Add stylish-haskell as a fixer (#1837)

* Add stylish-haskell as a fixer

`stylish-haskell` is a common formatting tool for the haskell toolchain.
It is not as advanced as `brittany` or `hindent`, but it is commonly
used for formatting of imports and data declarations. This adds it as a
fixer in ALE.

autoload/ale/fix/registry.vim

@@ -170,6 +170,11 @@ let s:default_registry = {
 \       'suggested_filetypes': ['haskell'],
 \       'description': 'Fix Haskell files with brittany.',
 \   },
+\   'stylish-haskell': {
+\       'function': 'ale#fixers#stylish_haskell#Fix',
+\       'suggested_filetypes': ['haskell'],
+\       'description': 'Refactor Haskell files with stylish-haskell.',
+\   },
 \   'refmt': {
 \       'function': 'ale#fixers#refmt#Fix',
 \       'suggested_filetypes': ['reason'],

autoload/ale/fixers/stylish_haskell.vim

@@ -0,0 +1,15 @@
+" Author: eborden <evan@evan-borden.com>
+" Description: Integration of stylish-haskell formatting with ALE.
+"
+call ale#Set('haskell_stylish_haskell_executable', 'stylish-haskell')
+
+function! ale#fixers#stylish_haskell#Fix(buffer) abort
+    let l:executable = ale#Var(a:buffer, 'haskell_stylish_haskell_executable')
+
+    return {
+    \   'command': ale#Escape(l:executable)
+    \       . ' --inplace'
+    \       . ' %t',
+    \   'read_temporary_file': 1,
+    \}
+endfunction