Interleaving line blocks in Vim

In my day-to-day work I sometimes need to “interleave” several line blocks together. What do I mean by interleave? An example will explain it best:

-- block 1
a
b
c

-- block 2
x
y
z

-- block 1 and block 2 interleaved
a
x
b
y
c
z

How to do that in (Neo)Vim

This can be done simply by using macros. Let’s go back to our initial example above and add line numbers so I can explain more clearly what I am doing

1  -- block 1
2  a
3  b
4  c
5  
6  -- block 2
7  x
8  y
9  z

If you want to have the lines of block 1 coming first, followed by a line of block 2, here is how you do it:

• Jump/go to line 2 and set the mark a, keystrokes ma in Vim normal mode
• Jump/go to line 6 and set the mark b, keystrokes mb in Vim normal mode
• Now we can record the macro. Make the following keystrokes: qa'bj:m 'a^Mmaq In words: q, a, single quote, b, j, colon, space, single quote, a, enter, j, m, a, q

qa starts to record a macro into register a (see :h q). Now every keystroke will be recorded until pressing q again, which stops the recording.

Note: The ^M above is actually one character an represents pressing the enter key. If you record the macro yourself this will be handled automatically, if you try to copy the macro from me you have to escape the key by pressing CTRL-V+Enter. Read this stackoverflow post for more info.

Note2: We are using the move command to move (see :h :m) one line after the other from block 2 to block 1. If you want to copy them instead use :t.

Note3: You don’t even have to type these keystrokes yourself. You can actually just copy mine! Just copy them in some register with the following two options:

• Vim normal mode command :let @a="'bj:m 'a^Mjma" will copy the string content into register a.
• Selecting everything in visual mode and pressing "ay will yank it into register a

When you have recorded the macro or otherwise filled the contents of the register you can simply execute the macro as often as there are lines. Let’s assume there are 15 lines, then in normal mode press 15@a to execute the macro 15 times, assuming you have saved your macro keystrokes to register a.

Usage

One typical example would be some enum like data type, like this sum type in Haskell:

data MyType
  = A
  | B
  | C

Often we want to write a comment for each value, e.g from some table or other external source and we can copy these into our code:

data MyType
  = A
  | B
  | C

comment1
comment2
comment3

All we have to do now is bring the commentaries and the data constructors together, but first the commentaries need to be commented out (easy with Vim’s visual block mode):

data MyType
  = A
  | B
  | C

  -- comment1
  -- comment2
  -- comment3

Now our macro can do the job. Mark the line data MyType with a and the empty line above the first comment with b and execute it:

data MyType
  -- comment1
  = A
  -- comment2
  | B
  -- comment3
  | C