Handle ghc panic in haskell (#4145)

* Add primitive handling of ghc panic.

* PascalCase in function.

* Add simple test.

autoload/ale/handlers/haskell.vim

@@ -19,6 +19,16 @@ let s:temp_regex_prefix =
 \   . substitute(s:temp_dir, '\\', '\\\\', 'g')
 \   . '\.\{-}'
 
+function! s:PanicOutput(lines) abort
+    return [{
+    \   'lnum': 1,
+    \   'col': 1,
+    \   'text': 'ghc panic!',
+    \   'type': 'E',
+    \   'detail' : join(a:lines, "\n"),
+    \}]
+endfunction
+
 function! ale#handlers#haskell#HandleGHCFormat(buffer, lines) abort
     " Look for lines like the following.
     "
@@ -34,6 +44,14 @@ function! ale#handlers#haskell#HandleGHCFormat(buffer, lines) abort
 
     let l:corrected_lines = []
 
+    " If ghc panic error, put the whole message in details and exit.
+    let l:panic_position = match(a:lines,'ghc: panic!')
+    let l:panic_end = match(a:lines,'Please report this as a GHC bug:')
+
+    if l:panic_position >= 0
+        return s:PanicOutput(a:lines[l:panic_position : l:panic_end])
+    endif
+
     " Group the lines into smaller lists.
     for l:line in a:lines
         if len(matchlist(l:line, l:pattern)) > 0