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

Yitzchak Gale gale at sefer.org
Sun Aug 24 14:32:01 EDT 2008

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.


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