(if (too (many parentheses)) (use parinfer) ‘())

When I first started learning Common Lisp I was a little annoyed by the syntax. After practicing writing Lisp code I have grown to like the simplicity of the basic language. However, like C/C++, even though you don’t have to format the code (i.e. add whitespace) it helps readability immensely if you do. In such cases a decently smart editor helps a lot, and the parinfer plugin is really, really neat.

For me, formatting Lisp code like it was Python improves it’s readability by a huge amount. Syntax highlighting, for Lisp, can’t do so much since Lisp syntax is minimal, but indenting things like meaty if statements, nested function calls and lengthy ‘cond’ constructs makes things easy to follow.

Some editor features, for example paredit, tailored to Lisps, are very helpful but, as the plugin manual itself notes:

ParEdit helps **keep parentheses balanced** and adds many keys for moving S-expressions and moving around in S-expressions. Its behavior can be jarring for those who may want transient periods of unbalanced parentheses, such as when typing parentheses directly or commenting out code line by line.

Jarring is right. I use Atom and had paredit running and had a session once where I thought my keyboard was broken because I couldn’t type parens. And not being able to type parens is pretty traumatic when learning Lisp.

Parinfer brings a very useful concept to the domain of Lisp editing. Like a most editor assists, it will close parentheses for you when you open them, and place the cursor in the right place. However, it does a bit of sophisticated parsing to handle the following condition:

(defun func (x)
  (setf x (+ x 3))
  (if (< x 3) t nil))
(defun func (x)
  (setf x (+ x 3)))      <--- note we now need a close parens here
  ; (if (< x 3) t nil))  <--- parinfer automatically generates it
                         <--- when we comment out the last line

(As a side-note, WordPress’ syntax highlighting for the tag does not have Lisp as an option, but I got the desired effect by claiming this was Python!)

Parinfer does take a little getting used to if you’ve been managing parens and indentation by yourself. I had to learn in which part of my S-exp to hit the “return” key to get the desired effect – the two outcomes being advancing to a new level of indentation (nesting) under the parent expression or dropping out to a new, separate expression

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