How to check last char is { in vimscript?

Question

I want to create a {} completion myself, and I did:

inoremap <CR> if_open_curly_on_left()? <CR>}<ESC>O : <CR>
function! s:if_open_curly_on_left() abort
    let col = col('.') - 1
    return !col || getline('.')[col - 1]  =~# '{'
endfunction

which is modified by other's code, since I don't really understand the meaning inside the function.

When I typed enter I got:

How to fix this?

Answer 1

The logic is wrong here:

getline('.')[col - 1]  =~# '{'

This is saying anywhere before my cursor do I have a { character.

Probably want something that says does the end have a { character

getline('.')[col - 1]  =~# '{$'
getline('.')[col - 1] == '{'

Although in theory that could still be weird if you put your insert cursor between {} characters and hit <cr>. So I imagine this needs to be adjusted a bit to become more robust

Maybe you the logic and be something like:

col('.') == (col('$') - 1) && strpart(getline('.'), -1) == '{'

This will check that your in the final column and it ends with {

Aside:

Why col('.')-2 to get the previous character?

TL;DR: column position is 1-indexed and strings/arrays are 0-indexed

col('.') give you the current cursor position with the first column being 1

getline('.') will return a string or an array of characters with the first index being 0

Assume the following line and you cursor on the r:

bar

So if we do a getline('.') and split it e.g. split(getline('.'), '\zs') we get:

['b', 'a', 'r']

Since we are on the r character then col('.') gives us 3 which is outside the bounds of our array by one. We compensate for the column starting on position one by subtracting one, col('.') - 1, to give the current position. To give the "previous character" then subtract by 2

Answer 2

After some search this work(, but might be simplified further):

" My try
inoremap <silent><expr> <CR>
    \ <SID>if_open_curly_on_left()? "\<CR>}\<ESC>O":
    \ "\<CR>"
function! s:if_open_curly_on_left() abort
    let col = col('.') - 1
    return getline('.')[col - 1] == '{'
endfunction