tcfail103.hs:15:13: error:
    • Couldn't match type ‘s’ with ‘t’
      Expected: ST s Int
        Actual: ST t Int
      ‘s’ is a rigid type variable bound by
        the type signature for:
          g :: forall s. ST s Int
        at tcfail103.hs:13:9-21
      ‘t’ is a rigid type variable bound by
        the type signature for:
          f :: forall t. ST t Int
        at tcfail103.hs:10:1-12
    • In the expression: readSTRef v
      In an equation for ‘g’: g = readSTRef v
      In the expression:
        do v <- newSTRef 5
           let g :: ST s Int
               g = readSTRef v
           g
    • 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)