tcfail103.hs:15:23:
    Couldn't match type ‘t’ with ‘s’
      ‘t’ is a rigid type variable bound by
          the type signature for: f :: ST t Int at tcfail103.hs:10:5
      ‘s’ is a rigid type variable bound by
          the type signature for: g :: ST s Int at tcfail103.hs:13:14
    Expected type: STRef s Int
      Actual type: STRef t Int
    In the first argument of ‘readSTRef’, namely ‘v’
    In the expression: readSTRef v
    Relevant bindings include
      g :: ST s Int (bound at tcfail103.hs:15:9)
      v :: STRef t Int (bound at tcfail103.hs:12:5)
      f :: ST t Int (bound at tcfail103.hs:11:1)