[Haskell-beginners] Type inference in ST monad?

[Aurimas]

On 08/19/2013 04:59 PM, Brandon Allbery wrote:
> On Mon, Aug 19, 2013 at 1:36 AM, Aurimas <aurimas.anskaitis at vgtu.lt 
> <mailto:aurimas.anskaitis at vgtu.lt>> wrote:
>
>     Thanks, this is clearly the most easy way to fix the problem. But
>     the question remains - why the line
>     "do r1 <- uniformR (-1.0, 1.0 :: Double) gen"  cannot be annotated
>     with ST s Double?
>
>
> Maybe you need ScopedTypeVariables? But I'd be suspicious that, given 
> the forall in the definition of ST, that such a type annotation 
> *always* creates a new `s` and some other way to "force" the type is 
> needed.
>
ScopedTypeVariables does not solve the problem. As you noticed, new s is 
created and runST is not happy about this. It is
strange that this type can be "forced" replacing given line by a 
function returning ST s a.
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