Fatal error: exception Graphics.Graphic_failure("Cannot open display ") - ocaml

Question

I was trying to run the code and it keeps showing the same error.
I start by compiling with ocamlc -o cardioide graphics.cma cardioide.ml and it appears to work, but then I do ./cardioide to execute it and the message Fatal error: exception Graphics.Graphic_failure("Cannot open display ") appears...
I've searched all across the internet and i can't find the solution, can someone please help me?
Thank you
open Graphics
let () = open_graph "300x20"
let () =
moveto 200 150;
for i = 0 to 200 do
let th = atan 1. *. float i /. 25. in
let r = 50. *. (1. -. sin th) in
lineto (150 + truncate (r *. cos th))
(150 + truncate (r *. sin th))
done;
ignore (read_key ())
Error message:
Fatal error: exception Graphics.Graphic_failure("Cannot open display ")

Answer 1

The string argument to the open_graph function is not the size or title, but actually implementation-dependent information that is passed to the underlying graphical subsystem (in X11 it is the screen number). In modern OCaml, optional arguments are passed using labels, but Graphics was written long before this feature was introduced to the language. Therefore, you have to pass an empty string there (if you don't want to pass any specific to implementation of the underlying graphical subsystem information), e.g.,
open_graph ""
will do the work for you in a system-independent way.
Besides, if you want to resize the window, then you can use the resize_window function. And to set the title, use set_window_title.
For the historic reference, the string parameter passed to the open_graph is having the following syntax (it is no longer documented, so there is no reason to believe that it will be respected):
Here are the graphics mode specifications supported by
Graphics.open_graph on the X11 implementation of this library: the
argument to Graphics.open_graph has the format "display-name
geometry", where display-name is the name of the X-windows display
to connect to, and geometry is a standard X-windows geometry
specification. The two components are separated by a space. Either
can be omitted, or both. Examples:
Graphics.open_graph "foo:0" connects to the display foo:0 and creates a
window with the default geometry
Graphics.open_graph "foo:0 300x100+50-0" connects to the display foo:0 and
creates a window 300 pixels wide by 100 pixels tall, at location (50,0)
Graphics.open_graph " 300x100+50-0" connects to the default display and
creates a window 300 pixels wide by 100 pixels tall, at location (50,0)
Graphics.open_graph "" connects to the default display and creates a
window with the default geometry.

Answer 2

Put a 'space' in the argument to get the window you want (should be 200 for your cardioide):
let () = open_graph " 300x200"

Answer 3

I met the same problem and it was because I used the Windows subsystem for linux(WSL) so it needs a XServer to run graphical application. And the Ubuntu Wiki For WSL helped solve the problem. I downloaded and installed MobaXterm (it has free community version!) and it automatically detects the WSL and runs it inside the app. Try the same code before and a graphical window will pop up!