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{-# LANGUAGE TypeFamilies, GADTs, EmptyDataDecls, TypeOperators #-}
module Test where
data Zero
data Succ a
data FZ
data FS fn
data Fin n fn where
FZ :: Fin (Succ n) FZ
FS :: Fin n fn -> Fin (Succ n) (FS fn)
data Nil
data a ::: b
type family Lookup ts fn :: *
type instance Lookup (t ::: ts) FZ = t
type instance Lookup (t ::: ts) (FS fn) = Lookup ts fn
data Tuple n ts where
Nil :: Tuple Zero Nil
(:::) :: t -> Tuple n ts -> Tuple (Succ n) (t ::: ts)
proj :: Fin n fn -> Tuple n ts -> Lookup ts fn
proj FZ (v ::: _) = v
proj (FS fn) (_ ::: vs) = proj fn vs
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