thrist-0.3: examples.hs

{-# LANGUAGE TypeOperators #-}

module Main
    where

-- import the main Thrist module
import Data.Thrist


-- identity function on thrists in terms of foldrThrist:
idThrist :: Thrist arr a c -> Thrist arr a c
idThrist = foldrThrist Cons Nil


 -- Explore creating a reverse function using foldlThrist:
 --  The function takes a Thrist of tuples and reverses it (also swapping the
 --  fst and snd values). We use the Flipped newtype to allow us to pass the
 --  reversed Thrist through the foldl function, and then unwrap it at the end:
reverseTuples :: Thrist (,) a b -> Thrist (,) b a
reverseTuples = unflip . foldlThrist rev (Flipped Nil)
    where rev (Flipped t) (a, b) = Flipped $ Cons (b, a) t


-- Composing a thrist of functions:
--  Shows one way of composing a thrist of functions (a -> b) using foldl
func :: [(Int, Char)] -> Int
func = foldlThrist (flip(.)) id thristOfFuncs
    -- ...and here is our type-threaded list of functions:
    where thristOfFuncs :: Thrist (->) [(Int, Char)] Int
          thristOfFuncs = Cons head $ Cons fst $ Cons (+2) Nil

 -- type-checker infers Integer here, instead of a polymorphic Num class unless
 -- I explicitly put it in the signature. Is that normal? Interesting?
--funcPoly :: (Num a) => [(a, b)] -> a
funcPoly = foldlThrist (flip(.)) id thristOfFuncs
    where thristOfFuncs :: (Num a) => Thrist (->) [(a, b)] a
          thristOfFuncs = Cons head $ Cons fst $ Cons (+2) Nil


-- a demo of the `appendThrist` function:
appendedThrists :: (Num a) => Thrist (->) [(a, b)] a
appendedThrists = (Cons head Nil) `appendThrist` (Cons fst Nil) `appendThrist` (Cons (+2) Nil) `appendThrist` Nil