Fix ob-haskell.el to work with custom ghci prompts

* lisp/ob-haskell.el (haskell-prompt-regexp):  Define defvar
`haskell-prompt-regexp`.  This variable will override the variable set by
inf-haskell, when `org-babel-execute:haskell` is called.
(org-babel-execute:haskell):  Make sure that
`comint-prompt-regexp` is set appropriately to enable  correct parsing
of "λ"-prompts.  Set `comint-preoutput-filter-functions` appropriately to
enable correct parsing of coloured ghci prompts.

The problem was that code sent back from the inf-haskell buffer to
 org-babel wasn't parsed correctly in `org-babel-comint-with-output`.
 This occured when the user uses the commonly used "λ"-prompt.

TINYCHANGE

lisp/ob-haskell.el

@@ -59,14 +59,23 @@
 
 (defvar org-babel-haskell-eoe "\"org-babel-haskell-eoe\"")
 
+(defvar haskell-prompt-regexp)
+
 (defun org-babel-execute:haskell (body params)
   "Execute a block of Haskell code."
+  (require 'inf-haskell)
+  (add-hook 'inferior-haskell-hook
+            (lambda ()
+              (setq-local comint-prompt-regexp
+                          (concat haskell-prompt-regexp "\\|^λ?> "))))
   (let* ((session (cdr (assq :session params)))
          (result-type (cdr (assq :result-type params)))
          (full-body (org-babel-expand-body:generic
 		     body params
 		     (org-babel-variable-assignments:haskell params)))
          (session (org-babel-haskell-initiate-session session params))
+       	 (comint-preoutput-filter-functions
+	       (cons 'ansi-color-filter-apply comint-preoutput-filter-functions))
          (raw (org-babel-comint-with-output
 		  (session org-babel-haskell-eoe t full-body)
                 (insert (org-trim full-body))