Instance function not in scope?

Main.purs

module Main where

import Prelude
import Data.Either (Either(..))
import Data.Profunctor (class Profunctor, arr)
import Data.Profunctor.Choice (class Choice)
import Data.Profunctor.Strong (second)
import Data.Tuple (Tuple(..))

-- State transformers
newtype StateTrans s i o = ST (Tuple s i → Tuple s o)

instance stFunctor ∷ Functor (StateTrans s a) where
  map f (ST st) = ST $ second f <<< st

instance stProfunctor ∷ Profunctor (StateTrans s) where
  dimap f g (ST st) = ST $ second g <<< st <<< second f

instance stChoice ∷ Choice (StateTrans s) where
  left (ST f) = ST lf
    where
    lf (Tuple s (Left a)) = let (Tuple s' b) = f (Tuple s a)
                            in Tuple s' (Left b)
    lf (Tuple s (Right c)) = Tuple s (Right c)
  -- Fails to compile with: Unknown value left
  right f = arr mirror <<< left f <<< arr mirror
    where
    mirror Left x = Right x
    mirror Right x = Left x