Question

I am following this draw_string_in_box example in Developing Applications with OCaml

The sample code given is as follows:

let draw_string_in_box pos str bcf col = 
 let (w, h) = Graphics.text_size str in
 let ty = bcf.y + (bcf.h-h)/2 in 
 ( match pos with 
       Center -> Graphics.moveto (bcf.x + (bcf.w-w)/2) ty 
     | Right  -> let tx = bcf.x + bcf.w - w - bcf.bw - 1 in 
                 Graphics.moveto tx ty 
     | Left   -> let tx = bcf.x + bcf.bw + 1 in Graphics.moveto tx ty  );
 Graphics.set_color col;
 Graphics.draw_string str;;

If I remove the parentheses around the “match” part, the code won’t work (nothing gets printed). Any idea why?

And more generally, when should I put parenthese around code bits like this?

Thanks.

Answer 1

One way to look at it is that after the arrow -> of a match statement you can have a sequence of expressions (separated by ;). Without the parentheses, the following expressions look like they’re part of the last case of the match.

You can also have a sequence of expressions (separated by ;) after let. With the parentheses, the following expressions look like they’re part of the let, which is what you want.

Personally I avoid using ;. That’s how I deal with this problem! Otherwise you have to figure the expression sequence goes with the innermost construct that takes a sequence.