runST type error

[Byron Cook]

Ok , I admit that I ran a patch on hugs-1.4's type checker .... so this
may be a bug in the patch... but the type checker seems to be a bit
relaxed about runST

this program (due to John Launchbury) typechecks and runs in our Hugs :

unsafeST x = runST x
 
cast :: a -> b
cast x
  = unsafeST $
    do let v = unsafeST $ newVar undefined
       writeVar v x
       readVar v

however, cast' does not typecheck

cast' x
  = runST (
      do let v = runST ( newVar undefined)
         writeVar v x
         readVar v
    )


byron