synIDattr returns nothing in function

Question

I have this part of code:

function! Test()
    echom synIDattr(synID(line('.'), col('.'), 1), 'name')
endfunction

autocmd TextChangedI <buffer> call Test()

I expected that every time I change some stuff the current "type" would be printed out. For example, if I'm in a commentary line of vim, I'd expect to see that at the bottom: vimLineComment. But nothing pops up.

If I enter

:echom synIDattr(synID(line('.'), col('.'), 1), 'name')

I'm getting my expected output. What am I doing wrong?

I've also tried this (which was written in :h synIDattr):

echo synIDattr(synIDtrans(synID(line("."), col("."), 1)), "name")

But it also doesn't work inside my Test() function:

function! Test()
    echo synIDattr(synIDtrans(synID(line("."), col("."), 1)), "name")
endfunction

autocmd TextChangedI <buffer> call Test()

If I insert echom "Test" I'm getting Test at the bottom everytime if I enter something. What am I doing wrong?

Answer 1

The provided code seemed to be working when I tried it and we established that OP is seeing the same behavior as I am. So that leaves a difference in expectations, I think, about what the correct behavior is.

Three important pieces of information:

1. Text colored by syntax highlighting is selected by rules, often regex patterns, and they specify where a matching block of text ends.
2. If you start appending text to the end of a line the cursor will usually be beyond the end of one of those defined blocks and so, naturally, you won't be inside a syntax item and nothing will be displayed by the autocmd/function.
3. In Normal mode the cursor is less likely to be beyond the end of a block since even after running $ it sits on top of the last character of the line rather than beyond it.

The example I gave in a comment was entering text before and after and endif statement in a vimscript function. With the former you'll see something like VimFuncBody but with the latter nothing will be displayed. Conversely, with the cursor at the end of the same line in Normal mode if you enter :call Test() you'll see VimCommand. Hopefully the three points above explain all this behavior.

Ah, but what about a line that begins with ", i.e. a line comment. Since the pattern for that extends to the end of the line (regex $) we should see vimlineComment when adding to the end of the comment but we dont! Well, no, because with " my comment the block ends on that final t ($ is a zero-width match) but the cursor is beyond that when we begin appending to the line.

(Admittedly, that last case is a little troubling because the text you type actually gets the appropriate comment color. Seems like a quirk of the implementation. Clearly dynamic text coloring and the output of synID*() don't operate by precisely the same rules and/or the same refresh timings.)