Re: [Haskell-cafe] Why doesn't this work?

I wrote:

...

A more basic issue is that fn is in the IO monad, but its use inside the mapM will need it to be in the ST monad.

Daniel Fischer wrote:

No, return (fn particle) :: ST s (IO ()) , so that's fine.

Ah, true. But I doubt that Andrew really meant to do the calculation in ST s (IO ()).

Indeed, filling in a few dummies, the code compiles with the 6.8 branch.

So then the only way to know why it didn't compile for Andrew is if we see more of the code. But that's only a technical issue. I hope that now Andrew has a better idea about what's wrong. If not - please let us know. Regards, Yitz