Ravi,
Ganesh and I were discussing today what would happen if one adds Id
as a primitive type constructor. How much did you have to change the
type checker? Presumably if you need to unify 'm a' with 'a' you now
have to set m=Id. Do you know if you can run into higher order
unification problems? My gut feeling is that with just Id, you
probably don't, but I would not bet on it.
Having Id would be cool. If we make an instance 'Monad Id' it's now
possible to get rid of map and always use mapM instead. Similarly
with other monadic functions.
Did you do that in the Bluespec compiler?
-- Lennart
On Mar 19, 2007, at 19:26 , Ravi Nanavati wrote:
On 3/19/07, Ian Lynagh <[EMAIL PROTECTED]> wrote: I'd really like to
be able to define an eta-reduced Id; I see two
possibilities:
* Allow "type Id =" (I prefer this to "type Id" as I think we are more
likely to want to use the latter syntax for something else later
on).
* Implementations should eta-reduce all type synonyms as much as
possible, e.g.
type T a b c d = X a b Int c d
is equivalent to
type T a b = X a b Int
and
type Id a = a
is equivalent to a type that cannot be expressed directly.
Any opinions?
A third possibility is to have "Id" be a special primitive type
constructor of kind * -> * that implementations handle internally.
If you wanted to give it different name you could use an eta-
reduced type synonym for that, of course.
That's the approach I took when I needed an identity type function
in the Bluespec compiler, and that worked out reasonably well. Part
of the reason that worked out, though, is that we already had a
normalization point during typechecking where certain special type
constructors (related to numeric types) were cleaned out, so adding
Id just extended that a little.
I don't know whether adding such a constructor would be an equally
simple change for Haskell implementations. And there's the separate
argument that requiring eta-reduction of all type synonyms might be
an interesting new feature in its own right (since I think you can
say other new things beyond type Id a = a).
- Ravi
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