{-# LANGUAGE GADTs #-}
{-# LANGUAGE PatternSynonyms #-}
{-# LANGUAGE ViewPatterns #-}

module ImportPattern (
  useP,
  collectArgs,
  isInt,
  isIntEndo,
  arrows,
  viewPL,
  viewPR,
  fac,
  showable,
  one,
  f,
  g,
  two,
  single,
  hd,
  h ) where

import Imported.ExportPattern
    ( T,
      Nat(Z, S),
      Showable(..),
      Type,
      pattern ExNumPat,
      pattern Head,
      pattern Single,
      pattern Pair,
      pattern One,
      pattern Succ,
      pattern (:>),
      pattern (:<),
      pattern Empty,
      pattern Int,
      pattern Arrow,
      pattern P )

-- pattern P = 42

useP P = 43

--

{-
data Type = App String [Type]

pattern Arrow :: Type -> Type -> Type
pattern Arrow t1 t2 = App "->" [t1, t2]

pattern Int = App "Int" []

pattern Maybe t = App "Maybe" [t]

-}

collectArgs :: Type -> [Type]
collectArgs (Arrow t1 t2) = t1 : collectArgs t2
collectArgs _ = []

isInt :: Type -> Bool
isInt Int = True
isInt _ = False

isIntEndo :: Type -> Bool
isIntEndo (Arrow Int Int) = True
isIntEndo _ = False

arrows :: [Type] -> Type -> Type
arrows = flip $ foldr Arrow

--
{-

pattern Empty <- (Seq.viewl -> Seq.EmptyL)
pattern x :< xs <- (Seq.viewl -> x Seq.:< xs)
pattern xs :> x <- (Seq.viewr -> xs Seq.:> x)
-}

viewPL (x :< Empty) = x
viewPR (Empty :> y) = y

--

{-
pattern Succ n <-
  (\x -> (x -1) <$ guard (x > 0) -> Just n)
  where
    Succ n = n + 1
-}

fac (Succ n) = Succ n * fac n
fac 0 = 1

--

{-
data Showable where
  MkShowable :: (Show a) => a -> Showable

-- Required context is empty, but provided context is not
pattern Sh :: () => (Show a) => a -> Showable
pattern Sh x <- MkShowable x
-}

showable :: (Show a) => a -> Showable
showable x = MkShowable x

--

{-
-- Provided context is empty
pattern One :: (Num a, Eq a) => a
pattern One <- 1
-}

one One = 2

--


--pattern Pair x y <- [x, y]


f (Pair True True) = True
f _ = False

g [True, True] = True
g _ = False


--
{-
data Nat = Z | S Nat deriving (Show)

pattern Ess p = S p
-}

two = S ( S Z)

--

-- pattern Single x = [x]

-- pattern Head x <- x : xs

single (Single x) = x
hd :: [a] -> a
hd (Head x) = x

--

{-
data T a where
  MkT :: (Show b) => a -> b -> T a

pattern ExNumPat x = MkT 42 x
-}

h :: (Num t, Eq t) => T t -> String
h (ExNumPat x) = show x