How to toggle Vim's search highlight visibility without disabling it

Question

What I'd like is to map one key, e.g. F4, so that pressing F4 will toggle the visibility of search highlights, and so that starting a new search enables visibility no matter the current visibility.

What I've tried:

1. Mapping F4 to :nohlsearch temporarily disables highlight visibility without turning the hlsearch setting off, but it does not toggle visibility back again.
2. Mapping F4 to :set hlsearch! does toggle on/off, but I don't want to toggle the hlsearch setting off, just the visibility setting. If hlsearch is off then it doesn't come back automatically with a new search.

There doesn't seem to be an opposite form of :nohlsearch and the command itself has problems being called from a function.

I've found similiar questions, but they don't provide an answer.

Update:
The first comment provides exactly what I was asking for, reproduced below:

let hlstate=0
nnoremap <F4> :if (hlstate == 0) \| nohlsearch \| else \| set hlsearch \| endif \| let hlstate=1-hlstate<cr>

(N.B. for anyone using this --- cramming the map onto one line instead of using a function is necessary since you can't effect a change on highlighting from inside a function.)

Related question for slightly different functionality: https://stackoverflow.com/a/16750393/1176650

Answer 1

Note, recent Vims (7.4.79) have the v:hlsearch variable available. This means you can improve your mapping to:

:nnoremap <silent><expr> <Leader>h (&hls && v:hlsearch ? ':nohls' : ':set hls')."\n"