Re: fixpoint combinator in monads

[Magnus Carlsson]

If you want to play with a monadic fixpoint combinator in HBC, you can
use

  module FixMonad where
  import UnsafeDirty(assign,deref,ref)

  fix :: Monad m => (a -> m a) -> m a
  fix f = do
      a <- f (deref r)
      assign r a `seq` return a
    where r = ref (undefined f)

Here is a stupid example using the list monad:

  import FixMonad

  ones :: Monad m => m [Int]
  ones = fix (\x -> return (1:x))

  main = print (ones :: [[Int]])

Disclaimer: No parts of this software have been tested on animals.

/Magnus