Add ormolu fixer.

This commit adds a fixer for the Haskell language, ormolu
(https://github.com/tweag/ormolu).

Signed-off-by: David Wood <david@davidtw.co>

autoload/ale/fix/registry.vim

@@ -380,6 +380,11 @@ let s:default_registry = {
 \       'suggested_filetypes': ['dhall'],
 \       'description': 'Fix Dhall files with dhall-format.',
 \   },
+\   'ormolu': {
+\       'function': 'ale#fixers#ormolu#Fix',
+\       'suggested_filetypes': ['haskell'],
+\       'description': 'A formatter for Haskell source code.',
+\   },
 \}
 
 " Reset the function registry to the default entries.

autoload/ale/fixers/ormolu.vim

@@ -0,0 +1,12 @@
+call ale#Set('haskell_ormolu_executable', 'ormolu')
+call ale#Set('haskell_ormolu_options', '')
+
+function! ale#fixers#ormolu#Fix(buffer) abort
+    let l:executable = ale#Var(a:buffer, 'haskell_ormolu_executable')
+    let l:options = ale#Var(a:buffer, 'haskell_ormolu_options')
+
+    return {
+    \   'command': ale#Escape(l:executable)
+    \       . (empty(l:options) ? '' : ' ' . l:options),
+    \}
+endfunction