Re: [PATCH] ob-haskell: Line Continuations Mangle Block Output

[Nick Daly <nick.m.daly@gmail.com>]

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Hi Kyle, thanks for the thoughtful analysis.

On Wed, May 20, 2020 at 12:51 AM Kyle Meyer <kyle@kyleam.com> wrote:
> So it looks like the member call above is returning nil because the
> prompt markers are corrupting the element.  If that's the case, it seems
> like the output cleansing should happen upstream of that call.
>
> What do you think?

After a bit of tinkering, I realized there are two things going on
here, only one of which I fully understand:

1. My core functional issue is that =comint-prompt-regexp= isn't set
   up to handle the "Prelude| " entries or the repeated prompts.  The
   other patches I submitted were unnecessary.

2. The =comint-prompt-regexp= gets default values from somewhere I
   don't understand and can't find with a quick source grep.

In ob-haskell, we set =comint-prompt-regexp= to the (undefined)
haskell-prompt plus "or optional-lambda":

  (defvar haskell-prompt-regexp)

  (defun org-babel-execute:haskell (body params) ...
  (setq-local comint-prompt-regexp
    (concat haskell-prompt-regexp "\\|^λ?> "))))

That causes an evaluation error that prevents the first source block
evaluation but, strangely, that also results in this mess in the
*haskell* buffer on subsequent evaluations:

: "^\\*?[[:upper:]][\\._[:alnum:]]*\\(?:
\\*?[[:upper:]][\\._[:alnum:]]*\\)*\\( λ\\)?> "

=comint-prompt-regexp='s variable documentation calls out much simpler
regexps that do basically the same thing as the one above and handles
the repeated "Prelude| " entries.  This one is based off the Canonical
Lisp example:

: "^[^>\n]+\\(> \\)?"

I've attached a patch against git master that results in fewer
undefined variable errors and depends less default-variable magic.
Now, =haskell-prompt-regexp= and =comint-prompt-regexp= are explicitly
set using defaults that can be M-x customized, and the default value
handles the repeated "Prelude| " entries without breaking the original
"λ> " prompt handling.

Thanks,
Nick

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diff --git a/lisp/ob-haskell.el b/lisp/ob-haskell.el
index bea162528..893e4220c 100644
--- a/lisp/ob-haskell.el
+++ b/lisp/ob-haskell.el
@@ -56,15 +56,25 @@
 
 (defvar org-babel-haskell-eoe "\"org-babel-haskell-eoe\"")
 
-(defvar haskell-prompt-regexp)
+(defvar haskell-prompt-regexp "^[^>\n]*\\(> \\)?"
+  "Filter out prompts from Haskell interpreters:
+
+GHC:
+
+- '^Prelude> '
+- '^Prelude| Prelude| Prelude> '
+
+Unknown Interpreter:
+
+- '^> '
+- '^λ> '")
 
 (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?> "))))
+              (setq-local comint-prompt-regexp haskell-prompt-regexp)))
   (let* ((session (cdr (assq :session params)))
          (result-type (cdr (assq :result-type params)))
          (full-body (org-babel-expand-body:generic