On Mon, Jan 19, 2009 at 1:13 PM, Daniel Fischer <daniel.is.fischer@web.de> 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