Before: Save g:ale_reasonml_refmt_executable Save g:ale_reasonml_refmt_options " Use an invalid global executable, so we don't match it. let g:ale_reasonml_refmt_executable = 'xxxinvalid' let g:ale_reasonml_refmt_options = '' call ale#test#SetDirectory('/testplugin/test/fixers') After: Restore call ale#test#RestoreDirectory() Execute(The refmt callback should return the correct default values): call ale#test#SetFilename('../reasonml_files/testfile.re') AssertEqual \ { \ 'read_temporary_file': 1, \ 'command': ale#Escape('xxxinvalid') \ . ' --in-place' \ . ' %t', \ }, \ ale#fixers#refmt#Fix(bufnr('')) Execute(The refmt callback should include custom refmt options): let g:ale_reasonml_refmt_options = "-w 80" call ale#test#SetFilename('../reasonml_files/testfile.re') AssertEqual \ { \ 'read_temporary_file': 1, \ 'command': ale#Escape('xxxinvalid') \ . ' ' . g:ale_reasonml_refmt_options \ . ' --in-place' \ . ' %t', \ }, \ ale#fixers#refmt#Fix(bufnr(''))