Hello,
 
>  a simple question concerning the Camltk API.
> 
>  What is the simplest way to specify the size of the font used in a widget.
> 
>  I've found Font_Size but when I try to use it, it raises Invalid_argument("Font_Size")
> 
> I'll appreciate any indication.

> let q = Button.create top 
>           [Text "quit"; Font_Size 12;
>            Command closeTk];;
> (* q exists but is not yet visible *)

This is since Font_Size is not a valid option for the button widget.
What you should do is:

     1. define a font using Font_Size 12:

	let font = Font.create [Font_Size 12]

     2. At the creation of the button, specify this font using Font option:

         let q = Button.create top 
           [Text "quit"; Font font;
            Command closeTk];;
        	
The options of Tk commands are gathered and form a huge type 
Camltk.options in Camltk. But a widget class does not take all of them
as valid options, but only a small subset. The validity of tk options
is tested at run-time, and if an invalid option is found, Invalid_argument
exception is raised.

In LablTk, this problem is almost resolved, by replacing option lists
to labeled optional arguments.

To find out which options are valid for a widget class is either

   * look the source code: otherlibs/labltk/Widgets.src defines the subsets
     of valid options for each widget class.

   * tk command manual

   * or, look at the type of the corresponding LablTk function, which
     details the set of possible options.

--
Jun

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