Hi Thomas, Ihor,

I'm not currently using ob-haskell, but I do have a version of GHC.  As
I may use ob-haskell one day, I decided to take a look.

Here are the versions I'm using:
   #+begin_src elisp
     (list (list "emacs" emacs-version)
           (list "org"   org-version)
           (list "ghc"   (string-trim (shell-command-to-string "ghc -V")))
           )
   #+end_src

   #+RESULTS:
   | emacs | 29.0.50                                                         |
   | org   | 9.6-pre                                                         |
   | ghc   | The Glorious Glasgow Haskell Compilation System, version 8.10.7 |


The following code block is incorrect:

   #+begin_src haskell :results output
     main :: IO () 
     main = putStrLn  "Hello, World!"
     main
   #+end_src

   #+RESULTS:
   : <interactive>:2:1-4: error:
   :     • Variable not in scope: main :: IO ()
   :     • Perhaps you meant ‘min’ (imported from Prelude)
   : Prelude> Hello, World!

The first line tries to evaluate 'main' which doesn't exist (yet).

The following modified block works, using the compiler.

   #+begin_src haskell :compile yes :results output
     main :: IO () 
     main = putStrLn  "Hello, World!"
   #+end_src

   #+RESULTS:
   : Hello, World!


The following works using the interpreter:
   #+begin_src haskell
     let { main :: IO () 
         , main = putStrLn  "Hello, World!"
         }
     main
   #+end_src

   #+RESULTS:
   : Hello, World!

A simpler version, that just print "Hello, World!" works:

   #+begin_src haskell
     putStrLn  "Hello, World!"
   #+end_src

   #+RESULTS:
   : Hello, World!

Just evaluating the string doesn't work!
   #+begin_src haskell
     "Hello world!"
   #+end_src

   #+RESULTS:

as we don't get any result ...

If I understand correctly, it seems to be a bug in ob-haskell;
`org-babel-comint-with-output' shouldn't be instructed to remove the
output if it matches the input, else, it will remove any constant.

Adding a type annotation is enough to make it works:

   #+begin_src haskell
     "Hello world!" :: String
   #+end_src

   #+RESULTS:
   : Hello world!
   
Or fixing `org-babel-interpret-haskell' (see attached patch):
   #+begin_src haskell
     "Hello world!"
   #+end_src

   #+RESULTS:
   : Hello world!

Another example that works too, with or without the patch:
   #+begin_src haskell
     concat ["Hello", ", ", "World", "!"]
   #+end_src

   #+RESULTS:
   : Hello, World!


I've attached the patch that I've used to fix ob-haskell.

Should I submit a patch for ob-haskell ?

Bruno