Simplify the code for most linters and tests with closures

ale_linters/scala/fsc.vim

@@ -1,29 +1,14 @@
 " Author: Nils Leuzinger - https://github.com/PawkyPenguin
 " Description: Basic scala support using fsc
-"
-function! ale_linters#scala#fsc#GetExecutable(buffer) abort
-    if index(split(getbufvar(a:buffer, '&filetype'), '\.'), 'sbt') >= 0
-        " Don't check sbt files
-        return ''
-    endif
 
-    return 'fsc'
-endfunction
-
-function! ale_linters#scala#fsc#GetCommand(buffer) abort
-    let l:executable = ale_linters#scala#fsc#GetExecutable(a:buffer)
-
-    if empty(l:executable)
-        return ''
-    endif
-
-    return ale#Escape(l:executable) . ' -Ystop-after:parser %t'
+function! s:IsSbt(buffer) abort
+    return index(split(getbufvar(a:buffer, '&filetype'), '\.'), 'sbt') >= 0
 endfunction
 
 call ale#linter#Define('scala', {
 \   'name': 'fsc',
-\   'executable_callback': 'ale_linters#scala#fsc#GetExecutable',
-\   'command_callback': 'ale_linters#scala#fsc#GetCommand',
+\   'executable_callback': {buf -> s:IsSbt(buf) ? '' : 'fsc'},
+\   'command': '%e -Ystop-after:parser %t',
 \   'callback': 'ale#handlers#scala#HandleScalacLintFormat',
 \   'output_stream': 'stderr',
 \})