fix indentation of multi-line type declaration (close #38) (#39)

Author: mateoxh

indent/haskell.vim

@@ -2,7 +2,7 @@
 " Filename: indent/haskell.vim
 " Author: itchyny
 " License: MIT License
-" Last Change: 2025/03/03 09:45:31.
+" Last Change: 2025/04/15 08:09:38.
 " =============================================================================
 
 if exists('b:did_indent')
@@ -80,6 +80,12 @@ function! GetHaskellIndent() abort
     return s:indent_eq()
   endif
 
+  if line =~# '\v\='
+    if getline(v:lnum - 1) =~# '\v^\s*-\>'
+      return indent(v:lnum - 1) - &shiftwidth
+    endif
+  endif
+
   " }, ], )
   if line =~# '\v^\s*[})\]]'
     return s:indent_parenthesis()
@@ -264,6 +270,11 @@ function! GetHaskellIndent() abort
     return s:indent('', '\v^\s*\zs.*<let>', 0, -1)
   endif
 
+  " ::
+  if nonblankline =~# '\v::\s*%(--.*)?$'
+    return indent(v:lnum - 1) + &shiftwidth
+  endif
+
   if nonblankline =~# '::'
     return s:indent('', nonblankline =~# '\v,\s*%(--.*)?$' ? '\S' : '\v\{\s*\<\w+\s*::', 0, match(nonblankline, '\S'))
   endif

test/typesig/typesig.in.hs

@@ -0,0 +1,4 @@
+foo ::
+Int
+-> Int
+foo = bar

test/typesig/typesig.out.hs

@@ -0,0 +1,4 @@
+foo ::
+  Int
+  -> Int
+foo = bar