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In the world of 3D-modeling, back in the day before sculpting and retopo were invented, there were two different styles of modeling. Point-by-point (where you create a point, join it up to your other points with a line, and join that line up with your other lines with a face) and box modeling, where you start with a box, and cut and push and add loops to it.
When I’m using paredit to work on Lisp code, I’m using it like a box modeler.
The core primitives for me are wrapping and raising.
I start by inserting an empty pair and as I write, I keep adding pairs in. In the following evolving example, the bar character | represents my cursor.
(|) (let ((|))) (let ((foo|))) (let ((foo (|)))) (let ((foo (bar|)))) (let ((foo (bar (|))))) (let ((foo (bar (+ 11 12|))))) (let ((foo (bar (+ 11 12))))|) (let ((foo (bar (+ 11 12)))) (|)) (let ((foo (bar (+ 11 12)))) (frobnicate foo foo|))
So far, so normal. Like any other paredit user. Now let’s say I later realize that I need one of those foos to be reversed.
Let me first show you what I would not do:
(let ((foo (bar (+ 11 12)))) (frobnicate (|) foo foo)) (let ((foo (bar (+ 11 12)))) (frobnicate (reverse|) foo foo)) (let ((foo (bar (+ 11 12)))) (frobnicate (reverse foo|) foo))
Using a forward-slurp-sexp.
Instead, I’d start by wrapping the foo, and then adding the new operator.
(let ((foo (bar (+ 11 12)))) (frobnicate (|foo) foo)) (let ((foo (bar (+ 11 12)))) (frobnicate (reverse |foo) foo))
If I later decide I don’t want that foo reversed after all, I can use raise-sexp on it. Wrapping and raising are the core primitives of working with the syntax tree as a tree, always preserving the syntactic structure, as opposed to seeing it just as text that happens to have a matching paren that you want to push rightwards by slurping.
There’s also splicing and splice-killing that are useful.