise print one line (the way you did) followed by recursive call to your function, with incremented a. So altogether:

let rec print_line (out:out_channel)(a:int)(b:int) : unit =
  if a >= b then
    ()
  else (
    output_string out ("line("^string_of_int(a)^","^string_of_int(a+1)^")\n");
    print_line out (a + 1) b
  )

answered Apr 4, 2011 at 22:47

akoprowski

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note that you can actually use let rec print_line out a b = as the first line, as the ocaml compiler is smart enough to know what the types of a b and out are.
– subsub
Commented Apr 6, 2011 at 13:15
An if doesn't need an else if the type of the expression is unit. So you can: if a < b then (output_string out ("line("^string_of_int(a)^","^string_of_int(a+1)^")\n"); print_line out (a + 1) b)
– Chris
Commented Apr 11 at 23:18

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