Setup a placeholder for 'FoldText()' function.

after/ftplugin/markdown/folding.vim

@@ -1,4 +1,8 @@
 " Helpers {{{1
+function! s:SID()
+  return matchstr(expand('<sfile>'), '<SNR>\d\+_')
+endfunction
+
 function! HeadingDepth(lnum)
   let level=0
   let thisline = getline(a:lnum)
@@ -17,12 +21,18 @@ function! HeadingDepth(lnum)
   endif
   return level
 endfunction
+
+function! FoldText()
+  return 'placeholder'
+endfunction
+
 " Setup {{{1
 setlocal foldmethod=expr
+let &l:foldtext = s:SID() . 'FoldText()'
 command! -buffer FoldToggle
 " Teardown {{{1
 let b:undo_ftplugin .= '
-  \ | setlocal foldmethod<
+  \ | setlocal foldmethod< foldtext<
   \ | delcommand FoldToggle
   \ '
 " vim:set fdm=marker:

test/folding.vim

@@ -22,6 +22,10 @@ describe 'setting filetype=markdown'
     Expect &l:foldmethod ==# 'expr'
   end
 
+  it 'applies foldtext=FoldText()'
+    Expect &l:foldtext =~# '<SNR>\d_FoldText()'
+  end
+
   it 'creates :FoldToggle command'
     Expect exists(':FoldToggle') == 2
   end
@@ -44,6 +48,11 @@ describe 'setting filetype!=markdown'
     Expect &l:foldmethod ==# 'manual'
   end
 
+  it 'resets foldtext to default'
+    setlocal filetype=
+    Expect &l:foldtext =~# 'foldtext()'
+  end
+
   it 'destroys :FoldToggle command'
     setlocal filetype=
     Expect exists(':FoldToggle') == 0
@@ -130,3 +139,26 @@ describe 'HeadingDepth'
   end
 
 end
+
+describe 'FoldText'
+
+  before
+    silent tabnew
+    setlocal filetype=markdown
+  end
+
+  after
+    silent tabclose!
+  end
+
+  " TODO: return to this after implementing foldexpr
+  " it 'uses "# level one" headings as is'
+  "   call PopulateBuffer([
+  "         \ '# Level one heading',
+  "         \ '',
+  "         \ 'Lorem ipsum dolor sit amet...',
+  "         \ ])
+  "   Expect foldtextresult('1') ==# '# Level one heading [2 lines]'
+  " end
+
+end