gen_eval is a homebrew version of instance_eval that eliminates the most annoying aspect of its use:

For example:

@x = 20
@y = 30
my_image.instance_eval {
    circle @x, @y, 20 
}

=> error @x not initialized

In the above code the programmer means to access the local @x yet instance_eval looks up @x in the receiver (my_image).

gen_eval, on the other hand, works as you’d expect, it looks up @x in the caller-context yet still invokes methods in the receiver-context.

This means we can now do things like this:

@x = 20
@y = 30
image.gen_eval {
    pixel @x, @y
    circle @x, @y, 20 
}