0

i have this code:

  qs [] = []
  qs [x] = [x]
  qs (x:xs) = qs [a | a <- xs, a < x] ++ [x] ++ qs [b | b <- xs, b >= x]

when i save it to some file .hs and load it to ghci i can execute qs with a list of numbers without a problem.

but in Org-mode it give me error: *** Exception: <interactive>:16:1-56: Non-exhaustive patterns in function qs

the code block looks like this:

#+BEGIN_SRC haskell
qs [] = []
qs [x] = [x]
qs (x:xs) = qs [a | a <- xs, a < x] ++ [x] ++ qs [b | b <- xs, b >= x]
qs [2,9,3,8,4,7,5,6]
#+END_SRC

it's the same code as in the .hs file except the last line that execute an actual qs function.
how can i successfully execute this block?

2

Well i got answer from #haskell in freenode (thanks dmwit!).
the problem is that the REPL run the code line by line (and not as a whole), thus every line create a new qs, shadowing the old one.
so what i need to do is add a parentheses before and after the code, like that:

#+BEGIN_SRC haskell
:{
qs [] = []
qs [x] = [x]
qs (x:xs) = qs [a | a <- xs, a < x] ++ [x] ++ qs [b | b <- xs, b >= x]
:}
qs [2,9,3,8,4,7,5,6]
#+END_SRC

after this change, the code is executing without a problem.

  • Yes, but this isn't really a good solution. What's really bad is I can't write functions without the :{ ... :} trick -- which means this isn't really tangle-able code, i.e., if you separate out the code into a .hs file, you've got to then strip out the trick. This is an old issue. There used to be let in front of function declarations and :set +m to allow multi-line input. But this defeats the purpose of a babel code block allowing true functions in the language to be evaluated. ob-haskell needs to be improved. – 147pm May 29 at 3:25

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.