idea!

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+When a property needs privdata to be set, it will fail at runtime when
+it's not available. Could this be detected at compile time instead?
+
+Here's an idea of a way to do it. Make propellor, whenever it adds/removes
+privdata, generate a haskell source file, Propellor/PrivData/Available.hs
+
+It would have one type-level function
+
+	data Available
+	type family HasPrivData source context
+	type instance HasPrivData "password" "foo.com" = Available
+	-- ^ supposed to be type level strings
+
+The file would generate instances of the type family or each available privdata
+value.
+
+`withPrivData` would use this type level function, and require it to return
+Availble. If it didn't, the type checker would blow up. 
+
+(Controlling the type error message content to make it clear what went wrong
+may be tricky.)
+
+For this to work, `withPrivData` would need some interesting changes to its
+type signature, so that it has available the type level strings describing
+the privdata it's supposed to get. Is that practical? I think so,
+actually..
+
+	withPrivData :: (HasPrivData source context) => source -> context -> (((PrivData -> Propellor Result) -> Propellor Result) -> Property i) -> Property HasInfo
+
+All that's needed is a way to provide a type level string from which a
+string value can be extracted that has the same string as the type. IIRC,
+that's supported by type level strings.
+
+--[[Joey]]