James M
This is easily testable.
deriving instance Typeable Entry deriving instance Typeable FileKind deriving instance Typeable 'FOLDER deriving instance Typeable 'FILE
convertBack :: SomeEntry -> Either (Entry FILE) (Entry FOLDER) convertBack (SomeEntry x) | typeOf x == typeOf (Folder "" "") = Right x | otherwise = Left x
If the goal is just to write the `convertBack' function with the type signature above you don't need Typeable. If you enable the GADT extension you can just pattern match on the Entry that is stored in the SomeEntry, even though they have different types: i.e. convertBack :: SomeEntry -> Either (Entry FILE) (Entry FOLDER) convertBack (SomeEntry f@(File _ _ _)) = Left f -- By pattern matching on f we -- can convince the compiler -- that f is of type Entry FILE convertBack (SomeEntry d@(Folder _ _)) = Right d Similarly we can write a function convertAsFile :: SomeEntry -> Entry FILE convertAsFile (SomeEntry f@(File _ _ _)) = f convertAsFile (SomeEntry d) = error "We cannot convert from a Entry FOLDER" however there is no way of making that function total (i.e. in the second case there is no proper way of constructing an Entry FILE from an Entry FOLDER) Because you can (locally) recover the type by pattern matching on a GADT I prefer to use GADTs to model existential types rather than using the ExistentialTypes extension. -- - Frank