T7438.hs:6:14: error: [GHC-25897]
    • Could not deduce ‘p1 ~ p2’
      from the context: b ~ a
        bound by a pattern with constructor:
                   Nil :: forall {k} (a :: k). Thrist a a,
                 in an equation for ‘go’
        at T7438.hs:6:4-6
      ‘p1’ is a rigid type variable bound by
        the inferred type of go :: Thrist a b -> p1 -> p2
        at T7438.hs:6:1-16
      ‘p2’ is a rigid type variable bound by
        the inferred type of go :: Thrist a b -> p1 -> p2
        at T7438.hs:6:1-16
    • In the expression: acc
      In an equation for ‘go’: go Nil acc = acc
    • Relevant bindings include
        acc :: p1 (bound at T7438.hs:6:8)
        go :: Thrist a b -> p1 -> p2 (bound at T7438.hs:6:1)
    Suggested fix: Consider giving ‘go’ a type signature