Allow ALEFix functions to be defined with only the buffer argument

autoload/ale/fix.vim

@@ -232,7 +232,10 @@ function! s:RunFixer(options) abort
     let l:index = a:options.callback_index
 
     while len(a:options.callback_list) > l:index
-        let l:result = call(a:options.callback_list[l:index], [l:buffer, copy(l:input)])
+        let l:Function = a:options.callback_list[l:index]
+        let l:result = ale#util#FunctionArgCount(l:Function) == 1
+        \   ? call(l:Function, [l:buffer])
+        \   : call(l:Function, [l:buffer, copy(l:input)])
 
         if type(l:result) == type(0) && l:result == 0
             " When `0` is returned, skip this item.

doc/ale.txt

@@ -770,10 +770,16 @@ The values for `g:ale_fixers` can be a list of |String|, |Funcref|, or
 |lambda| values. String values must either name a function, or a short name
 for a function set in the ALE fixer registry.
 
-Each function for fixing errors must accept two arguments `(buffer, lines)`,
-representing the buffer being fixed and the lines to fix. The functions must
-return either `0`, for changing nothing, a |List| for new lines to set, or a
-|Dictionary| for describing a command to be run in the background.
+Each function for fixing errors must accept either one argument `(buffer)` or
+two arguments `(buffer, lines)`, representing the buffer being fixed and the
+lines to fix. The functions must return either `0`, for changing nothing, a
+|List| for new lines to set, or a |Dictionary| for describing a command to be
+run in the background.
+
+Functions receiving a variable number of arguments will not receive the second
+argument `lines`. Functions should name two arguments if the `lines` argument
+is desired. This is required to avoid unnecessary copying of the lines of
+the buffers being checked.
 
 When a |Dictionary| is returned for an |ALEFix| callback, the following keys
 are supported for running the commands.

test/test_ale_fix.vader

@@ -35,6 +35,10 @@ Before:
     return {'command': 'cat - <(echo d)'}
   endfunction
 
+  function CatLineOneArg(buffer) abort
+    return {'command': 'cat - <(echo d)'}
+  endfunction
+
   function ReplaceWithTempFile(buffer, lines) abort
     return {'command': 'echo x > %t', 'read_temporary_file': 1}
   endfunction
@@ -43,6 +47,10 @@ Before:
     return ['a', 'b']
   endfunction
 
+  function RemoveLastLineOneArg(buffer) abort
+    return ['a', 'b']
+  endfunction
+
   function! TestCallback(buffer, output)
     return [{'lnum': 1, 'col': 1, 'text': 'xxx'}]
   endfunction
@@ -65,8 +73,10 @@ After:
   delfunction AddDollars
   delfunction DoNothing
   delfunction CatLine
+  delfunction CatLineOneArg
   delfunction ReplaceWithTempFile
   delfunction RemoveLastLine
+  delfunction RemoveLastLineOneArg
   delfunction TestCallback
   delfunction SetUpLinters
   call ale#fix#registry#ResetToDefaults()
@@ -315,3 +325,21 @@ Execute(ale#fix#InitBufferData() should set up the correct data):
   \   'should_save': 1,
   \ },
   \}, g:ale_fix_buffer_data
+
+Execute(ALEFix simple functions should be able to accept one argument, the buffer):
+  let g:ale_fixers.testft = ['RemoveLastLineOneArg']
+  ALEFix
+
+Expect(There should be only two lines):
+  a
+  b
+
+Execute(ALEFix functions returning jobs should be able to accept one argument):
+  let g:ale_fixers.testft = ['CatLine']
+  ALEFix
+
+Expect(An extra line should be added):
+  a
+  b
+  c
+  d