Ttdfjt

Got this:

let Func = function(folding_function) "folding_function is name of function
setlocal foldexpr=call(Func(v:lnum))

This is so user can set a custom function for folding in their config file. I can't get it to work, though.

Also tried:

call(Func, v:lnum)

and

call(Func, 'v:lnum')

Code in context

" Set settings which are local to a window. In a new tab they would be reset to
" Vim defaults. So we enforce our settings here when the cursor enters a
" Vimwiki buffer.
function! s:set_windowlocal_options()
 if !&diff " if Vim is currently in diff mode, don't interfere with its folding
 let foldmethod = vimwiki#vars#get_global('folding')
 if foldmethod =~? '^expr.*'
 setlocal foldmethod=expr
 let custom = vimwiki#vars#get_global('custom_fold_func')
 if custom
 let Func = function('VimwikiFoldLevelCustom')
 setlocal foldexpr=Func(v:lnum)
" setlocal foldexpr=VimwikiFoldLevelCustom(v:lnum)
 else
 setlocal foldexpr=VimwikiFoldLevel(v:lnum)
 endif
 setlocal foldtext=VimwikiFoldText()
 elseif foldmethod =~? '^list.*' || foldmethod =~? '^lists.*'
 setlocal foldmethod=expr
 setlocal foldexpr=VimwikiFoldListLevel(v:lnum)
 setlocal foldtext=VimwikiFoldText()
 elseif foldmethod =~? '^syntax.*'
 setlocal foldmethod=syntax
 setlocal foldtext=VimwikiFoldText()
 elseif foldmethod =~? '^custom.*'
 " do nothing
 else
 setlocal foldmethod=manual
 normal! zE
 endif
 endif

 if vimwiki#vars#get_global('conceallevel') && exists("+conceallevel")
 let &conceallevel = vimwiki#vars#get_global('conceallevel')
 endif

 if vimwiki#vars#get_global('auto_chdir')
 exe 'lcd' vimwiki#vars#get_wikilocal('path')
 endif
endfunction

The value of the foldexpr option is evaluated to get the foldlevel of a line. You don't need to add extra call to it. You can copy following code in a new file and source it to check how it works.

" vim:set foldmethod=expr noexpandtab:

function! FoldingFunction(lnum)
 return getline(v:lnum)[0]==#"t"
endfunction
let Func = function('FoldingFunction') "FoldingFunction is name of function
setlocal foldexpr=Func(v:lnum)

finish

 fold
 fold
 fold
 fold
 fold

update

Your code doesn't work because your Func is a function local variable, change it to g:Func fix the problem.