[Haskell-cafe] type families and type signatures

[Ganesh Sittampalam]

On Sun, 6 Apr 2008, Thomas M. DuBuisson wrote:

> Id is an operation over types yielding a type, as such it doesn't make
> much sense to me to have (Id a -> Id a) but rather something like (a ->
> Id a).  One could make this compile by adding the obvious instance:
>
>> type instance Id a = a
>
> Curiously, is this a reduction from a real world use of families?  I
> just can't think of how a (Fam a -> Fam a) function would be of use.

Yes, it's cut down from an example where (I think) I really need the type 
signature to specialise a general function that does do something useful. 
The generalised intstance above wouldn't be valid or sensible in that 
context.

Cheers,

Ganesh