Paredit Slurp and Barf Mnemonics

By Susam Pal on 28 Feb 2022

Of the several sophisticated Paredit commands to edit s-expressions, I believe, the following four commands are the most important ones:

These commands are bound to the key sequences C-(, C-), C-{, and C-}, respectively, by default on Emacs. These commands can make editing s-expressions very convenient. However, often on IRC networks and other online forums, I come across users who have difficulty remembering the key sequence each command is bound to. I will share some mnemonics in this post that make this easier to remember. Before discussing the mnemonics, let us see what these commands do. However, if you already know what these commands do, skip right ahead to the Mnemonics section.

Slurp Demo

Say, we have the following Lisp expressions in our editor: 

(* 10 20) (+ 30 40)

The block above shows where the cursor is placed. Let us now bring the expression (+ 30 40) inside the expression (* 10 20). To do so, we first place the cursor somewhere inside the first expression, say, as shown below: 

(* 10 20) (+ 30 40)

Now type C-) and the closing parenthesis of the first expression moves forward to slurp the next expression. The result looks like this: 

(* 10 20 (+ 30 40))

The key sequence C-) invokes the paredit-forward-slurp-sexp command that adds the expression following the current list into that list by moving the closing delimiter ahead. Similarly, there is the key sequence C-( that invokes the paredit-backward-slurp-sexp command which works similarly except that it makes the current list consume the preceding expression. Note that the slurp commands expand the current list to consume a neighbouring expression.

Barf Demo

Say, we have the following Lisp expression in our editor: 

(* 10 20 (+ 30 40))

We now want to move (+ 30 40) out of the first expression. To do so, we first place the cursor somewhere inside the outer expression but not inside the inner expression. Here is an example of where we place the cursor: 

(* 10 20 (+ 30 40))

Now type C-} and the closing parenthesis of the outer expression moves behind to barf the inner expression out. The result looks like this: 

(* 10 20) (+ 30 40)

The key sequence C-} invokes the paredit-forward-barf-sexp command that removes the last expression in the current list from that list by moving the closing delimiter behind. Similarly, there is the key sequence C-{ that invokes the paredit-backward-slurp-sexp command which works similarly except that it removes the first expression from the current list. Note that the barf commands shrinks the current list to remove an expression from that list.

Mnemonics

When I began using Paredit for the first time, several years ago, I could never remember which key sequences slurp and which ones barf. Then I made up these mnemonics:

I do not need these mnemonics anymore because after using these key sequences a few times, they get burnt into our muscle memory. However, in the initial days of using Paredit, these mnemonics were quite useful. Now whenever I find someone complaining about how easy it is to forget the key sequences bound to these commands, I share these mnemonics with them and they never have any problem again with the slurp and barf commands. These mnemonics have served me and many others I know quite well.

Update on 01 Mar 2022: Jon Snader, who maintains a very popular blog named Irreal, has discussed this blog post on his own blog. While he did not find my mnemonics particularly helpful, he has shared his own way of remembering the key sequences. Quoting Jon below:

To me, the braces suggest pointing out so Ctrl+} and Ctrl+{ are clearly for barfing.

That is quite an interesting and elegant mnemonic. See his post Using Paredit’s Slurp and Barf to read his complete views on this topic.

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