bmmoore · April 23, 2012 20:37 · Apr 23, 2012
diff --git a/gistfile1.txt b/gistfile1.txt
@@ -0,0 +1,47 @@
+{-# LANGUAGE
+  MultiParamTypeClasses,
+  TypeFamilies #-}
+
+class Field f where
+  type FType f :: *
+
+class (Field field) => Has field o where
+  get :: field -> o -> FType field
+
+data Name = Name
+instance Field Name where
+  type FType Name = String
+data Area = Area
+instance Field Area where
+  type FType Area = Float
+data AsString = AsString
+instance Field AsString where
+  type FType AsString = String
+
+data Srec = Srec { sn :: String, srepr :: String } 
+instance Has Name Srec where
+  get _ s = sn s
+instance Has AsString Srec where
+  get _ s = srepr s
+
+shape name = \self -> Srec
+  name
+  ("Shape "++name++" with area "++show (get Area self))
+
+{- inferred type:
+shape :: Has Area o => String -> o -> Srec
+ -}
+
+data SqRec = SqRec {sqn :: String,
+                    sqa :: Float,
+                    sqrepr :: String}
+instance Has Name SqRec where
+  get _ s = sqn s
+instance Has Area SqRec where
+  get _ s = sqa s
+instance Has AsString SqRec where
+  get _ s = sqrepr s
+
+square name side = \self ->
+  let super = shape name self in
+  SqRec (get Name super) (side*side) (get AsString super)
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