Fix #130 - Run linters when the filetype is changed

doc/ale.txt

@@ -276,6 +276,17 @@ g:ale_lint_on_enter                                       *g:ale_lint_on_enter*
   behaviour.
 
 
+g:ale_lint_on_filetype_changed                 *g:ale_lint_on_filetype_changed*
+
+  Type: |Number|
+  Default: `1`
+
+  This option will cause ALE to run whenever the filetype is changed. A short
+  delay will be used before linting will be done, so the filetype can be
+  changed quickly several times in a row, but resulting in only one lint
+  cycle.
+
+
 g:ale_lint_on_save                                         *g:ale_lint_on_save*
 
   Type: |Number|

plugin/ale.vim

@@ -78,6 +78,9 @@ let g:ale_lint_on_enter = get(g:, 'ale_lint_on_enter', 1)
 " This flag can be set to 1 to enable linting when a buffer is written.
 let g:ale_lint_on_save = get(g:, 'ale_lint_on_save', 1)
 
+" This flag can be set to 1 to enable linting when the filetype is changed.
+let g:ale_lint_on_filetype_changed = get(g:, 'ale_lint_on_filetype_changed', 1)
+
 " This flag may be set to 0 to disable ale. After ale is loaded, :ALEToggle
 " should be used instead.
 let g:ale_enabled = get(g:, 'ale_enabled', 1)
@@ -161,6 +164,13 @@ function! s:ALEInitAuGroups() abort
         endif
     augroup END
 
+    augroup ALERunOnFiletypeChangeGroup
+        autocmd!
+        if g:ale_enabled && g:ale_lint_on_filetype_changed
+            autocmd FileType * call ale#Queue(300, 'lint_file')
+        endif
+    augroup END
+
     augroup ALERunOnSaveGroup
         autocmd!
         if g:ale_enabled && g:ale_lint_on_save

test/test_ale_toggle.vader

@@ -15,6 +15,7 @@ Before:
   \ 'ALECursorGroup',
   \ 'ALEHighlightBufferGroup',
   \ 'ALERunOnEnterGroup',
+  \ 'ALERunOnFiletypeChangeGroup',
   \ 'ALERunOnSaveGroup',
   \ 'ALERunOnTextChangedGroup',
   \]