From 6a0d7f5c434c3564d0119befb6799fd77581050a Mon Sep 17 00:00:00 2001
From: Paul Oliver <contact@pauloliver.dev>
Date: Fri, 3 Jan 2025 11:01:20 -0800
Subject: Initial

---
 ch09_09.2.example.hs | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)
 create mode 100644 ch09_09.2.example.hs

(limited to 'ch09_09.2.example.hs')

diff --git a/ch09_09.2.example.hs b/ch09_09.2.example.hs
new file mode 100644
index 0000000..bcecf7a
--- /dev/null
+++ b/ch09_09.2.example.hs
@@ -0,0 +1,33 @@
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE TypeFamilies #-}
+
+import Data.Kind (Type)
+import Data.Proxy (Proxy (Proxy))
+import GHC.TypeLits (KnownSymbol, Symbol, symbolVal)
+
+data (a :: k1) :<< (b :: k2)
+
+infixr 5 :<<
+
+class HasPrintf a where
+  type Printf a :: Type
+  format :: String -> Proxy a -> Printf a
+
+instance KnownSymbol text => HasPrintf (text :: Symbol) where
+  type Printf text = String
+  format s _ = s <> symbolVal (Proxy @text)
+
+instance (HasPrintf a, KnownSymbol text) => HasPrintf ((text :: Symbol) :<< a) where
+  type Printf (text :<< a) = Printf a
+  format s _ = format (s <> symbolVal (Proxy @text)) (Proxy @a)
+
+instance (HasPrintf a, Show param) => HasPrintf ((param :: Type) :<< a) where
+  type Printf (param :<< a) = param -> Printf a
+  format s _ param = format (s <> show param) (Proxy @a)
+
+instance {-# OVERLAPPING #-} HasPrintf a => HasPrintf (String :<< a) where
+  type Printf (String :<< a) = String -> Printf a
+  format s _ param = format (s <> param) (Proxy @a)
+
+printf :: HasPrintf a => Proxy a -> Printf a
+printf = format ""
-- 
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