Re: [Haskell] pattern matching accross instance declarations

S. Alexander Jacobson wrote:

data MyExistantialType=forall v.(Show v)=>EType v

class MyExistentialTypeable a where toMyType::String->MyExistantialType

instance MyExistentialTypeable String where toMyType "String" = EType "foo"

instance MyExistentialTypeable Int where toMyType "Int" = EType "bar"

tVal (EType v)=show v main = print $ tVal $ toMyType "String"

The type class isn't necessary here, since toMyType has the same type in every instance anyway (namely String->MyExistentialType). You can simply write data MyExistantialType=forall v.(Show v)=>EType v toMyType "String" = EType "foo" toMyType "Int" = EType "bar" tVal (EType v)=show v main = print $ tVal $ toMyType "String" -- Ben