Add a string at the beginning of a line through builtin functions

Question

I am practicing some vimscript and, although there are very good plugins out there, I am writing a function for commenting/uncommenting code for the matter of experimenting.

So far, I wrote the following:

def g:MyComment(comment_symbol: string)
    if trim(getline("."))[0] == comment_symbol
        normal! ^2x
    else 
        ?????
    endif
enddef

augroup commenting_blocks_of_code
    autocmd!
    autocmd FileType c,cpp,java.      b:comment_symbol = '// '
    autocmd FileType sh,vim,python    b:comment_symbol = '# '
    autocmd FileType tex              b:comment_symbol = '% '
augroup END

The function works well for uncommenting one line.

However, I wish to add the string comment_symbol .. " " at the beginning of the current line in position ^ but I cannot figure out how, i.e. I have to fill the ????? line according to the mentioned requirements.

If possible, I would like to use builtin vim functions.

The next step would be to modify my function so that it can accept a start_line and an end_line as argument, i.e. I want to modify my function signature with def g:MyComment(start_line: number, end_line:number, comment_symbol: string) so that I can comment/uncomment a bunch of lines picked e.g. by a selection in Visual mode or by explicitly specifying a range.

Then, I can throw everything and install vim-commentary. :D

Answer 1

To put the value of b:comment_symbol at the beginning of a line, you can use

execute "normal 0i" . b:comment_symbol

Or, if you want to put your comment symbol before the first non-whitespace character (i.e. have your comments follow existing indentation), use

execute "normal I" . b:comment_symbol

Maybe there are more elegant solutions. I tried to find one that fits into the style of your code.

Having said that, I think that the :substitute command is much better suited for this task:

substitute/^/\=b:comment_symbol/

I also don't think that whitespace should be part of b:comment_symbol. Instead, it should be appended when inserting or removed when removing the comment. As your code is currently written, removing a C, C++ or Java comment will leave in one space while removing a comment from the (python) line #foo will chop off the f.