Before: Save g:ale_haskell_stylish_haskell_executable " Use an invalid global executable, so we don't match it. let g:ale_haskell_stylish_haskell_executable = 'xxxinvalid' call ale#test#SetDirectory('/testplugin/test/fixers') After: Restore call ale#test#RestoreDirectory() Execute(The stylish-haskell callback should return the correct default values): call ale#test#SetFilename('../haskell_files/testfile.hs') AssertEqual \ { \ 'read_temporary_file': 1, \ 'command': ale#Escape('xxxinvalid') \ . ' --inplace' \ . ' %t', \ }, \ ale#fixers#stylish_haskell#Fix(bufnr(''))