How to map f+ combination to def function ?

Hi,

I just want to map  f+ or f- to increase or decrease guifont size,

what's the best way to do this ? 

def AugmentGuifontSize(): void
    var size = &guifont->split(':')[1]->strpart(1,3)
    echomsg size
enddef
impossible ??  nnoremap f+   :call AugmentGuifontSize()<CR>  

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