Add support for post-processing fixer output

autoload/ale/fix.vim

@@ -109,6 +109,15 @@ function! s:HandleExit(job_id, exit_code) abort
     endif
 
     let l:chain_callback = get(l:job_info, 'chain_with', v:null)
+    let l:ProcessWith = get(l:job_info, 'process_with', v:null)
+
+    " Post-process the output with a function if we have one.
+    if l:ProcessWith isnot v:null
+        let l:job_info.output = call(
+        \   ale#util#GetFunction(l:ProcessWith),
+        \   [l:buffer, l:job_info.output]
+        \)
+    endif
 
     " Use the output of the job for changing the file if it isn't empty,
     " otherwise skip this job and use the input from before.
@@ -226,6 +235,7 @@ function! s:RunJob(options) abort
     \   'chain_with': l:chain_with,
     \   'callback_index': a:options.callback_index,
     \   'callback_list': a:options.callback_list,
+    \   'process_with': a:options.process_with,
     \}
 
     if l:read_temporary_file
@@ -329,6 +339,7 @@ function! s:RunFixer(options) abort
             \   'chain_with': l:Chain_with,
             \   'callback_list': a:options.callback_list,
             \   'callback_index': l:index,
+            \   'process_with': get(l:result, 'process_with', v:null),
             \})
 
             if !l:job_ran

doc/ale.txt

@@ -459,6 +459,15 @@ are supported for running the commands.
                       for commands which need to modify some file on disk in
                       order to fix files.
 
+  `process_with`        An optional callback for post-processing.
+
+                      The callback must accept two arguments,
+                      `(buffer, output)`, which can be used for converting
+                      the output from a command into lines to replace the
+                      buffer's contents with.
+
+                      A |List| of |String|s must be returned.
+
   `chain_with`          An optional key for defining a callback to call next.
 
                       The callback must accept two or three arguments,

test/fix/test_ale_fix.vader

@@ -133,6 +133,25 @@ Before:
     return empty(l:lines) ? '' : l:lines[-1]
   endfunction
 
+  function! FixWithJSONPostProcessing(buffer) abort
+    let l:ProcessWith = 'JSONPostProcessor'
+
+    " Test with lambdas where support is available.
+    if has('lambda')
+      let l:ProcessWith = {buffer, output -> JSONPostProcessor(buffer, output)}
+    endif
+
+    return {
+    \ 'command': 'echo ' . ale#Escape('{"output": ["x", "y", "z"]}'),
+    \ 'read_buffer': 0,
+    \ 'process_with': l:ProcessWith,
+    \}
+  endfunction
+
+  function! JSONPostProcessor(buffer, output) abort
+    return json_decode(a:output[0]).output
+  endfunction
+
 After:
   Restore
   unlet! g:ale_run_synchronously
@@ -160,6 +179,8 @@ After:
   delfunction GetLastMessage
   delfunction IgnoredEmptyOutput
   delfunction EchoLineNoPipe
+  delfunction FixWithJSONPostProcessing
+  delfunction JSONPostProcessor
 
   call ale#test#RestoreDirectory()
 
@@ -612,3 +633,12 @@ Execute(A temporary file shouldn't be piped into the command when disabled):
 
 Expect(The new line should be used):
   new line
+
+Execute(Post-processing should work):
+  let g:ale_fixers.testft = ['FixWithJSONPostProcessing']
+  ALEFix
+
+Expect(The lines in the JSON should be used):
+  x
+  y
+  z