#1278 Allow linters to be defined pretty much anywhere

autoload/ale/linter.vim

@@ -3,6 +3,7 @@ call ale#Set('wrap_command_as_one_argument', 0)
 " Description: Linter registration and lazy-loading
 "   Retrieves linters as requested by the engine, loading them if needed.
 
+let s:runtime_loaded_map = {}
 let s:linters = {}
 
 " Default filetype aliases.
@@ -38,6 +39,7 @@ let s:default_ale_linters = {
 
 " Testing/debugging helper to unload all linters.
 function! ale#linter#Reset() abort
+    let s:runtime_loaded_map = {}
     let s:linters = {}
 endfunction
 
@@ -250,20 +252,20 @@ function! ale#linter#Define(filetype, linter) abort
     call add(s:linters[a:filetype], l:new_linter)
 endfunction
 
+" Prevent any linters from being loaded for a given filetype.
+function! ale#linter#PreventLoading(filetype) abort
+    let s:runtime_loaded_map[a:filetype] = 1
+endfunction
+
 function! ale#linter#GetAll(filetypes) abort
     let l:combined_linters = []
 
     for l:filetype in a:filetypes
-        " Load linter defintions from files if we haven't loaded them yet.
-        if !has_key(s:linters, l:filetype)
+        " Load linters from runtimepath if we haven't done that yet.
+        if !has_key(s:runtime_loaded_map, l:filetype)
             execute 'silent! runtime! ale_linters/' . l:filetype . '/*.vim'
 
-            " Always set an empty List for the loaded linters if we don't find
-            " any. This will prevent us from executing the runtime command
-            " many times, redundantly.
-            if !has_key(s:linters, l:filetype)
-                let s:linters[l:filetype] = []
-            endif
+            let s:runtime_loaded_map[l:filetype] = 1
         endif
 
         call extend(l:combined_linters, get(s:linters, l:filetype, []))

doc/ale.txt

@@ -2359,6 +2359,21 @@ ale#linter#Define(filetype, linter)                       *ale#linter#Define()*
   containing `.` characters will be split into individual parts, and files
   will be loaded for each filetype between the `.` characters.
 
+  Linters can be defined from vimrc and other files as long as this function
+  is loaded first. For example, the following code will define a Hello World
+  linter in vimrc in Vim 8: >
+
+  " Plugins have to be loaded first.
+  " If you are using a plugin manager, run that first.
+  packloadall
+
+  call ale#linter#Define('vim', {
+  \   'name': 'echo-test',
+  \   'executable': 'echo',
+  \   'command': 'echo hello world',
+  \   'callback': {buffer, lines -> map(lines, '{"text": v:val, "lnum": 1}')},
+  \})
+<
 
 ale#linter#Get(filetype)                                     *ale#linter#Get()*
 
@@ -2374,6 +2389,13 @@ ale#linter#Get(filetype)                                     *ale#linter#Get()*
   components.
 
 
+ale#linter#PreventLoading(filetype)               *ale#linter#PreventLoading()*
+
+  Given a `filetype`, prevent any more linters from being loaded from
+  |runtimepath| for that filetype. This function can be called from vimrc or
+  similar to prevent ALE from loading linters.
+
+
 ale#statusline#Count(buffer)                           *ale#statusline#Count()*
 
   Given the number of a buffer which may have problems, return a |Dictionary|

test/test_ale_info.vader

@@ -32,6 +32,7 @@ Before:
 
   call ale#engine#ResetExecutableCache()
   call ale#linter#Reset()
+  call ale#linter#PreventLoading('testft')
   let g:ale_linters = {}
   let g:ale_fixers = {}
   let g:ale_linter_aliases = {}

test/test_linter_retrieval.vader

@@ -5,6 +5,9 @@ Before:
   let g:testlinter1 = {'name': 'testlinter1', 'executable': 'testlinter1', 'command': 'testlinter1', 'callback': 'testCB1', 'output_stream': 'stdout', 'read_buffer': 1, 'lint_file': 0, 'aliases': [], 'lsp': '', 'add_newline': 0}
   let g:testlinter2 = {'name': 'testlinter2', 'executable': 'testlinter2', 'command': 'testlinter2', 'callback': 'testCB2', 'output_stream': 'stdout', 'read_buffer': 0, 'lint_file': 1, 'aliases': [], 'lsp': '', 'add_newline': 0}
   call ale#linter#Reset()
+  call ale#linter#PreventLoading('testft')
+  call ale#linter#PreventLoading('javascript')
+  call ale#linter#PreventLoading('typescript')
 
 After:
   Restore
@@ -145,6 +148,7 @@ Execute (Buffer-local overrides for aliases should be used):
   AssertEqual [g:testlinter1, g:testlinter2], ale#linter#Get('testft1')
 
 Execute (Linters should be loaded from disk appropriately):
+  call ale#linter#Reset()
   AssertEqual [{'name': 'testlinter', 'output_stream': 'stdout', 'executable': 'testlinter', 'command': 'testlinter', 'callback': 'testCB', 'read_buffer': 1, 'lint_file': 0, 'aliases': [], 'lsp': '', 'add_newline': 0}], ale#linter#Get('testft')