transducermisc.elm

type alias Reducer input state = input -> state -> state

dimapR : (a -> b) -> (b -> a) -> Reducer x a -> Reducer x b
dimapR f g reducer x b = 
  f (reducer x (g b))


contramap : (b -> a) -> Transducer a b r
contramap f reducer b = 
  reducer (f b)
  
keeping : (a -> Bool) -> Transducer a a r
keeping predicate reducer a r = 
  if predicate a
  then 
    reducer a r
  else
    r
  
dropping : (a -> Bool) -> Transducer a a r
dropping predicate = 
  keeping (predicate >> not)  
  
divide : (c -> (a, b)) -> Reducer a r -> Reducer b r -> Reducer c r
divide f reducerA reducerB c = 
  let 
      (a, b) = f c
  in 
      reducerA a >> reducerB b
  

conquer : Reducer a x
conquer _ x = x
  
  
type alias Transducer a b r = Reducer a r -> Reducer b r


map : (a -> b) -> Transducer a x y -> Transducer b x y
map f transducer reducer = 
  transducer (contramap f reducer)
   
compose : Transducer a b r -> Transducer b c r -> Transducer a c r
compose = (>>)

constant : Transducer a b r
constant _ = conquer

id : Transducer a a r
id x = x
  
  
dimap : (a -> b) -> (b -> a) -> Transducer x y a -> Transducer x y b
dimap f g transducer reducer y b = 
  f (transducer (dimapR g f reducer) y (g b))