Support ghc-mod sig command (#68)

after/ftplugin/haskell/ghcmod.vim

@@ -49,6 +49,7 @@ endif
 command! -buffer -nargs=0 -bang GhcModType call ghcmod#command#type(<bang>0)
 command! -buffer -nargs=0 -bang GhcModTypeInsert call ghcmod#command#type_insert(<bang>0)
 command! -buffer -nargs=0 -bang GhcModSplitFunCase call ghcmod#command#split_function_case(<bang>0)
+command! -buffer -nargs=0 -bang GhcModSigCodegen call ghcmod#command#initial_code_from_signature(<bang>0)
 command! -buffer -nargs=? -bang GhcModInfo call ghcmod#command#info(<q-args>, <bang>0)
 command! -buffer -nargs=0 GhcModTypeClear call ghcmod#command#type_clear()
 command! -buffer -nargs=? -bang GhcModInfoPreview call ghcmod#command#info_preview(<q-args>, <bang>0)
@@ -61,6 +62,8 @@ command! -buffer -nargs=0 -bang GhcModExpand call ghcmod#command#expand(<bang>0)
 let b:undo_ftplugin .= join(map([
       \ 'GhcModType',
       \ 'GhcModTypeInsert',
+      \ 'GhcModSplitFunCase',
+      \ 'GhcModSigCodegen',
       \ 'GhcModInfo',
       \ 'GhcModInfoPreview',
       \ 'GhcModTypeClear',

autoload/ghcmod.vim

@@ -36,6 +36,16 @@ function! ghcmod#split(line, col, path, module) "{{{
   return split(l:parsed[5], '\n')
 endfunction "}}}
 
+function! ghcmod#sig(line, col, path, module) "{{{
+  " `ghc-mod sig` is available since v5.0.0.
+  let l:cmd = ghcmod#build_command(['sig', a:path, a:module, a:line, a:col])
+  let l:lines = s:system('sig', l:cmd)
+  if len(l:lines) < 3
+    return []
+  endif
+  return [l:lines[0], l:lines[2 :]]
+endfunction "}}}
+
 function! ghcmod#type(line, col, path, module) "{{{
   let l:cmd = ghcmod#build_command(['type', a:path, a:module, a:line, a:col])
   let l:output = ghcmod#system(l:cmd)

autoload/ghcmod/command.vim

@@ -73,6 +73,27 @@ function! ghcmod#command#split_function_case(force) "{{{
   delete _
 endfunction "}}}
 
+function! ghcmod#command#initial_code_from_signature(force) "{{{
+  let l:path = s:buffer_path(a:force)
+  if empty(l:path)
+    return
+  endif
+
+  let l:initial_code = ghcmod#sig(line('.'), col('.'), l:path, ghcmod#detect_module())
+  if empty(l:initial_code)
+    call ghcmod#util#print_warning('Cannot generate initial code')
+    return
+  endif
+
+  let [l:sort, l:codes] = l:initial_code
+  if l:sort == 'instance'
+    let l:sw = exists('*shifwidth') ? shiftwidth() : &shiftwidth
+    let l:indent = repeat(' ', l:sw)
+    call map(l:codes, 'l:indent . v:val')
+  endif
+  call append('.', l:codes)
+endfunction "}}}
+
 function! ghcmod#command#type_insert(force) "{{{
   let l:path = s:buffer_path(a:force)
   if empty(l:path)

test/data/sig/SigFunction.hs

@@ -0,0 +1,3 @@
+module SigFunction where
+
+func :: [a] -> Maybe b -> (a -> b) -> (a,b)

test/data/sig/SigInstance.hs

@@ -0,0 +1,9 @@
+module SigInstance where
+
+newtype D = D (Int,String)
+
+class C a where
+    cInt :: a -> Int
+    cString :: a -> String
+
+instance C D where

test/test_command_sig_codegen.vim

@@ -0,0 +1,23 @@
+let s:unit = tinytest#new()
+
+function! s:unit.test_command_sig_function()
+  edit test/data/sig/SigFunction.hs
+  call cursor(3, 1)
+  GhcModSigCodegen
+  call self.assert.match('^func x y f = ', getline(line('.')+1))
+endfunction
+
+function! s:unit.test_command_sig_instance()
+  edit test/data/sig/SigInstance.hs
+  call cursor(9, 1)
+  GhcModSigCodegen
+  call self.assert.match('^\s*cInt x = ', getline(line('.')+1))
+  call self.assert.match('^\s*cString x = ', getline(line('.')+2))
+endfunction
+
+function! s:unit.teardown()
+  " Discard changes
+  bdelete!
+endfunction
+
+call s:unit.run()