adds fixer support for hfmt (#1027)

Add support for fixing Haskell with hfmt

autoload/ale/fix/registry.vim

@@ -117,6 +117,11 @@ let s:default_registry = {
 \       'suggested_filetypes': ['rust'],
 \       'description': 'Fix Rust files with Rustfmt.',
 \   },
+\   'hfmt': {
+\       'function': 'ale#fixers#hfmt#Fix',
+\       'suggested_filetypes': ['haskell'],
+\       'description': 'Fix Haskell files with hfmt.',
+\   },
 \}
 
 " Reset the function registry to the default entries.

autoload/ale/fixers/hfmt.vim

@@ -0,0 +1,16 @@
+" Author: zack <zack@kourouma.me>
+" Description: Integration of hfmt with ALE.
+
+call ale#Set('haskell_hfmt_executable', 'hfmt')
+
+function! ale#fixers#hfmt#Fix(buffer) abort
+    let l:executable = ale#Var(a:buffer, 'haskell_hfmt_executable')
+
+    return {
+    \   'command': ale#Escape(l:executable)
+    \       . ' -w'
+    \       . ' %t',
+    \   'read_temporary_file': 1,
+    \}
+endfunction
+