Detect and use CM files for smlnj (#884)

* Detect and use CM files for smlnj

* Split into two checkers

- one for CM projects
- one for single SML files

* Fix some typos

* Fix error caught by writing tests

We want to actually use `glob` to search in paths upwards from us.
(Previously we were just searching in the current directory every time!)

* Fix errors from former test run

* Write tests for GetCmFile and GetExecutableSmlnj

* Typo in 'smlnj/' fixture filenames

ale_linters/sml/smlnj_cm.vim

@@ -0,0 +1,25 @@
+" Author: Jake Zimmerman <jake@zimmerman.io>
+" Description: SML checking with SML/NJ Compilation Manager
+
+" Let user manually set the CM file (in case our search for a CM file is
+" ambiguous and picks the wrong one)
+"
+" See :help ale-sml-smlnj for more information.
+call ale#Set('sml_smlnj_cm_file', '*.cm')
+
+function! ale_linters#sml#smlnj_cm#GetCommand(buffer) abort
+    let l:cmfile = ale#handlers#sml#GetCmFile(a:buffer)
+    return 'sml -m ' . l:cmfile . ' < /dev/null'
+endfunction
+
+" Using CM requires that we set "lint_file: 1", since it reads the files
+" from the disk itself.
+call ale#linter#Define('sml', {
+\   'name': 'smlnj-cm',
+\   'executable_callback': 'ale#handlers#sml#GetExecutableSmlnjCm',
+\   'lint_file': 1,
+\   'command_callback': 'ale_linters#sml#smlnj_cm#GetCommand',
+\   'callback': 'ale#handlers#sml#Handle',
+\})
+
+" vim:ts=4:sts=4:sw=4