More advanced options for fzf#complete

README.md

@@ -71,18 +71,25 @@ inoremap <expr> <c-x><c-k> fzf#complete('cat /usr/share/dict/words')
 ```
 
 For advanced uses, you can pass an options dictionary to the function. The set
-of options is almost identical to that for `fzf#run`, except that it takes
+of options is pretty much identical to that for `fzf#run` only with the
-`reducer` function instead of `sink` or `sink*`.
+following exceptions:
+
+- `reducer` (funcref)
+    - Reducer transforms the output lines of fzf into a single string value
+- `prefix` (string; default: `\k*$`)
+    - Regular expression pattern to extract the completion prefix
+- Both `source` and `options` can be given as funcrefs that take the
+  completion prefix as the argument and return the final value
+- `sink` or `sink*` are not allowed
 
 ```vim
-" Reducer function transforms the output of fzf into a single string value
+function! s:first_line_in_uppercase(lines)
-function! FirstLineInUpperCase(lines)
   return toupper(a:lines[0])
 endfunction
 
 inoremap <expr> <c-x><c-k> fzf#complete({
   \ 'source':  'cat /usr/share/dict/words',
-  \ 'reducer': function('FirstLineInUpperCase'),
+  \ 'reducer': function('<sid>first_line_in_uppercase'),
   \ 'options': '--no-multi --reverse --margin 15%,0',
   \ 'left':    20})
 ```

plugin/fzf.vim

@@ -436,10 +436,21 @@ function! s:complete_insert(lines)
   endif
 endfunction
 
+let s:TYPE = {'dict': type({}), 'funcref': type(function('call'))}
+
+function! s:eval(dict, key, arg)
+  if has_key(a:dict, a:key) && type(a:dict[a:key]) == s:TYPE.funcref
+    let ret = copy(a:dict)
+    let ret[a:key] = call(a:dict[a:key], [a:arg])
+    return ret
+  endif
+  return a:dict
+endfunction
+
 function! fzf#complete(...)
   if a:0 == 0
     let s:opts = copy(get(g:, 'fzf_window', s:default_window))
-  elseif type(a:1) == type({})
+  elseif type(a:1) == s:TYPE.dict
     if has_key(a:1, 'sink') || has_key(a:1, 'sink*')
       echoerr 'sink not allowed'
       return ''
@@ -455,8 +466,12 @@ function! fzf#complete(...)
   let s:eol = col('.') == eol
   let &ve = ve
 
+  let prefix = s:pluck(s:opts, 'prefix', '\k*$')
   let s:query = col('.') == 1 ? '' :
-        \ matchstr(getline('.')[0 : col('.')-2], '\k*$')
+        \ matchstr(getline('.')[0 : col('.')-2], prefix)
+  let s:opts = s:eval(s:opts, 'source', s:query)
+  let s:opts = s:eval(s:opts, 'options', s:query)
+
   call feedkeys("\<Plug>(-fzf-complete-trigger)")
   return ''
 endfunction