j is abstract. It seems like it should be easier if it is concrete, but I find that this tends to lead to assumptions and encourages scanning instead of reading.
When I sit down to a program, I have an idea about what I want to do. I don't have home/end keys on my keyboard and I note that pressing Fn-Left and Fn-Right is annoying because I don't usually push them (I use emacs sometimes). So when I sit down to edit I want to add emacs keys.
I can see at http://kparc.com/z.txt that hx handles home/end, and that cX is control-X, so I think it should be something like:
ca:{hx 0 -1};ce:{hx 0 1}
Now I look at the characters spent: Is it possible I could say this simpler? I don't think so, but I'd love someone to tell me.
How about delete? I think what I want to do is select the next character and remove it. So where is the cursor? Well I remembered that hx moves the cursor, so I read it's definition:
hx:{L i+d*x}
Okay, so I remember d is "dimensions" and "i" is something, and L I haven't seen yet. So I go look at L:
L:{K@B/0|x&d-1}
Now I have two new words: K and B to look up:
B:{c[x]&y+b x}
K:{J(;|\(*k),)[H]x}
Okay, B is straightforward: b is indexed by line, so b x returns the offset of line x. We add this to y (which is the second value of |x&d-1) and take the min of it and the offset for c. I might think at this point B converts the x/y coordinate into a linear offset. But what about K?
K is using J and H (but it turns out we only need J):
J:{k::2#0|x&-1+#a}
And here we see the definition of k: x&-1+#a is the smallest of x and the second to last character in the file (a), and the max of that and zero (0|). This reads like "bounds check x to the shape of a". Taking 2 from that simply takes two values and assigns them to k so if x is only one-value k will still have two.
k is the cursor selection.
From here, I can make an attempt at implementing delete. My first attempt looked something like:
cd{a::((0,k)_a),(((k+1),(#a)-(k+1)))_a}
Oh! But I've seen a lot of these terms before! Maybe I can do better. I note that kx is documented as the callback for keystrokes
http://kparc.com/z.txt and can come up with:
cd:{K j+!2;kx""}
This seems about as simple as I can make it. I'd like to see someone do better, but
knowing that K is a setter for the cursor selection and j is the offset of the cursor, then j+!2 simply returns offset and offset+1; K will select it and kx"" will remove it.
Now maybe if k were called cursorSelection I might have gotten my first cd faster, but I would've missed the opportunity to see how these functions and variables were interconnected and I might not have written the second one.
I noticed however, that not scrolling really helped this exercise. I just moved my eyes around, and jotted a few symbols down on my notepad. I feel like if I had to scroll or switch windows I would not have been able to do this.
As for your last question: What does one need to master it? For now, I would say practice. Try writing a program in as dense a manner as possible. Remove as much redundancy as you can. Read it and re-read it until you feel like it can be no more abstract.
I am working on a much better answer, but I hope that one will do for now.