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

Alfonso Acosta wrote:

I haven't tried to run the code, but my first bet is that, due to the rank-2 polymorphism of ST, you should use parenthesis instead of $ in the case of runST.

Perhaps if Andrew is using an old compiler. That is no longer a problem in recent versions of GHC. 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. Regards, Yitz