From 018ebecade82009a3410f19982dd435b6e0715d8 Mon Sep 17 00:00:00 2001
From: Tom Smeding <tom@tomsmeding.com>
Date: Sun, 14 Apr 2024 10:31:46 +0200
Subject: Rename inductive naturals to INat

---
 src/Data/INat.hs | 95 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 95 insertions(+)
 create mode 100644 src/Data/INat.hs

(limited to 'src/Data/INat.hs')

diff --git a/src/Data/INat.hs b/src/Data/INat.hs
new file mode 100644
index 0000000..f7523ee
--- /dev/null
+++ b/src/Data/INat.hs
@@ -0,0 +1,95 @@
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE PatternSynonyms #-}
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE StandaloneDeriving #-}
+{-# LANGUAGE TypeApplications #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE TypeOperators #-}
+{-# LANGUAGE UndecidableInstances #-}
+{-# LANGUAGE ViewPatterns #-}
+{-# OPTIONS_GHC -fplugin GHC.TypeLits.KnownNat.Solver #-}
+module Data.INat where
+
+import Data.Proxy
+import Numeric.Natural
+import GHC.TypeLits
+
+
+-- | Evidence for the constraint @c a@.
+data Dict c a where
+  Dict :: c a => Dict c a
+
+-- | An inductive peano natural number. Intended to be used at the type level.
+data INat = Z | S INat
+  deriving (Show)
+
+-- | Singleton for a 'INat'.
+data SINat n where
+  SZ :: SINat Z
+  SS :: SINat n -> SINat (S n)
+deriving instance Show (SINat n)
+
+-- | A singleton 'SINat' corresponding to @n@.
+class KnownINat n where inatSing :: SINat n
+instance KnownINat Z where inatSing = SZ
+instance KnownINat n => KnownINat (S n) where inatSing = SS inatSing
+
+-- | Explicitly bidirectional pattern synonym that converts between a singleton
+-- 'SINat' and evidence of a 'KnownINat' constraint. Analogous to 'GHC.SNat'.
+pattern SINat' :: () => KnownINat n => SINat n
+pattern SINat' <- (snatKnown -> Dict)
+  where SINat' = inatSing
+
+-- | A 'KnownINat' dictionary is just a singleton natural, so we can create
+-- evidence of 'KnownNat' given an 'SINat'.
+snatKnown :: SINat n -> Dict KnownINat n
+snatKnown SZ = Dict
+snatKnown (SS n) | Dict <- snatKnown n = Dict
+
+-- | Convert a 'INat' to a normal number.
+fromINat :: INat -> Natural
+fromINat Z = 0
+fromINat (S n) = 1 + fromINat n
+
+-- | Convert an 'SINat' to a normal number.
+fromSINat :: SINat n -> Natural
+fromSINat SZ = 0
+fromSINat (SS n) = 1 + fromSINat n
+
+-- | The value of a known inductive natural as a value-level integer.
+inatVal :: forall n. KnownINat n => Proxy n -> Natural
+inatVal _ = fromSINat (inatSing @n)
+
+-- | Add two 'INat's
+type family n +! m where
+  Z +! m = m
+  S n +! m = S (n +! m)
+
+-- | Convert a 'INat' to a "GHC.TypeLits" 'G.Nat'.
+type family FromINat n where
+  FromINat Z = 0
+  FromINat (S n) = 1 + FromINat n
+
+-- | If an inductive 'INat' is known, then the corresponding "GHC.TypeLits"
+-- 'G.Nat' is also known.
+knownNatFromINat :: KnownINat n => Proxy n -> Dict KnownNat (FromINat n)
+knownNatFromINat (Proxy @n) = go (SINat' @n)
+  where
+    go :: SINat m -> Dict KnownNat (FromINat m)
+    go SZ = Dict
+    go (SS n) | Dict <- go n = Dict
+
+-- * Some type-level inductive naturals
+
+type I0 = Z
+type I1 = S I0
+type I2 = S I1
+type I3 = S I2
+type I4 = S I3
+type I5 = S I4
+type I6 = S I5
+type I7 = S I6
+type I8 = S I7
+type I9 = S I8
-- 
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