sexp-rewrite

sexp-rewrite (abbreviated sexprw) is an Emacs package for doing pattern-based rewriting of sexp-structured code---ie, code in Lisp, Scheme, and primarily Racket.

Some examples of pattern-based rewriting are:

• Turn a chain of if expressions into a cond expression.

• Rewrite an application of map with lambda to for/list---or andmap to for/and, etc.

• Turn a letrec into a sequence of internal definitions.

The pattern language is simple enough that you can easily define your own rewriting rules.

Transformations preserve comments and are halfway decent at preserving formatting, with the occasional help of appropriate spacing annotations.

Try it out

Visit sexp-rewrite.el and evaluate the buffer (M-x eval-buffer). Then visit racket-rewrites.el and evaluate that buffer too.

Most of the rewrite rules (or "tactics") in racket-rewrites.el have examples after them. Go to the example labeled "example for if-to-cond..." and place the cursor at the first left parenthesis---that is, at (if (< x 10)....

Run the if-to-cond tactic by entering M-x sexprw-execute-tactic and then if-to-cond. The if expression gets rewritten to a cond expression---but only the first if; there are still if expressions left in the else branch. Now run the cond-else-absorb-if tactic. There's a keybinding for sexprw-execute-tactic that makes executing a specific tactic quicker: C-c C-s x. Then type cond-else-absorb-if at the prompt (you can use tab completion to save typing). The if expression gets "absorbed" into the cond. There's just one if left, but it's inside a let, so let's leave it alone for now.

What a bother to have to type in the tactic names. Fortunately, there's an even quicker way.

Undo twice (C-/ C-/) to reset the example to the original form, and make sure the cursor is back at the beginning of the example. Now enter C-c C-s e (which runs the sexprw-auto-expression command). The command automatically tries a bunch of tactics until one works, and it reports the name of the tactic in the minibuffer.

Faster?

Undo once (C-/) to reset the example again. Now type C-c C-s r e, which repeatedly tries tactics (up to about a hundred times) until they stop working. This time we get the first two if expressions in one shot, as well as the if under the let, which can be converted to an => clause.

Note that the rewrite that produced the => can be unsafe: the else-branch expression was originally in the scope of the let-bound variable, but after rewriting it is in a separate clause. That's usually not a problem, because the variable is always known to be false in the else, branch (unless it's mutated...) so there's no reason to refer to it. But it's always a good idea to keep an eye on the tactics applied to make sure they don't break your code.

Keybindings

The prefix for all sexp-rewrite commands is C-c C-s.

The following keybindings invoke sexp-rewrite tactics:

• <prefix> x : runs sexprw-execute-tactic, which applies the given tactic

• <prefix> e : runs sexprw-auto-expression

• <prefix> r e : runs sexprw-auto-expression repeatedly until no tactic applies

• <prefix> d : runs sexprw-auto-definition

• <prefix> r d : runs sexprw-auto-definition repeatedly until no tactic applies

The following keybindings manipulate sexpagons:

• <prefix> k : runs sexprw-kill-next-sexpagon-sexp
• <prefix> w : runs sexprw-kill-sexpagon-region
• <prefix> y : runs sexprw-yank-sexpagon
• <prefix> M-SPC : runs sexprw-collapse-space/move-sexps

The following other keybindings are also provided:

• <prefix> s : runs sexprw-search-pattern, which searches forward for a term matching the given sexprw pattern

• <prefix> [ : runs sexprw-squarify, which converts parentheses to square brackets for all following terms at the given level

• <prefix> ( : runs sexprw-roundify, which converts square brackets to parentheses for all following terms at the given level

Defining Tactics

See REFERENCE.md for a description of the tactic language.

Known bugs and limitations

This library has a vague notion of sexp syntax. Notions like improper lists are not supported, and . is treated as an atom. Reader abbreviations like ' for quote are not recognized.