Re: [Haskell] pattern matching accross instance declarations

[Ben Rudiak-Gould]

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
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