Re: [Haskell-cafe] Different return type?

18 Jan 2009

      Hi Daniel,

When would I use either?  What are the trade-offs?

Thanks

-John

On Mon, Jan 19, 2009 at 1:13 PM, Daniel Fischer wrote:
...
Am Montag, 19. Januar 2009 02:44 schrieb John Ky:
...
Hi,
Possibly a silly question but is it possible to have a function that has
a
different return type based on it's first argument?
For instance
data Person = Person { name :: String, ... }
data Business = Business { business_number :: Int, ...}
key person = name person
key business = business_number business
Thanks
-John
Well, you could use
{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies,
TypeSynonymInstances #-}
module Key where
data Person = Person { name :: String }
data Business = Business { business_number :: Int}
class Key a b | a -> b where
   key :: a -> b
instance Key Person String where
   key = name
instance Key Business Int where
   key = business_number
or with type families:
{-# LANGUAGE TypeFamilies #-}
class Key2 a where
   type Res a
   key2 :: a -> Res a
instance Key2 Person where
   type Res Person = String
   key2 = name
instance Key2 Business where
   type Res Business = Int
   key2 = business_number
but apart from that and parametrically polymorphic functions (of type a ->
[a]
or the like), I don't think it's possible, it would need dependent types.
HTH,
Daniel