1 files changed, 54 insertions, 0 deletions
diff --git a/src/Nats.hs b/src/Nats.hs
new file mode 100644
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--- /dev/null
+++ b/src/Nats.hs
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+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE StandaloneDeriving #-}
+{-# LANGUAGE TypeApplications #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE TypeOperators #-}
+{-# LANGUAGE UndecidableInstances #-}
+{-# OPTIONS_GHC -fplugin GHC.TypeLits.KnownNat.Solver #-}
+module Nats where
+
+import Data.Proxy
+import Numeric.Natural
+import qualified GHC.TypeLits as G
+
+
+data Dict c a where
+  Dict :: c a => Dict c a
+
+data Nat = Z | S Nat
+  deriving (Show)
+
+data SNat n where
+  SZ :: SNat Z
+  SS :: SNat n -> SNat (S n)
+deriving instance Show (SNat n)
+
+class KnownNat n where knownNat :: SNat n
+instance KnownNat Z where knownNat = SZ
+instance KnownNat n => KnownNat (S n) where knownNat = SS knownNat
+
+unSNat :: SNat n -> Natural
+unSNat SZ = 0
+unSNat (SS n) = 1 + unSNat n
+
+unNat :: Nat -> Natural
+unNat Z = 0
+unNat (S n) = 1 + unNat n
+
+snatKnown :: SNat n -> Dict KnownNat n
+snatKnown SZ = Dict
+snatKnown (SS n) | Dict <- snatKnown n = Dict
+
+type family GNat n where
+  GNat Z = 0
+  GNat (S n) = 1 G.+ GNat n
+
+gknownNat :: KnownNat n => Proxy n -> Dict G.KnownNat (GNat n)
+gknownNat (Proxy @n) = go (knownNat @n)
+  where
+    go :: SNat m -> Dict G.KnownNat (GNat m)
+    go SZ = Dict
+    go (SS n) | Dict <- go n = Dict