Automatically delete closing pair without causing unbalanced pairs

Question

I am writing a VimL function that, in insert mode, allows a <BS> through the opening part of a pair ([, {, etc.) to delete the closing part of the pair if it is the next character after the cursor (much like auto-pairs and vim-autoclose). This function will be different because the closing part of the pair will not be deleted if there are unbalanced pairs.

Examples:

The format is <initial> => <result-after-backspace> where my cursor is in front of the |.

((|)    =>   (|)

((|))   =>   (|)

(|)     =>   |

(defn   =>   (defn
  (|)          |)

How can I determine whether or not the pairs are unbalanced before I go ahead and delete the closing pair?

I have tested both auto-pairs and vim-autoclose. With both plugins the closing part of the pair is always deleted.

Answer 1

If I understand your question correctly, the key is to determine whether the character on the cursor is a matched delimiter.

Consider the normal mode commands v%"zy and v%%"zy.

• If the cursor is on a matched delimiter, then the first command will yank a string of length at least 2—the delimiter, its pair, and any text in the middle—into @z. The second will yank a string of length exactly 1—again, the delimiter itself—into @z.
• If the cursor is on an unmatched delimiter, then the first command will yank a string of length 1—the delimiter itself—into @z. Abort.
• If the cursor is not on a delimiter and there is a delimiter in the line, then the second command will yank a string of length greater than 1 into @z. Abort.
• If the cursor is not on a delimiter and there is no other delimiter in the line, then the first command will yank a string of length exactly 1—the character itself—into @z. Abort.

Thus, we have the following result:

The cursor is on a matched delimiter if and only if the normal mode command v%"zy yanks a string of length at least 2 into @z and the normal mode command v%%"zy yanks a string of length exactly 1 into @z.

Hints for VimL: :norm ... and strlen(@z) will help here.

Answer 2

Ok so I ended up going through paredit.vim and pulling out some functions to create this: vim-pear. It inserts and deletes delimiters in pairs while maintaining a balanced state.