Haskell: "Could not deduce" error with runST

When I try to compile this:

module Main where

import qualified Data.Vector.Unboxed.Mutable as MV
import Control.Monad.ST

myRead mv = runST $ MV.read mv 0

I get the following error message:

Could not deduce (t ~ U.MVector s a)
    from the context (U.Unbox a)
      bound by the inferred type of myRead :: U.Unbox a => t -> a
      at src/Main.hs:53:1-32
      `t' is a rigid type variable bound by
          the inferred type of myRead :: U.Unbox a => t -> a
          at src/Main.hs:53:1
    Expected type: U.MVector (PrimState (ST s)) a
      Actual type: t
    In the first argument of `MV.read', namely `mv'
    In the second argument of `($)', namely `MV.read mv 0'
    In the expression: runST $ MV.read mv 0

Can I make a read from a mutable vector pure with runST? If so, how? I assume it entails a type signature for myRead, but everything I've tried has just lead to more and more incomprehensible error messages.

EDIT: Highlighting some context I just put in a comment below: The context here is that I have a function that takes in a mutable vector, does some computations using the mutable vector as a temporary scratch space, then needs to return a float value. Because I don't care about the changes to the mutable vector, I was wondering if there was a way to ignore its "state change" and simply return one of the values from inside it.