Can someone explain new box()?

EvanReese

I see new box() used in the std and other places by people who are better at coding than me, but still haven't wrapped my head around it or when I'd want to use it. I've read the explanation here in the FS documentation, and AI says to use Box Types "When you need to wrap a single value and potentially add additional metadata or behavior or for implementing generic programming patterns", but I'm still not clear on it. Anyone care to explain it a bit or link me to a good primer?

ilya_baran

Boxes are a way around the issue that FeatureScript values are generally unchangeable — if you have:

var x = { a : 12 };
myFunction(x);
println(x);

You'll always see {a : 12} no matter what myFunction does. (It does not work this way in Java or JavaScript, for example).

If you want a function to be able to change an argument, you need to put it in a box, like this:

var x = new box({ a : 12 });
myFunction(x);
println(x[]);

…
// In myFunction:
x[] = 3;

Then this code will print 3 because myFunction changed "what is in the box".

This can lead to useful patterns like the pseudo random number generator I posted here:

https://forum.onshape.com/discussion/comment/18945#Comment_18945

Hope this helps.

ilya_baran

Boxes are a way around the issue that FeatureScript values are generally unchangeable — if you have:

var x = { a : 12 };
myFunction(x);
println(x);

You'll always see {a : 12} no matter what myFunction does. (It does not work this way in Java or JavaScript, for example).

If you want a function to be able to change an argument, you need to put it in a box, like this:

var x = new box({ a : 12 });
myFunction(x);
println(x[]);

…
// In myFunction:
x[] = 3;

Then this code will print 3 because myFunction changed "what is in the box".

This can lead to useful patterns like the pseudo random number generator I posted here:

https://forum.onshape.com/discussion/comment/18945#Comment_18945

Hope this helps.

EvanReese

oooh! Thanks Ilya, that made it click. Lots of possibilities.