On Thu, 11 Oct 2007, Tom Davies wrote:
Andrew Wagner
writes: If you change your type declarations to 'newtype' declarations, I believe you would get the effect that you want, depending on what you mean by 'equivalent'. In that case, Foo and Bar would essentially be strings, but you could not use either of them in a place where the other is expected, nor where a String is expected. See http://haskell.org/haskellwiki/Newtype for more information. Hope this helps!
I wanted to avoid wrapping the string with a constructor.
I suppose what I'm really asking for is for each type to implicitly define a 'type class with no methods', and to be able to create new instances of that type class which simply behave as the underlying type.
For custom types you can add phantom type parameters. This way you can make types distinct while being able to apply the same functions to their values. Say data SpecialString usage = SpecialString String Then you can define a generic function like take :: Int -> SpecialString usage -> SpecialString usage or a specialised function like foo :: SpecialString Foo -> SpecialString Foo