module ArrowEnv(module Arrow, module ArrowEnv) where
import Arrow
-- more arrow classes (requires multiple parameter type classes)
-- hugs -98 or ghc -fglasgow-exts -fallow-undecidable-instances

class Arrow q => StateArrow s q where
  update :: (s->s) -> q a a
  -- Alternatives:
  -- fetch  :: q a s
  -- store  :: q s ()
  -- update f = arr (\x->((),x)) >>> first (fetch >>> arr f >>> store) >>> arr snd

  -- updateAndReturn :: (s->s) -> q a s
  -- update state and return the old state. 
  -- Impractical to use as a is thrown away.

class Arrow q => ReaderArrow r q where
  poke   :: (r->r) -> q a b -> q a b  
  peek   :: q a (r,a)