Change the backspace behavior in neovim

Question

I have the following function that inserts tabs if cursor is at the start of a line and required amount of spaces otherwise.

function! SpecialTab() abort
    if (col('.') == 1) || (matchstr(getline('.'), '\%'.(col('.') - 1).'c.') =~ '\t')
        return "\<Tab>"
    else
        return repeat("\<Space>", (&tabstop - (virtcol('.') % &tabstop) + 1))
    endif
endfunction
inoremap <expr> <Tab> SpecialTab()

If you use :set backspace=indent,eol,start with :set expandtab, one backspace can delete a tabs worth of spaces if you originally inserted a tabs worth of spaces by tapping the key in the first place. But if you manually typed required amount of spaces, one backspace just deletes one space.

I want to get that with my function. I don't have enough experience to understand C source code and I don't understand how backspace characteristics is maintained. Any help would be appreciated.

Edit

I have just checked that vim actually doesn't check whether the spaces were inserted with pressing <Tab> or <Space>. It just behaves that way until you escape from the insert mode. After you have escaped from insert mode and returned back to insert mode, if you try to use <BS> you'll see that it is deleting &tabstop amount (i.e. assuming the spaces are integer multiple of &tabstop) of spaces no matter how you inserted it.

Answer 1

We discuss only non indention space here.

During inserting, if you backspace after a series of tab and space (mixed), vim only check the last key stroke, if it's tab, it delete softtabstop number of spaces, other wise only 1 space deleted. If you backspace immediately after enter insert mode, vim delete softtabstop number of spaces. I don't believe vim knows if existing space are created by tab or space, it's a bit crazy.

Your backspace behaves different with default because you send 4 space to vim instead of a real tab. To fix that, you must fake a tab by adding an extra tab and backspace after 4 spaces, so vim will believe it's a tab, not space.

" vim: tabstop=4 shiftwidth=4 softtabstop=4 smarttab shiftround noexpandtab noet list listchars=tab\:+.,space\:-

function! SpecialTab() abort
    if (col('.') == 1) || (matchstr(getline('.'), '\%'.(col('.') - 1).'c.') =~ '\t')
        return "\<Tab>"
    else
        return repeat("\<Space>", (&tabstop - (virtcol('.') % &tabstop) + 1)) . "\<Tab>\<Backspace>"
    endif
endfunction
inoremap <expr> <Tab> SpecialTab()

update

I missed something, it's not working.

update

for unknow resason send \<space>\<space>\<space>\<space>\<tab>\<backspace> will insert a real tab. I have no idea why it behaves like this. But if i send \<space>\<space>\<space>\<space>a\<tab>\<backspace>\<backspace>, it works.

" vim: tabstop=4 shiftwidth=4 softtabstop=4 smarttab shiftround noexpandtab noet list listchars=tab\:+.,space\:-

function! SpecialTab() abort
    if (col('.') == 1) || (matchstr(getline('.'), '\%'.(col('.') - 1).'c.') =~ '\t')
        return "\<Tab>"
    else
        return repeat("\<Space>", (&tabstop - (virtcol('.') % &tabstop) + 1)) . "a\<tab>\<backspace>\<backspace>"
    endif
endfunction
inoremap <expr> <Tab> SpecialTab()

update

check comment for the reason why \<space>\<space>\<space>\<space>\<tab>\<backspace> result in a tab.