Question

Following code has been picked up from this blog

function! Privatize()
  let priorMethod = PriorMethodDefinition()
  exec "normal iprivate :" . priorMethod  . "\<Esc>=="
endfunction

function! PriorMethodDefinition()
  let lineNumber = search('def', 'bn')
  let line       = getline(lineNumber)
  if line == 0
    echo "No prior method definition found"
  endif
  return matchlist(line, 'def \(\w\+\).*')[1]
endfunction

map <Leader>p :call Privatize()<CR>

I tried but I fail to understand PriorMethodDefinition method. Can someone walk me through this code?

Answer 1

PriorMethodDefinition returns the name of the first method definition above the cursor.

It does this by searching backwards for a line containing the text ‘def’. The search function returns the line number and getline is used to retrieve the content of that line.

The function checks that it has found a valid line, before using a regular expression to get the name of the method and return it.

You can read more about these functions if you’re curious about the specifics – see:

:help search
:help getline
:help matchlist

Edit: you can also read about the regular expression pattern

:help pattern

But I found it a little confusing at first, so allow me to explain it a little. Here’s the expression used:

'def \(\w\+\).*'

This will search for any text matching the following pattern: “the text def followed by one or more ‘word’ characters \w\+ followed by zero or more characters .*“. The part matching the word characters is placed into a group (or atom), designated by the escaped parens \( & \). More info on the definitions of word characters etc can be found in the help link above.

The matchlist function returns a list of matches, the first [0] of which is the full text matching the regex, followed by submatches (ie our group). We are interested in the first such submatch, hence the [1].